Rebuilding the BC-348 Receivers
AC Power Supply Enhancement -
- Amateur Vintage Military Radio Stations -
by: Henry Rogers WA7YBS/WHRM
Let's face it! Almost every, single BC-348 has fallen victim to the "cut and hack" mania that afflicted many hams back in the fifties and sixties. If you're lucky, the BC-348 you happen to find might only have an AC power supply installed. However, even this simple modification required a lot of changes to the BC-348 circuit and today, if you want to return the BC-348 back to its original configuration, you can't just drop in the DM-28 dynamotor and expect the receiver to function. This article will detail not only what is required to retrofit the DM-28 into a modified BC-348 but also details many other aspects to consider in the restoration of these great receivers. Part One of this article is specifically for the BC-348-Q. The "Q" version was the most popular of the BC-348 series with probably well over 40,000 receivers having been built by Wells-Gardner. Of note is that the "Q" version is virtually identical to the "N" and the "J" versions. These three versions use Single-Ended tubes and feature a circuit that is very easy to work on. Part Two of this article deals specifically with the BC-348-C version that uses the "Grid Cap" tubes. The BC-348-C is very similar to all of the "Grid Cap" tube versions. The earlier versions of the BC-348 receiver are much more difficult to work on. Three additional sections address the BC-348 owner that wants to keep his AC power supply operation but wants the best performance from his BC-348, how to do a successful wrinkle finish re-paint job and a photo gallery of Amateur Vintage Military Radio Stations.
A Brief History of the BC-348 Series Radio Receivers
The BC-348 Aircraft Receiver was designed by RCA in the late thirties. It was an updated version of the earlier BC-224 12 volt version that RCA designed in 1935. As aircraft power systems developed, the change-over to 28vdc systems was taking place by the late-thirties. At the time the BC-348 was introduced, the BC-224 was also upgraded to have the same external appearance. The BC-348 operates on 24-28vdc with the high voltage (~+220vdc) provided by an internal dynamotor. Many tens of thousands were built during WWII by many different contractors building many different versions within that time period. The "Early" circuit used eight tubes with the heaters originally wired in series-parallel for 24vdc operation (two parallel strings of four 6 volt tube heaters in series would operate on 24vdc.) The "early" circuit provided two RF amplifiers, a Mixer, a Local Oscillator, an IF amplifier stage, a combination 2nd IF amp and BFO, a combination 3rd IF amp and Detector/AVC followed by a type 41 audio output stage (this was changed to a 6K6 in some later versions.) Early versions also will have a 991 neon lamp acting as a regulator on the local oscillator and will provide an antenna trim control. A selectable crystal filter was also included in the circuit. The dual dial lamps were adjustable for brightness and were wired in series through a potentiometer and fixed resistor. Frequency coverage was from 200-500kc (not on the B or C version) and 1.5-18mc. The audio output impedance was internally selectable at "low Z" which was around 300 Z ohms or "high Z" which was around 4000 Z ohms. Some BC-348s will have a decal on the front panel indicating if the "low Z" was optioned.
During the middle of WWII, the BC-348-J, N and Q "Later Version" of the receiver was introduced. This new version had to be interchangeable with the earlier BC-348 so physically both versions appear almost identical. Inside, however, several changes were made to reduce the cost of building the BC-348 but still maintaining its high level of performance. The "later" version reduced production costs by simplifying the construction and component mounting which additionally made depot repairs of these "later" receivers much easier. The "later" circuit used two RF amplifiers, a converter stage, three IF amplifiers, a duplex diode/triode provided Detector, AVC and BFO functions and a 6K6 provided the audio output.
When the receiver was installed on its FT-154 shock mount and installed in the aircraft, an eight pin Jones plug mated with a receptacle and cable that exited from the rear of the mount containing the 28vdc input, the remote stand-by relay function and an audio output line. The BC-348 was generally interconnected with the transmitter to control boxes allowing the transmitter's control relay to provide antenna switching, receiver stand-by and providing side tone monitoring which allowed for full "break-in" keying.
Since there are so many variations, military radio collectors have generally divided the BC-348 into two groups, "early" types (B, C, E, H, K, L, O, P & R) referred to as the "Grid Cap Tubes" version and the "later" versions (J, N & Q) referred to as the "Single-Ended Tubes" version. Many different contractors built BC-348s but Wells-Gardner Co. probably built the greatest quantity of receivers and is the most commonly seen contractor-manufacturer. Estimated production of the entire BC-348 line exceeds 100,000 receivers making it one of the most commonly found WWII aircraft receivers today.
The BC-348 became available as surplus in the 1950s allowing hams the ability to purchase a great performing receiver at a reasonable price - NOS and still in the crate for about $75. The down side is that today it is almost impossible to find a BC-348 that hasn't been modified. Typically, the dynamotor will have been removed and an AC power supply added. This mod was fairly simple and usually didn't compromise the receiver's performance (other than adding some hum to the audio output if the power supply wasn't well-filtered.) Many other dubious modifications were published in various surplus conversion books of the day that actually did compromise the original BC-348's great performance. Additionally, many hams would drill the case with lots of .25" holes thinking the receiver needed extra cooling. Originally, the BC-348 operated at altitudes up to 25,000 feet and higher where the temperature inside the aircraft was below zero - cooling the receiver was not a problem. But even on the ground in other applications, the BC-348 didn't run hot and adding ventilation holes only ruined the cabinet.
Today, with the increasing popularity of hams running completely military radio stations, the BC-348 is often being used as the station receiver. Sometimes this is because the original military configuration requires its use, other times because the operator knows that it's such a great performing receiver - a receiver that "does so much, so well, with so little." Fabulous military looks, excellent sensitivity and good selectivity (when the IF is properly aligned and you actually use the Crystal Filter) have made the BC-348 a favorite among the military radio enthusiasts, knowledgeable hams and collectors of WWII airborne radio equipment.
Finding a Good Candidate for Restoration
|What to Look For - After you've looked at a lot of
BC-348 receivers, it becomes pretty easy to eliminate the "victims" from
the "survivors." "Victims" will have extra controls, perhaps a "hacked
in" S-meter, generally sloppy workmanship and sometimes lots of extra
holes drilled in the cabinet. Of course, I'm exaggerating a little bit,
but the ideal "survivor" BC-348 will have all the original controls and
no extra ones. Everything will appear correct on the exterior. Pull the
receiver out of the case and check the chassis for obviously missing
components - it is common to find the combination audio output
transformer-filter choke missing. It's also possible that an extra stage
of audio amplification might have been added. What you want to see is
all of the original components present with the exception of an AC power
supply that just fits into the area where the dynamotor originally was
and is mounted to the chassis using the original dynamotor threaded
bosses. This is about as close as you can normally come to finding a
"perfect" BC-348 - at a reasonable price, anyway.
The photo to the right shows a typical "looks good" candidate. No extra controls, no extra holes. Everything looks pretty good on the exterior with the possible exception of the RCA phone jack for the Antenna input. The photos below show what surprises are lurking inside this particular BC-348.
photo right: BC-348-Q sn 20966 looks like a good restoration candidate.
|Once the receiver is pulled out of the case it becomes apparent that
BC-348-Q sn 20966 was modified by someone lacking the technical ability
and experience to do a competent job. Note that the top of the chassis
looks complete and original except for the AC power supply that was
built onto the old dynamotor chassis but underneath extensive
modifications have removed many of the original parts. Of note, the
complete Jones-type plug assembly is missing along with C-70A/B*, the
dual filter capacitor. Fortunately, the modifier utilized the existing
wiring harness and terminal connections to the AC PS. This particular
BC-348 is certainly a very good candidate for complete
restoration.** Actually, it's in much better condition than most BC-348
receivers you find. But, as with most examples that turn up today, I'll
be collecting parts for this restoration for awhile. In fact, the
plastic bag tied to the front handle contains some of the replacement
parts I've already located.
When you find a BC-348 restoration candidate, you'll want to know "up front" how much work and how many parts you are going to have to find in order to complete the restoration since this is going to affect the final selling price. It is imperative that the prospective BC-348 chassis be inspected thoroughly. If the receiver is being purchased online, be sure to insist on complete photos of the top and bottom of the chassis so you are aware of any missing parts or other problems ahead of purchasing the receiver.
I'm just adding a letter prefix to the original numerical designation to
help in recognizing the type of component.
** Take a look at the "Expected Performance" section and "Rebuilding a Homebrew AC Power Supply for Good Filtering" to see how the restoration of SN 20966 turned out. This BC-348-Q has been restored to original condition with the exception of a well-filter AC Power Supply. When (or if) a DM-28 dynamotor becomes available, SN 20966 can now easily be converted to fully original DC operation.
|What to Look For in Your Preliminary Detailed Inspection
- What you should find upon closer inspection of the modified circuit is
that the original series-parallel wiring of the tube heaters has been
changed to all of the tube heaters connected in parallel. This allowed
the tube heater string to operate on 6.3vac rather than the original
+24vdc. This was the most common AC power supply configuration. You will
find that three resistors have probably been removed R-82, R-84 and
R-85. These resistors limit the surge current on start-up when operating
the (cold) heater string on +24 to +28vdc. You should also find that the
two dial lamps will have been wired in parallel for 6.3vac operation.
Additionally, with the dial lamps there is a series resistor (R-81) and
a potentiometer control (R-111) - "DIAL LIGHTS" - that sometimes have
been removed and a switch substituted for potentiometer and resistor.
It's common to find C-70A/B, the dual filter capacitor, missing. It's a
large square metal box mounted under the chassis.
How little damage is found in how the AC power supply has been fitted into the receiver will depend upon the technical abilities and experience of the original modifier. Most of the time the wiring harness isn't too badly damaged and it will only require a little dressing up, re-routing back to original and re-installation of the spade lugs in order to have the proper connections to the dynamotor terminals. Sometimes you may have to re-install some wiring in order to have the harness wires at the proper place for connection to the dynamotor terminals.
photo right: The underneath of SN 11227 before the DM-28 retrofit. This BC-348-Q AC PS mod had done minimal damage.
The normally found AC power supply modification has some way provided for the AC power cord to exit the receiver. This usually involves removal of the 8 pin Jones-type male plug SO-143 that was how the 24vdc was originally routed into the receiver along with remote stand-by and audio outputs. If SO-143 is missing an original will have to be found. Additionally, the remote stand-by and audio output functions will normally be disconnected. You will usually find the remote stand-by wired to be permanently in "receive."
Originally, the BC-348 was powered up with the AVC-OFF-MVC switch. Normally this is also used by modifiers as the AC power switch. Hopefully, a non-original ON-OFF toggle switch wasn't installed into the front panel. Why this was done on so many BC-348 receivers is a mystery. I think many modifiers thought the rotary switch wouldn't handle switching the AC. I've only encountered one BC-348 that had the rotary switch damaged and that was probably because of a wiring problem in the modification. Normally, the rotary switch handles the AC current fine. When everything is returned to original, the 24vdc will be applied by using the AVC-OFF-MVC switch.
Fortunately, most modifiers seem to mount the AC power supply chassis to the original dynamotor threaded bosses. This does no damage to the chassis. Occasionally, an ambitious modifier will "cut and hack" a power supply into the chassis - both top and bottom - but this is rare.
This should be all of the modifications that will be found in a BC-348 J, N or Q series that has only the AC power supply installation.
Locating Missing Parts
|Parts Sets - Since the BC-348 is a very common
receiver with thousands available it becomes practical and even
desirable to purchase at least one "parts set." Sometimes one receiver
has to be purchased for great external cosmetics and another because the
chassis is perfect. It is very common to have to use at least two and
often three different BC-348 receivers to build-up one complete,
operational and original receiver. It is often much faster to purchase a
"parts set" rather than to spend an eternity looking for a specific part
that never turns up separated from a receiver. Unfortunately, even the
BC-348 receivers that are in a condition that relegates them to "parts
set" status, are nowadays fairly expensive. However, the time that is
saved must be part of the equation in figuring how much to spend on a
Advertising - Sometimes this works but usually it is time consuming with very little success. I've had moderate success with QTH.com but other advertising avenues seem to not be worth the time involved.
E-Bay - E-Bay works quite well for locating parts. Be prepared to pay absolutely the highest price for any part that happens to turn up for auction. In comparison to time involved in finding parts versus paying e-Bay prices, it's probably worth the higher prices since the items can be located and purchased fairly fast. Who wants their restoration to drag on and on?
Commonly Missing Parts
|The following resistors will more than likely be missing:
Number 82 - Resistor - 190 ohms 2W CC - parallel with RF Amplifier
tubes heater string and the Converter tube heater
|The following parts are often missing from modified BC-348 receivers...|
Jones-type male plug, 8 pin, SO-143 - This assembly is made up of several parts that include the contact pins, the bakelite insulators, the two-piece aluminum housing, a bakelite block centering screw and four mounting screws and lock washers. Some parts lists identify this connector as SO-103. The original connections to the plug were +24vdc, -24vdc, AF output (worked against chassis) and Remote Standby. The -24vdc was connected directly to chassis adjacent to SO-143. In the original configuration, the FT-154 shock mount was wired into the aircraft system via P-103 which was mounted to FT-154 and mated with SO-143. When the BC-348 was installed onto the shock mount, then SO-143 engaged P-103 and the receiver was then connected into the aircraft power and control system.
IMPORTANT NOTE: When doing any work on the SO-143 connector where disassembly is required, be sure to carefully examine how it is assembled. You must make notes for correct reassembly! This connector can be assembled several different ways, with different pin lengths, different heights and none of them will fit into the PL-103 receptacle on the shock mount. Only one way of assembly is correct. So, be sure to carefully observe how the unit comes apart, so it goes back together the right way. Be sure to test your BC-348 mounted SO-143 connector for proper fit into the FT-154 mounted PL-103 receptacle before initial disassembly.
Dual Filter Capacitor, Part Number 70 (C-70A/B) - This is a dual filter capacitor with a 6uf capacitor on the B+ line and a 1uf capacitor as a cathode bypass for the 1st AF amplifier tube. This is a large square metal can that is mounted under the chassis and somewhat under the dynamotor bottom cover. Probably because of the proximity to the modification AC power supply, this capacitor is many times removed. Modifiers probably felt that the filtering in their AC power supply was sufficient and just added a replacement capacitor for the cathode bypass unit. It is original to have this unit installed, so if it's missing, an original will have to be found.
Be sure to test the replacement C-70A/B. These were high quality units that rarely are bad. If you are uncomfortable using 60 year old components, then you can rebuild C-70A/B by installing new capacitors inside the can. The bottom can be removed by cutting along the bottom seam. Then the capacitors can be removed, new components installed and the bottom re-attached. I usually just test and install an original unit.
Most of the capacitors installed in the BC-348 are high quality metal cased units that rarely fail. I usually only replace defective ones (rare) and then I try to utilize tested-good original components from a parts set. If you feel like all of the capacitors need to be replaced with new components, then you can replace with SBE orange drops if the under chassis appearance is not a concern or the originals can be rebuilt (restuffed) although this is a difficult process.
Audio Output Transformer & Filter Choke, Part Number 115A/B (T-115A/B) - This is the combination audio output transformer and filter choke. It is unique to the BC-348 series. If it is missing an original will have to be found. The audio output transformer on the J, N and Q versions will have two terminals to select either 300 ohms Z (low Z) or 4000 ohms Z (high Z) for the audio output impedance. The two top terminals shown in the photo to the right are marked LO (on left) and HI. Moving the wire from one terminal to the other accomplishes the change in output Z. Some receivers will have a decal on the front panel the shows that "LOW Z" has been wired for the output Z. What Z you select depends on what you intend to operate as your audio reproducer. The LS-3 is popular but its original Z is 8000 ohms. The LS-3 was never originally used with the BC-348. Hi-Z headsets will require the 4000 ohms Hi-Z and the 300 ohms Lo-Z works fine into a 600 ohm Z load - lo-Z 'phones or 600 ohm matching transformer.
Why some modifications removed T-115A/B and then added an extra stage of audio amplification is probably due to the belief that insufficient audio was present for loud speaker operation. This is a myth. The J, N and Q versions produce ample volume to drive a properly matched loud speaker. You can run an original 8K ohm Z LS-3 with the Hi-Z tap selected and the volume will be very loud. If you have modified an LS-3 to have a 600 ohm Z input, it will have plenty of volume using the Lo-Z tap. One has to remember, the original intent was to use earphones, but the LS-3 speaker does work fine with the BC-348 using the original T-115A/B. It's possible that some of the old modifications sought to match the receiver to 8 or 4 ohm Z loud speakers and that is why they went on to add an extra AF output stage with a standard AF output transformer. Totally unnecessary, of course, but that was the "modification craze" - do whatever you can to "undo" what the real radio engineers had designed.
|The Elusive FT-154 Shock Mount - The
FT-154 shock mount is almost never found with a BC-348 receiver. The
shock mounts were probably left on the aircraft when the receivers were
pulled which accounts for why the FT-154 is almost always found
separated from the receiver. Although you can certainly operate the
BC-348 without the shock mount, the FT-154 does provide a proper way to
support the receiver - on cushioned mounts - and safe way to apply power
to the receiver - via the rear connector (if you also have PL-103.)
Also, the FT-154 does somewhat change the total overall appearance of
the BC-348 in a positive way.
I have encountered two different BC-348 receiver cases that have had the bottom plate that has the pointed engagement pins removed. This was done by drilling out the pop-rivets and removing the entire bottom plate. This leaves the case with out anything between its bottom and the table it's setting on. Both cases had glued-on rubber feet on the bottom of the case. Of course, this totally ruins the receiver case for future mounting on the correct FT-154. Fortunately, a BC-348 case is not too difficult to find. Don't worry that the inside paper tag might not match your version receiver. Most receivers were indiscriminately mounted in any case handy while in the repair depot so today most BC-348 receivers are not in matching cases.
The current prices for the shock mount reflects its rarity. An FT-154 might sell for $150 to $200+ but sometimes you'll find them reasonably priced (even on eBay.)
|PL-103 - This receptacle is mounted to
back of the FT-154 Shock Mount and is what the SO-143 plugs into when
the receiver is mounted to FT-154. There are two versions that were
made, the PL-P-103, which is straight back exit type and the PL-Q-103,
which is a right-angle exit. The +/-28vdc input, the audio output and
the remote standby line are all connected to the BC-348 via PL-103. The
photo to the right shows a close up of the PL-P-103. As can be seen, this
connector assembly has several parts to it. You can also reference the
photo of the FT-154 shock mount to see where and how the PL-103 mounts.
Be aware that the original mounting screws were "shouldered" and center
PL-103 for proper alignment with the receiver SO-143 plug. Be careful
using lock washers with these screws since some washers will not allow
the screw "shoulder" to center the mounting.
The cables exit out the back of the connector and then are routed down
through a metal tube that either points directly to the rear or points
to the side. The wrinkle finish metal cover goes over the entire
connector and only the metal tube can be seen exiting from under the
There are some vertical exit plugs for the BC-348, the PL-P-103 (same identification as the straight back exit which is confusing,) and these plugs do work fine. It's tight clearance from the bottom of the outlet tube to the table surface but you'll have about a half an inch which is plenty of room for the cable. Originally, these plugs were used when the BC-348 receiver was going to be mounted in a manner other than directly on a table top or the table had a hole for routing the cables underneath. At present these are the only easily available PL-103 and they show up regularly on eBay or can sometimes be purchased from Fair Radio Sales.
If You Can't Find a PL-103 - The correct PL-103 receptacle is very difficult to find and usually very expensive. While you are searching for this part, you'll probably want to run your BC-348. Buy a packet of "Push-On Connectors - Female .187" x .032" - these crimp onto the wire ends and then will push onto the flat finger contacts of the SO-143 plug on the BC-348. They are a fairly "tight fit" and won't pull off easily. You can make up a harness that has 14 ga. wires for the 28vdc input (LV+ and LV-) and 20 ga. wires for the Standby Relay function and the Audio Output (if you want to use this output rather than the phone jacks on the front of the receiver.) This set-up will look totally original unless you look at the rear of the receiver and the Push-On Connectors work great. When a PL-103 receptacle is found, you can use the same harness by removing the Push-On Connectors and then soldering the harness wires to the proper pins of the PL-103.
|Finding the DM-28 and What to Look For -
The correct dynamotor for the BC-348 is the DM-28 which operates on
24vdc to 28vdc input, originally supplied by the aircraft
battery-charger system. The dynamotor provides an average of
220vdc output. It takes about 1.8A under full load to run the dynamotor.
But the initial start current, that is, the current required to actually
get the armature moving, is closer to 8A or 9A. This current is only
required for an instant and as soon as the armature begins to move the
current drops rapidly. The DM-28 assembly consists of the dynamotor, the
filter assembly, the terminal strip and the chassis. You'll need to find
the entire DM-28 assembly, as shown in the photo to the right.
DM-28 prices vary from source to source. E-Bay prices were normally
around $125 for the complete assembly, although lately (10-2010) the
prices have risen to over $200 for the complete unit. Swap meet prices
might be less, if you are lucky enough to find a DM-28 at a swap meet.
Sometimes just the dynamotor turns up but, without all of the other
parts, it will only be useful as a spare unit. Most restorers feel that
the time saved in an eBay purchase makes up for the higher prices - at
least you will have the DM-28 and can proceed with the project.
|Servicing the DM-28 - More than likely, your newly acquired DM-28 will not have had anyone checking the lubrication of the bearings in the last several decades, if ever. You are going to have to pull the end-caps of the dynamotor to check the bearings. Once the end-caps are off, you can also inspect the commutators. There will be darkening of copper where the brushes ride on the commutator but there should be very little wear. Remove the brushes but be very careful to observe and mark their orientation because you have to re-install them exactly the same way. Clean the commutators using 400 grit aluminum oxide paper. Wipe the bright copper segments with denatured alcohol to clean and clean between the segments with a stiff paint brush. Now, unscrew the bearing covers. You will probably see old, brownish-yellow grease that is not really in the ball bearings anymore. Clean the ball bearings with solvent to remove the old grease. Blow out with compressed air, if you have it available. Repack the ball bearings with new wheel bearing grease. The newer wheel bearing greases are usually red in color and good for very high temperatures. Rotate the armature as you work in the grease. When the bearing is full, install the bearing covers.||Check the condition of the rubber mounts - they should be elastic
and not all dried out and hard. If you find hardened motor mounts they
will have to be replaced otherwise the dynamotor will run noisily. Now
you can re-install the brushes. Apply 27vdc to LV+ and LV- terminals and
the dynamotor should start running very smoothly and very quietly.
Remove the voltage and replace the end-caps. The dynamotor is now ready
to install into the receiver.
If you find excessive wear on the
commutators they will have to be turned on a lathe. This requires
removing the armature from the frame. Turn the armature on the centers
of the shaft to avoid having to remove the bearings. The bearings are a
hard press fit to the shaft and difficult to remove. The BC-348 manual
has more details on rebuilding the DM-28 if your's happens to be in
particularly poor condition. Normally, the dynamotors are in pretty good
shape and really only need to be serviced for them to then operate
smoothly and quietly.
Other Useful Information
|VT Designation to Standard Tube Identification for
the BC-348 J, N and Q Versions
VT-116 = 6SJ7
VT-150 = 6SA7
VT-233 = 6SR7
VT-117 = 6SK7 VT-152 = 6K6GT
|Schematics - These are available from several sources. I'm using the schematic that is in "Surplus Schematic Handbook" by Kenneth B. Grayson W2HDM, which was part of the CQ Technical Series, published by Cowan Publishing Corporation in 1960. My references to various components are from this schematic except that I have added a letter prefix to identify what kind of component it is. So, R-84 is listed on the schematic and parts list as 84. Manuals and schematics are available free from many online sources. BAMA has available the correct manual for the BC-348 J, N and Q versions. The schematic available on BAMA for the J, N and Q is actually for the earlier versions and is not usable for the J, N and Q versions. I'm including this link to my scan of the correct schematic for the BC-348 J, N and Q from the "Surplus Schematic Handbook." (1.7M) BC348Q Schematic|
Retrofitting the Parts and Testing the BC-348-Q, N & J
|Before you Begin the Retrofit - If the modified BC-348 you've selected for the retrofit is in great condition and appears mostly complete, it's probably a good idea to get the receiver operational on its AC power supply. It probably did work at one time, especially if it looks like a nicely done job, so getting the receiver operational before the retrofit of the dynamotor will reduce the amount of work you have to do later.|
|How to Change the Tube Heaters Parallel Wiring
Back to Series Parallel - Look at the two drawings that are
shown below. One shows the typical Parallel hook-up that is found in
most AC modifications. The other shows the correct, original
Series-Parallel connections for the tube heater strings. Most of the
time, the conversion to Parallel only has a few jumps and a couple of
extra wires to remove. Once you know what to look for by looking at the
drawings below, it becomes fairly easy to remove the tube heater
modifications. You'll have to install R-84 and R-85 as shown in the
photo left. The terminal board (pin 3) is the +24vdc tube heaters
connection (sometimes pin 1 in some receivers - see note below with
drawing.) The resistor from pin 3 to pin 1 is R-85, 5 ohms 1W and
connects +24vdc to the tube heaters for the AF output, 1 & 2 RF amps and
the Converter tubes. Also connected to pin 3 is R-84, 7 ohms 1W that
also connects to pin 7 of the IF amplifier tube and then on through the
other IF amp tubes and the Det-AVC tube. There is a shield that mounts
over the IF amp tube socket so R-84 leads must be sleeved and mounted
close to the chassis with the one lead coming up vertically to the
terminal board. This provides good clearance for the shield.
Underneath the RF/AF chassis (remove the front panel access cover to access) you will have to remove the Parallel wiring modifications and return the wiring to Series-Parallel. You will also have to add R-82, 190 ohms 2W from pin 2 of the 1RF amp tube to ground. There is a ground lug on the tube socket mounting that was originally used to mount R-85. Note that the wire jump from the AF output tube socket to the Converter tube has to exit through a chassis eyelet and then connects to a terminal on the metal housing for the Converter tube. Usually no changes were done here since the return connection to chassis is from pin 7 of this tube. Reference the wiring diagram shown below for how the original series-parallel is accomplished. Also shown for reference is the typical modified parallel wiring diagram.
Once the tube heater wiring is returned to original, "ohm out" the connections to verify it is correct. Then install the tubes and apply +24vdc to the tube heater terminal and verify the correct voltage on each tube heater, approximately 6vdc. You will probably see about 5.8vdc or so if you use +24.0vdc but remember that a fully charged battery set-up will provide about 27.5vdc. The design allows for wide variation from a low of 24vdc up to about 28vdc.
Original +24vdc Series-Parallel Wiring - This wiring diagram shows how the tube heater wiring is accomplished for Series-Parallel operation on +24vdc. The wire from pin 7 of the 2nd RF amp tube socket passes through an eyelet in the chassis and is then connected to a terminal on the outside of the Converter tube housing. Sometimes the connections to the terminal strip (where the resistors are mounted) are inverted - that is - connections going to pin 1 go to pin 3 and vice versa.
Typical Modified Tube Heater Wiring for 6.3vac Operation - This wiring diagram will show what is usually found in BC-348 receivers that have been modified for operation on an AC power supply that provided 6.3vac for the tube heaters. This is how most of the AC modified BC-348 receivers seem to be wired but there are other variations that accomplish the same thing. As mentioned in the Series-Parallel description about the terminal strip - it is common to find pin 1 and pin 3 connections inverted from what is shown here.
|Wiring the Dial Lights Back to Series - Once the heaters are in the proper configuration, the next step is to change the dial lamps wiring back to series. Remove the dial cover to access the dial lamp sockets. There will be two wires installed between each lamp socket. Remove the non-original wire which should leave one original wire connecting the two sockets to each other. The left side lamp socket should also be connected to the wire that runs through the panel on the left side. Then install the wire that comes through the panel on the right side to the right side lamp socket. This wire should then connect to R-111, the "DIAL LIGHTS" potentiometer.||From the pot a non-original wire is usually connected to the chassis lug. Remove that wire and install R-81. R-81 is a 60 ohm 2W wire wound resistor but it isn't too critical, (I installed 100 ohms because that was all I could find and it works fine.) R-81 connects between R-111 and chassis. Install #47 lamps and test with +24vdc applied to the +LV input. The dial lamp circuit is connected directly to the switched +24vdc input and jumped to the tube heater wiring inside the dynamotor chassis.|
|Rebuilding the Wiring Harness Back to Original
Identify each of the wires from the harness that are going to have to
connect to the dynamotor terminals. There should be five wires. HV+
is the B+ line (consists of two white with red tracer wires.) HV-
is the B- and bias line.
Blank or "not ID'd" is the connection for the tube heaters
- it is internally jumped to LV+
inside the dynamotor chassis. LV+ is the switched +24vdc and dial
lamps line. LV- line is connected to chassis. Normally, modifiers
didn't change any of the wiring placement in the harness so, hopefully,
all that will be required is to install spade lugs on the wire sets. You
may have to add wire if the length of the original wire has been cut to
fit into the modified AC power supply. Every modified BC-348 is somewhat
different but try to keep to original wiring routing as much as
possible. If the either LV line needs to be lengthened be sure to use
16ga. wire on these lines. Once you have the order and length correct,
then install the spade lugs. These have to be soldered to the wire ends,
as original. Remove any plastic insulation on the spade lug ends and
then solder to the wire ends. To make the harness look original
install cable lace made of waxed string. Connect wire harness to the
dynamotor terminal board to complete the installation.
|Powering the BC-348 with 24vdc to 28vdc - The original power source for the BC-348 was the aircraft battery-charger buss that provided around 24vdc when the airplane was on the ground. This was usually when all the equipment was pre-flight tested and the BC-375 transmitter set-up for the frequencies to be used by the radioman. Once the aircraft was in flight the battery-charger system provided nearly 28vdc to the radio equipment. If you are going to operate the BC-348 on batteries expect the voltage to vary from fully charged at about 27vdc down to 23.5vdc (when the batteries need to be charged.) If you are going to operate the receiver on a DC power supply, provide about 27vdc for good operation of the tube heaters and the dynamotor. The DC power supply will have to be capable of providing around 6A for the surge current to start the dynamotor. After the dynamotor is up to speed the current draw is usually less than 2A.||For initial testing connect +27vdc to pin 3 or 4 on SO-143 and connect the negative lead to pin 8 or 7. Be sure to use a test jump between pin 2 and 6 to take the receiver out of stand-by. With the +27vdc on, switch the AVC-OFF-MVC switch to AVC. The dynamotor will come on along with the dial lights. After about 30 or 40 seconds the receiver should come to life. Use earphones (of the proper Z for what you have T-115A/B wired for) and tune in some stations on SW BC. Check all of the controls and see that they function. Tune in stations on all bands to test. Switch to MVC and turn on the CW OSC. You can now tune in some SSB or CW stations on the ham bands. Note that in AVC, the VOLUME will be set rather low since the AVC is controlling the receiver sensitivity and VOLUME controls the audio gain. In MVC you'll have to advance the VOLUME higher since VOLUME now controls sensitivity and the audio gain is at maximum. This gives the proper ratio of RF to BFO injection for good CW or SSB demodulation.|
|IF Alignment - The BC-348 uses an Intermediate Frequency of 915kc. The passband selectivity is directly related to how the IF is aligned. There are other factors, such as the amount of coupling in the IF transformers and other factors of design, but an accurate alignment of the IF system will result in the best selectivity that the BC-348 is capable of. You will have to determine the exact crystal frequency of the Crystal Filter first. This can be determined by switching in the Crystal Filter and then sweeping an RF Signal Generator coupled to the converter grid. The crystal should be very close to 915kc. You'll hear a "peaking" of the background noise as you sweep through the crystal frequency. At the exact peak of the response is the exact crystal frequency. Whatever the crystal frequency is, that is where the RF Signal Generator should be set for the IF alignment.|
|To assure that the Crystal Filter will operate correctly and be a
useable asset to operation, you should perform the IF alignment with the
Crystal Filter ON. This will narrow the IF passband to about 1KC. The
end result is you'll have a Crystal Filter that works great when the alignment is
adjust all of the IF transformers, both top and bottom adjustments, for
the peak response to the RF Signal Generator input frequency. Monitor
the audio output using an AC voltmeter. You'll have to use 1000~
modulation on your RF Signal Generator for the meter to read and operate
the receiver in MVC for the best results.
You can also use an unmodulated signal and monitor the AVC line, with the receiver in AVC. If you have a digital frequency counter monitoring the RF Signal Generator frequency, check it often as you proceed with the IF adjustments. Go through all of the IF adjustments twice to assure accurate adjustment. This method of alignment will result in the best IF selectivity for the BC-348. Also, the Crystal Filter will operate correctly since the IF is adjusted to exactly the crystal frequency with the Crystal Filter in the circuit.
RF Alignment - The accuracy of the RF alignment affects how well the BC-348 tracks - that is, how accurate the dial readout is. Since the receiver was adjusted at the factory with more or less fixed components, there are no padder adjustments for the tracking except for the LF band. The tracking is adjusted with trimmer capacitors on each of the tuning ranges. These are adjusted at the top of each of the bands. How accurately you adjust the RF Signal Generator frequency input, the receiver's tuning dial and each of the trimmers, results in how accurate the final tracking is. Most BC-348 receivers adjust up to be very accurate - within the limits of an analog readout and a tuning range that spans several megacycles.
Let's start out by recognizing that the Grid-Cap Tubes (GCT) version of the BC-348 is a completely different receiver if compared to the later Single-Ended Tubes (SET) version. The SET version was not given a new designation because the SET version was a "cost reduced" design that had to be interchangeable for the end user. But we are looking at the circuit in order to restore the GTC version back to original dynamotor operation and the first thing that will be encountered is that the earlier receiver circuit is 90% different from the later versions. Even the component designations are different for the few identical components encountered. Also, within all of the GCT versions there are many slight variations in components used and construction, so the first thing you'll need is the proper documentation for your particular GCT version and, if a "parts set" is needed for the restoration, it should be a similar vintage GCT version receiver.
Accessing Your Restoration Candidate
|Despite their differences, much of what is presented in Part One is applicable to
the earlier GCT versions of the BC-348. Since the
receivers were considered interchangeable for the end-user, the exterior
of the receiver is almost identical to the later versions. This means
that a lot of the chassis, cabinet and some panel parts are interchangeable.
Also, the general retrofit techniques presented in Part One are applicable to the
earlier receivers. But, a look at the circuit or schematic will show you
that we are going to be working on a very different receiver. Here
are some of the main differences:
1. Early versions (GCT version) use entirely different tubes. The RF amps and one IF amp are 6K7s, a 41 is used for the audio output, the LO is a 6J5 and the Mixer is a 6L7. These tubes functions were replaced with 6SK7, 6K6 and 6SA7 tubes in the later receivers.
2. A 991 Neon bulb is used a a voltage regulator for the LO on GCT receivers. Not used in SET receivers.
3. A 6F7 (large seven pin glass tube) is used as the 2IF and BFO functions. This tube will have a separate metal shield installed. This tube's functions were replaced by the 6SR7 and the 6SJ7 in the SET versions.
4. A 6B8 is used for the 3IF and Det/AVC functions. This metal octal with a grid cap tube was replaced by the 6SR7 and 6SJ7 tubes in the SET version.
5. The BFO and the Crystal Filter are fully shielded in square cans mounted on the top of the chassis.
6. A tracking RF gain control is coupled to the tuning condenser. Not used on SET versions.
7. The receiver construction uses component mounting that is "old-style RCA" using four component mounting boards with wires routed through a harness down to the tubes. This was simplified for the SET versions to mount most of the components on or near the tube sockets with only two component boards utilized. The use of so many component boards means much of the wiring is in harnesses that makes "wire tracing" very difficult.
8. Capacitor mounting under the dynamotor uses several individual metal body, oil filled, dual capacitors mounted together and connected to the circuit through wires in the harness. The SET version simplifies the component placement and use.
9. Placement of the audio output tube is significantly different with early versions locating the type 41 on the main chassis near the audio output transformer. The SET version locates the 6K6 on the small chassis on the right front of the receiver (with the two RF amplifier tubes - where the mixer tube is on early versions.) NOTE: Some GCT versions have a type 6K6 audio output tube.
There are several more changes such as layout, component designations and manufacturers used, wiring harness, etc. Since different tubes are utilized, the original series-parallel wiring is slightly different than the single-ended tubes version and the conversion to AC was slightly different. Dial lamp conversion is slightly different because of the +28vdc source changed in later receivers. You will have to make use of the correct wiring diagram for the early receivers. This is not a schematic but a pictorial of how the wires are connected to the various components, component boards and the harnesses.
The victim,...I mean the receiver (shown above) to be retrofitted with the DM-28 dynamotor belongs to KØDWC who intends to run this BC-348-C in the ARC-8 configuration (with a dynamotor operated ART-13 transmitter.) The BC-348-C version was built by RCA in 1941. It was common to convert the "B" or "C" variations to cover 200kc to 500kc and then re-designate those receivers as "R." However, this particular "C" has not had the LF conversion making it somewhat rare.
Unfortunately, like much of the rare military gear that is found today, this early BC-348-C was at one time owned (and hacked up) by an avid reader of CQ magazine publications. CQ advocated the wholesale destruction of much of the military surplus gear under the guise of "modifications for ham radio operation" - in other words, mods that were poorly engineered and, in most cases, unnecessary. "Modification for modification's sake" was the CQ motto and many hams bought into the idea that they could "out engineer" the Professional Radio Engineering Teams that designed military radio gear to perform reliably under the worst conditions imaginable. Also, that they (the hams) could perform the mechanical and technical work better than the professional radio assemblers and technicians that were employed by the major radio manufacturing companies. Today we see these "hamster" results in the sloppy workmanship of hacked up modified gear we have to try to restore back to original operation and performance. A properly designed AC power supply conversion that was well thought out and well filtered was all that was necessary as a modification and this would not be a serious problem to convert back to dynamotor operation (or use "as-is.") However, this BC-348-C example we are restoring has had several other modifications besides the AC power supply and the quality of design and rework are strictly of the "hamster" caliber. Luckily for military radio enthusiasts, most BC-348 receivers encountered are usually not nearly so compromised by modifications as this example was.
|photo left: Note the
metal octal tube on the AC power supply. This is a 6F6 audio output
modification. Also note the metal octal next to the band switch drive.
This is the 6C5 "hacked" in to replace the 41 audio output tube and
convert that stage to a 1st audio amplifier stage to drive the 6F6. Note the
cable routed to where the DIAL LIGHTS control was. This is now a
separate AF Gain control and ON/OFF switch.
photo right: Lots of extra cables and components added here to further confuse anyone who attempts to restore this example back to original. The fuse mount and the SO-143 were missing. Replacements are installed but not wired - yet.
|Preliminary Inspection - The photos above show the BC-348-C chassis in "as found"
condition. A poorly designed and inadequately filtered AC power supply
is obvious when looking at the photo above right. Also, note that an audio output stage has been added using a
6F6 tube. Additionally, the original type 41 audio output tube was
removed, the large six pin socket removed and an octal socket installed
so the stage could be changed to a 6C5 First AF Amplifier to drive the
6F6. Due to the poorly filtered AC supply and the additional
amplification, the hum level in this BC-348 was exceptionally loud. Totally unnecessary modifications
resulting in poor performance!
The original audio output circuit is sufficient to drive a matched load to
Unfortunately, the modifier decided to grind off the bosses and drill-out the threads that are normally used to mount the dynamotor (or the typical AC power supply.) This means that helicoils and spacers will have to be installed to assure that the DM-28 mounts to the chassis correctly. Also, the DIAL LIGHTS control has been replaced with an "on-off" switch/AF Gain combo (another circuit modification.) Luckily, KØDWC supplied a SO-143 connector and an original type audio output transformer (note that neither is wired into the circuit.)
Note in the photo of the underneath that a filter choke was mounted to
the side panel. This required removal of the original Candohm
resistor pack to allow clearance. The shielded cables are additions for the 6F6
mod and the twisted pair of white wires go to the mod'd AF/Gain and
All of the front panel modification holes had already been repaired by KØDWC so they are not easily seen. The other "hamster" mods included the addition of an SO-239 connector for the antenna and ground connections and an additional switch in the upper left panel area (repair of the holes might be visible in the front panel photos.)
All things considered, this BC-348 would probably have been a "parts set" had it not been the rare "C" version without the LF conversion. Although the receiver has been severely altered, it can be restored back to its original configuration using a DM-28 for it power source.
Commonly Missing Parts and Required Documentation
|Fortunately, many of the
hard to find parts are the same in both versions of the BC-348.
DM-28 Dynamotor - Same in both versions, the DM-28. Covered in Part One.
SO-143 - Power Input Connector is the same in both versions. Covered in Part One.
115A/B Audio Output Transformer/Choke - This unit is somewhat different in the GCT versions and is designated 101A/B/C or 123A/B/C. One side of the choke is connected to chassis internally, .05uf capacitor connects to pin 6 (C-101 - BFO TC) and the audio output transformer connects to different pins. You can substitute 115A/B for the earlier 101A/B/C or 123A/B/C but be aware that it ha to be connected differently. More details below in "Substituting 115A/B for missing 101A/B/C."
FT-154 & PL-103 - The same shock mount and connector for both versions. Covered in Part One.
Filter Capacitor 70 - This dual filter capacitor is only in the Q, N and J versions. The GCT version uses several oil filled, dual capacitors connected via the harness to the circuit.
Resistors - Most GCT versions have R-501A/B removed to operate the tube heaters on 6.3vac. R-501A/B consists of a 3.0 ohm 1.5 watt WW resistor and a 190 ohm 1.5 watt WW, both in a package that looks somewhat like a Candohm-type resistor. The series resistor (3.0 ohm) limits the surge current on cold heaters and the parallel resistor (190 ohm) will slightly raise the heater voltage on the audio output tube heater. Some versions have different component designations but the values and function are the same. The load resistor and potentiometer for the dial lamps have the same values as the later versions but the designations are R-503 for the 60 ohm load and R-500 for the 200 ohm potentiometer (but designations do vary with the particular version within the GCT classification.)
|Nomenclature Note - Since the GCT versions are different receivers as far as the circuit and tube types
used, different designations are used for all of the resistors and
capacitors that perform similar functions in the Q, N and J versions.
Also note that since the GCT version was built longer and by more
contractors than the SET versions, the component designations are
somewhat different within the various GCT types. Check your particular
GCT version against all of the earlier documentation to decide what is
correct for your BC-348. As
with our reference to the various components in Part One, I add "R"
or "C" to resistors or capacitors so the reader can easily identify the
type of component referenced. In the manual or schematic these
components are only identified by the number.
Schematic - The GCT version schematic is available on the BAMA site. Also available is the complete manual for the early BC-348 series that has wiring diagrams, parts lists and a lot of other useful information. It's a large file at around 9meg but the wiring diagrams are required, so download it.
Use the Wiring Diagrams - Wiring diagrams are not schematics. Wiring diagrams are pictorial drawings that show the exact routing of all of the wires and their connections to all of the receiver components. The GCT version of the BC-348 is very difficult to "trace out" the wires when trying to find out how a particular receiver has been modified. This is because most of the wiring is routed through harnesses with connections to the different component boards. The easiest way to trace the wiring is to have copies of the wiring diagrams that are in the military manuals. These diagrams show where the original wiring was connected and how it was routed through the harnesses. Using the wiring diagrams will give wire colors but these changed depending on the contractor so you can't go just by the indicated color. You will still have to confirm the correct wires and connections using an ohm meter.
Changing the Tube Heater Wiring back to Series-Parallel
|Original Series-Parallel Connections for +28vdc
Operation - Since the GCT version makes extensive use of harness
the tube heater wiring, at first, seems confusing. This is mainly due to
R-501A/B. This WW resistor package is
mounted at the back of the chassis (under the dynamotor) so wires have
to be routed via the harnesses to make those connections. Two different
component boards are utilized in the routing.
The simplified drawing below shows the correct original wiring for a Series-Parallel connection of the tube heaters to allow operation on +24vdc to +28vdc. If your version uses the 6K6 Audio Output tube then the heater connections are to pins 2 and 7 on that socket.
|Modified Parallel Connections for 6.3vac Operation - The
simplified drawing below shows the Parallel 6.3vac AC hook-up for the tube
heaters that I found in this BC-348-C. Note that R 501A/B is removed so
that the 6.3vac heater voltage connects direct in parallel to all of the
tubes. On this example, the three wires that originally connected to
R-501A/B were all soldered together and taped. It may not be how all
examples are modified for AC filament operation, since this receiver
had so many other modifications. Generally though, there aren't too many
different ways to accomplish the parallel heater connection. If your
version uses the 6K6 Audio Output tube then the heater connections are
to pins 2 and 7 on that socket.
|Connecting the Dial Lights in Series for GCT
Versions - The
wiring for the dial lamps starts from the +28vdc connection on pin 1-Front
the AVC/MVC switch then proceeds through the two lamps in series then
exits from the dial housing to the 200 ohm pot and the 60 ohm load. The
usual mod is to move one of the series wires to parallel the incoming
connection on the left side lamp socket and then run another wire from
the other connection on that lamp to the right side lamp to create the
parallel wiring for the lamps. The exiting wire on the right side is
usually just connected to the chassis. To return to series wiring,
remove the added parallel wire between the two lamp sockets. Then move
the original wire that was moved to parallel the incoming voltage
connection on the left side socket and move it to the other lamp socket
connection. That should be all that is necessary under the dial cover.
What is needed from the right side wire is a connection to the 200 ohm DIAL LIGHTS potentiometer
and 60 ohm load resistor and then to chassis.
This completes the circuit. Be sure that you use #47 lamps in the dial.
Micamold Capacitor and Allen-Bradley Resistor Replacement - On the GCT versions only, Micamold capacitors are used for many bypass and coupling functions. Micamolds are not mica capacitors but are rather paper-wax capacitors inside a molded case. They suffer from the same problems that Sprague "Black Beauties" suffer from, that is, excessive leakage current (only Micamolds are much worse than "Black Beauties" when it comes to reliability.) Inspect your BC-348 carefully and replace any Micamolds that show obvious swelling in the center. This is always an indication of heat build-up due to leakage current. If you plan on using your BC-348 as a station receiver, it's probably a good idea to replace all of the Micamold capacitors in the receiver as they will usually result in frequent rework issues if left in the circuit.
The resistors used in most of the GTC versions are Allen-Bradley manufacture. For some reason, the A-B resistors made during WWII (and for sometime afterwards) exhibit chronic drift problems. It is not predictable as to whether high values, e.g., over 100K, will drift higher or whether the lower values will suffer value drift. It's best to check all of the resistors for drift and replace any that are more than 20% out of tolerance.
VT Tube Designations for the Grid-Capped Tubes Versions VT-48 = 41 VT-93 = 6B8 VT-70 = 6F7 VT-65 = 6J5 VT-91 = 6J7 VT-86 = 6K7
Rebuilding the Dynamotor Connections from the Harness -
Most of the time the harness spade lugs had to be removed for solder
connections to the AC power supply. See this subject in Part One as it
is an identical procedure for the GCT versions.
Rebuilding the DM-28 Dynamotor - See this subject covered in Part One. The procedure is the same.
Alignment - This is basically the same although the locations of the IF and RF tracking adjustments are somewhat different. Different type trimmer capacitors are used in the earlier versions but they are a better quality component than the types used in the SET versions. The same basic procedure is used for either type receiver. 915kc IF frequency. Be sure to do the IF alignment with the Crystal Filter ON (in the circuit.) This will assure you have a working Crystal Filter that will be an asset to operation of your BC-348.
Power Up - Same as covered in Part One.
|photo left: Top of the chassis with the
BC-348-C finished and the DM-28 installed. When compared to the before
photo, note that now a type 41 tube is installed for
the audio output. This required re-installation of a fiber-type large
six pin tube socket, as original.
photo right: Underneath the BC-348C. Much cleaner wiring now that it is back to original. Note that 501A/B is two modern WW resistors that are mounted on a vintage fiber component board.
Performance Expectations for the Single-Ended Tubes Versions
|The BC-348-Q is an amazing receiver and the more you use it, the
more you'll appreciate the fact that this great performing receiver "does so
much, so well, with so little." Being an aircraft receiver, weight had to be kept
to a minimum but this little receiver has two RF stages, three IF
stages, a Crystal Filter, a BFO and a "1940s accurate" dial readout.
It's surprising that even today the old myths of "not sensitive" or "too broad" still turn up in contemporary reviews of the BC-348. The receiver's performance is directly related to how thoroughly it was rebuilt and how accurately it was aligned along with operating the receiver with the correct impedance speaker. I've used my two BC-348-Q receivers (and a BC-348-R) many times as the station receiver and have found them to be fine receivers. Can adjacent frequency activity be heard? Sure. But does it prevent solid copy? Of course not. Remember, you have a Crystal Filter. With the Crystal Filter "ON" the IF passband will be around 1KC wide. The Crystal Filter will reduce adjacent frequency interference on AM - just be sure to tune the incoming signal "on the nose." Obviously, the critics who keep promoting the "Broad as a Barn" reputation of the BC-348 have never used the Crystal Filter or they have never aligned the IF section of their receiver correctly. I've used the BC-348-Q on CW also. It's a very good CW receiver with very little drift. Of course, the lack of CW activity these days doesn't offer up much of a challenge to the BC-348-Q's selectivity. SSB is also demodulated quite well since in MVC the VOLUME is actually an RF Gain control.
I always find the performance of the BC-348-Q to be a pleasant surprise. Probably because, over the years, I have always heard so many negative comments about the receiver's lack of selectivity and limited features - but this just isn't the case. The BC-348-Q is a great performer that always delivers solid copy even in tough conditions.
Performance Expectations for the Grid-Cap Tubes Versions
GCT versus SET - After working on several receivers from both the SET version and the GCT version it is apparent that each version has its own particular advantages. The advantages are in the mechanical construction and the ease of rework though, not necessarily in performance. For example, the SET versions are much easier to work on since the majority of components are mounted in the circuitry rather than on component boards. However, the GCT versions are a little more stoutly built and have more shielding in the IF/Crystal Filter area and the BFO.
The GCT versions include an ANTENNA TRIM control that is not on the SET versions. This would be an advantage when using an end-fed wire antenna that was untuned. Most ham stations though use the transmitting antenna for receive, so they provide a tuned antenna to the receiver and an antenna trim control is not really needed.
Both receiver-types have plenty of audio and easily drive a matched loudspeaker. If you plan on using a stock LS-3 (8K Z,) be sure you have the audio transformer tap connected to the HI-Z terminal. If you've modified an LS-3 to have a 600 ohm Z input then you'll have to connect to the LO-Z terminal on the audio transformer. Some users believe that there are performance differences when comparing the two versions of the BC-348 but I've found that careful attention to detail when performing the restoration and, most importantly, the final alignment, results in either version performing very well. The two BC-348 versions were considered interchangeable by the military and my experience seems to confirm that there is virtually no difference in performance between properly operating receivers of either type. The main reason that the military and contractors went to the SET version was cost. The SET was much cheaper to build, easier to work on and, in the end, performed just about the same as the earlier GCT versions.
AC Power Supply Enhancement for the BC-348
|How to Build (or Rebuild) a "Homebrew" AC
Power Supply for Good Filtering Many users don't want to run their BC-348 on the
original dynamotor. Though convenience is certainly a factor, so is the
dynamotor noise. The noise from the dynamotor is not loud at all and in the original aircraft installation, the dynamotor
wasn't heard since the radioman used 'phones and the ambient noise was
much, much louder. Today, if the station transmitter that the BC-348 is
to be used with operates on AC, then dynamotor
operation probably isn't something desired. Today, there is absolutely no excuse
for "hacking up" an original dynamotor operated BC-348 just to modify it
for AC operation because practically every single BC-348 for sale has
already been converted for AC operation. However, most of the AC power supplies that are
installed in BC-348s are inadequately filtered and hum is quite audible
and sometimes very annoying, especially when using earphones.
There are two methods to achieve good filtering. If a Pi-network is used for the filter, that is, one choke and two electrolytic capacitors, then the value of the capacitors will have to be rather high. Two 40uf electrolyics would be typical for a Pi-network filter. If a dual section filter is used, that is two chokes and three capacitors, then the value of the three electrolyics can be reduced to 10uf each.
In order to have very good filtering, two filter chokes and three filter capacitors should be used. This dual section filter will reduce the hum level to "imperceptible." You might have to replace the power transformer since the extra filter choke will drop the B+ somewhat. Most transformers were providing higher B+ than necessary anyway and the builder used a series load resistor to drop the voltage to about +230vdc. Another trick with two filter chokes is to use "choke input" which eliminates the input filter capacitor. This will drop the B+ significantly but it will improve regulation. With "choke input" the value of the electrolytics should be increased to 20uf. Whether you will be able to use "choke input" will depend on your power transformer's HV winding supplying a high enough AC to the rectifier to compensate for the voltage drop in the filter since the lack of an input capacitor doesn't have the rectifier output "charging to peak." There are many solutions to the filtering problem and the proper B+ voltage.
Remember, the original design of the BC-348 was as a DC operated receiver so filtering was minimal and trying to just use a typical Pi-filter arrangement for the power supply will result in too much audible hum unless a significant increase in the capacitance value is used. Even then, the hum is still present although usually reduced to a level where it isn't too annoying if the volume is kept at a fairly high level.
|The original design of the BC-348 audio output stage was to ground
the cathode and to negative bias the grid (fixed bias.) To achieve a
negative bias the negative from the dynamotor was connected through the
choke inside the audio output transformer unit to chassis. The
resistance of the choke was about 250 ohms which elevated the negative
above chassis and provided about minus 16vdc bias voltage. When
designing your AC power supply, you will have to connect the CT of the
power transformer to the B- line which then connects to the choke inside
the audio output transformer unit. Then, in order to achieve good
filtering, the negatives of the filter capacitors should connect to B-,
not to chassis. This will provide good filtering for the
-16vdc bias voltage and also will result in a power supply that has no
perceptible hum. The exception is the last filter capacitor in the dual
section filter which should be connected to chassis.
When it comes to construction, the main problem is lack of space in the GCT versions since the chassis extends under the dynamotor. This isn't the case with the SET version and removal of C-70 allows plenty of room to fit a well-filtered power supply. If C-70 is left in place, the SET version has same space challenges as the GCT version. So, depending on your receiver, fitting a power transformer, two chokes, rectifier tube and three filter capacitors onto a small chassis with no depth available is a challenge. I utilize a small one-half inch tall chassis that just fits into the dynamotor area which mounts using the original dynamotor threaded bosses. This gives much more clearance and allows for mounting larger components underneath without interference from C-70 or from the chassis extension on the GTC versions.
Going solid-state certainly will save some room as you now don't need a tube socket or a rectifier tube. Don't worry about cathode stripping with full B+ appearing on cold tubes. That happens with dynamotor operation, too (as soon as the dynamotor is up to speed, full B+ voltage is present.) Solid State rectifiers will result in higher B+ voltage so be sure to compensate for this in your design. Be sure to prevent magnetic coupling between the "iron" by proper orientation of the transformer and chokes. It is possible to build the AC PS in "two levels" - that is, to use a second chassis over the main chassis to gain more area for components (or one angled bracket might suffice.) Most builders don't consider that there is ample space above the main chassis and a secondary chassis can solve a lot of space problems. You can easily build as high as the top of the side support of the chassis which has clearance to fit into the cabinet.
|Dual Section Filtered Power Supply
- To the right is the schematic for a dual section filtered power
supply that I build and installed into a GTC version BC-348-R SN 10662 (Belmont
contract.) This receiver had an AC power supply that utilized a Pi-network
filter with 10uf filter capacitors. The builder had connected the CT and
filter negatives to chassis and connected B- to chassis. This
effectively eliminated the filter choke and negative bias from the
circuit. Though the hum wasn't excessive, the audio distortion at louder
volume was. A redesigned power supply was needed for this otherwise
great condition BC-348-R.
Unfortunately, the power transformer HV winding 230-0-230vac and just provided +230vdc B+ using the Pi-network. If I installed a dual section filter, the B+ would probably be around +200vdc or less under load. I had to select a different power transformer, one with 270-0-270vac would be perfect but all I could find in the junk boxes was a 290-0-290vac - close enough!
Since the dual section filter wouldn't drop enough voltage, I had to utilize a series dropping resistor. I calculated the value by assuming the BC-348 B+ current requirement is 60mA. The IR drop was going to be about +60vdc, so the value should be close to 1000 ohms and I²R=P showed that the resistor had to dissipate about 4.0 watts. On the test bench, the power supply B+ was +235vdc when powering the receiver.
I built the power supply onto a small chassis that measured 6.25"W x 3.75"D x 0.625"T. The height of the chassis just allows the left side receiver to cabinet mounting screw to have clearance over it. The bottom edge of the chassis will have to be "notched" for clearance for the dynamotor mounting bosses (see photo lower right.) Concerning the layout, I find that nearly all old power supplies mounted the power transformer in the left corner of the chassis which presents several problems when one is trying to utilize a terminal strip for connecting the AC PS to the receiver. I find that mounting the power transformer on the right side allows more room underneath for components and tie strips. I also use solid-state diodes for the rectifier to provide more space for important filtering components.
After all of the sheet metal work was finished but before any components were mounted, I like to put a finish on the aluminum so it doesn't look so "homebrew." I clean the chassis, then use 400 grit aluminum oxide paper to impart a slight grain to the surface. Then I heat the chassis using a heat gun until it's fairly warm - about 120 degrees. Then I spray the chassis with Easy-Off Oven Cleaner. The active ingredient in EOOC is Sodium Hydroxide which reacts with the aluminum to create a flat finish. Leave the EOOC on the chassis for about five minutes and then rinse it off with cold water. Don't rub the finish or you'll change it's flat look. Use a heat gun to dry the water off. After this treatment the aluminum chassis will look like a professionally made piece. >>>
The Dynamotor harness was intact in SN 10662 so I utilized an original terminal strip from a "junk" DM-28 base. This was mounted under the chassis with spacers so it would place the terminal screws at the proper height for the opening in the side panel of the receiver to allow access for connecting and disconnecting the harness.
AC was routed into the SO-143 connector and through the wiring harness, utilizing the AVC-MVC switch for "ON/OFF" function (as original) and then to the AC power supply via the dynamotor harness. This routing doesn't add any AC hum to the receiver output. I have tried both a direct AC connection to the power supply and routing through the harness - there is no difference, so no hum is "added" by using the AC connection as shown in the schematic.
Be aware that on GCT versions, the dial lights connect to the AVC-MVC switch to provide +28vdc to the lamps but now this point will be at 115vac. You'll have to move the dial lights connection to an adjacent tube socket for the proper 6.3vac.
If you want to keep the Remote Standby operational, more than likely you 'll have to go back into the BC-348 and find where the two wires for the remote receiver standby line are tied together (they originally were connected to pins 2 and 6 of the SO-143 connector.) Since I utilized the SO-143 connector for AC power input, I also can access the remote standby line for the receiver from there. Check the section in Part One about the PL-103 connector and how to input the wires using push-on connectors the just fit the blade contacts of the SO-143 connector. This makes for a very clean hook-up for the BC-348.
photo above: BC-348-R SN: 10662 Belmont Radio contract from 1942. This shows the final installation of the dual section filter AC power supply. This receiver was in very nice physical condition and the original modification to AC operation didn't do any damage. There were some design issues with the original mod AC PS so I built this new AC power supply as part of the restoration of this receiver. Final operation of this receiver is impressive and the hum level is entirely imperceptible.
Other Problems - Upon power up the BC-348-R operated fine. I measured the B+ at +235vdc which is just where it should be. After the receiver had operated for a while I tuned around the 40M band and noted that I had difficulty tuning in SSB signals. This isn't usually the case with BC-348 receivers. There was a large amount of backlash in the tuning. Looking at the tuning condenser drive gear I noted that there was no "anti-backlash" set into the mesh with the straight-tooth drive gear from the gear train. Luckily, it's easy to set this anti-backlash. First set the tuning condenser for "full mesh." Then loosened the two front mounting screws on the tuning condenser. Then slightly lift the front of the tuning condenser and the condenser drive gear will disengage from the small straight-tooth drive gear. Now while holding the front gear move the back gear to set in about "two teeth" of anti-backlash and hold this tightly with you fingers while re-engaging the anti-backlash gear into the straight-tooth gear. Then re-tighten the condenser mounting screws. This cleared up the tuning problem.
Cleaning and Restoration of the Black Wrinkle Finish - To thoroughly clean the wrinkle finish paint I use Glass Plus and a soft brass brush. It sounds harsh but the wrinkle finish can take the abuse. I first remove the knobs and access cover. Then I saturate the front panel with Glass Plus and gently work the solution with a medium soft brass brush - "tooth brush size." I wipe the dirty solution off with paper towels and repeat the process a second time. This time the paper towels should be fairly clean after wiping off the Glass Plus. Any touch-up painting should be done now. I then let the front panel dry thoroughly. Then I apply Armor-All in a very heavy coating and let it set on the front panel for about thirty minutes. After a wipe-down, the panel should look fabulous (for an original.) The same process is applied to the knobs and the access panel. Be careful of any acceptance stamps as this orange ink is somewhat soluble in water or Glass Plus.
Here's another way to provide good filtering...
Choke Input Filtered Power Supply - The schematic shown to the left is an AC PS that I installed in BC-348-Q SN 20966, the receiver I wanted to use with my ART-13 transmitter. Since the ART-13 operated on an AC PS, I didn't want the receiver to be dynamotor operated. I naturally used a BC-348 that had years before been AC converted and then just restored the receiver to be as original as possible but with just a well-filtered AC PS installed. This will provide me with a BC-348-Q that is very quite to use and still performs almost exactly like the original dynamotor operated receiver.
Note that choke input is used. This was because the power transformer had an HV winding that provided about +300vdc with a dual section filter. An easy correction was to go with the choke input which gave about +230vdc output with good filtering characteristics. I did utilize the SO-143 connector for my AC input. Especially important to note is that the negative connection on the first filter capacitor is connected to B- and not to chassis. Most choke input power supplies are not designed to develop a negative bias and the transformer CT and the filter capacitor negative are connected to chassis. However, the BC-348 requires a bias voltage for the audio output tube and this is accomplished by floating the CT or B- and connecting to chassis through a choke with 250 ohms of DCR. To achieve the proper charging loop, the first filter capacitor must be connected to B- to provide good filtering. The second filter capacitor should be connected to chassis though. The second filter capacitor is a 10uf because C-70A/B (in the BC-348) adds a 6uf filter capacitor in parallel, so the total capacitance is 16uf. This type of "hook-up" using a choke input dual section filter reduces the hum to the "imperceptible level."
The Dynamotor harness was intact in SN 20966, so I installed a screw terminal strip underneath the power supply chassis to preserve this hook up method just in case a DM-28 turns up in the future. AC input is through the SO-143 connector pin 3/4 which is the +28vdc input (with the AC neutral wired direct as shown in the schematic.) The wire from Pin 3/4 is routed in the harness to the AVC/MVC switch and then continues as a "switched" +28vdc wire to the Dynamotor harness connection pin 3. Routing the AC input via this wire will add no hum to the audio output.
Shown to the left is the restored BC-348-Q SN 20966. It's hard to believe that it's the same receiver shown in the first section of this article, "Finding a Good Candidate for Restoration." As can be seen, the front panel is original with the "M1" stamp and the very faint "AN - Army-Navy" stamp (just below "BAND SWITCH.") This receiver's front panel was given the same treatment described above using Glass Plus worked with a soft brass brush followed by a heavy application of Armor-All.
|photo left: Top of SN
20966 showing the installation of the AC power supply with a dual
section filter and solid state rectifiers. Care has to be taken to
assure that the long screw on the left side (that secures the radio to
the cabinet) will have clearance past the chokes when
the AC power supply is installed. The screw is not installed in this
photo but it does have the necessary clearance.
Repainting the BC-348 - Both Versions
First off,...don't attempt this unless your receiver is in really poor
cosmetic condition because IT'S A LOT OF WORK! However, some
BC-348s are poor on the outside and really nice on the inside, so a
repaint is sometimes a necessity.
You're going to need about three or four cans of VHT High-Temp Black Wrinkle Finish Paint. It is carried at most O'Reilly auto parts stores. Be prepared, it's expensive at around nine dollars per can. Use a "spray can handle" as this allows better control over the spray pattern. Also, use high quality paint stripper. Remove all of the knobs, the handles, the small panel, the dial cover and the phone jack "toilet-seat" covers. Unsolder the wires to the dial lamps and to the Antenna terminal. Remove the front panel. Keep all of the screws together because they will also be painted. Drill out the rivets that mount the ID tag to the front panel. You will also have to drill out the rivets that hold the two dial lamp sockets to the panel. You can now strip the paint from the front panel, the handles, the knobs and the smaller access panel.
The front panel, all knobs, the two pieces to the dial cover, the phone jack toilet seat covers and the small access panel are to be painted black wrinkle finish. The handles and the screw heads will be painted black satin finish. Some of the original hardware was actually a dark bronze finish but this was an oxide finish that is difficult to accomplish. It can sometimes be duplicated with ink dyes that combine brown and black to achieve the proper color. Most restorers just paint the screw heads satin black.
First, though, a word or two on how to successfully paint wrinkle finish paint. The original paint used on the BC-348 and all military and commercial gear from the fifties on back was a two-part process that required a base coat of nitrocellulose lacquer that was allowed to dry. Then a catalyst was sprayed onto the lacquer base. The panels were then put into ovens that baked the paint and the wrinkle developed in the oven. That's why most vintage wrinkle paint jobs have the wrinkle on the outside and smooth gloss black paint on the inside. The inside wasn't sprayed with the catalyst.
Today, we use paint that is a one-part process that is heat activated. I do all of my painting outside so the heat comes from incandescent lamps. First, the paint has to be applied with very heavy successive coats separated by just a few minutes. I use three heavy coats applied at different angles for each coat and separate each application by a couple of minutes. Next, to apply heat I use 100 watt light bulbs in clip-on utility lamps with aluminum shades. These are available at Home Depot for less than $10 each. I usually use three lamps suspended over the panel about 10" to 12" above the panel. The three lamps will heat the paint and the panel and the wrinkle process will start in about 10 minutes. I standby with a hand-held heat gun and after about 10 minutes, when the wrinkling starts, I use the heat gun to apply more heat around the corners and places where the lamps don't heat the panel very well. For stubborn areas, the heat gun will force the wrinkle but don't apply too much heat or the paint will "gloss" and the wrinkle won't match. After about another 5 to 10 minutes the wrinkling should be complete and now remove all heat and let the panel cool down for about 30 minutes. When the panel is cool, the paint will be set-up enough to handle. Let the paint cure overnight before mounting any screws or parts to the panel. If you can wait a little longer, like a week, the paint will be much harder and resistant to scratching or other types of damage.
Now comes the tedious part. You'll notice that the silver nomenclature on an original front panel is bare aluminum. Originally, after the panel was painted the paint over the nomenclature was "ground off." Originally, the nomenclature was much higher and when just "ground" down slightly, the bare metal showed through as silver lettering. This time we can't do that since the lettering was already ground down once and now the lettering is just above the paint level. I make a small tool that consists of an industrial razor blade piece about .25" wide that is mounted to a small wooden handle. With this tool I can carefully "shave" the wrinkle finish paint off of the nomenclature but the lettering still isn't bright. >>>
|>>> I also make another tool that is another small
handle that has the end cut at an angle that has a flat surface. I
use 400 grit aluminum oxide paper held to the angled part of the handle with tape to polish
the lettering. This leaves the lettering looking very bright and
original. Sometimes there's minor slip-ups but I use a small paint brush
to apply Artist's Acrylic Mars Black paint to touch-up scratches or
other blemishes I've caused during the process.
To remount the ID tag, you'll have to make "fake rivets." I use 4-40 SS binder head slotted screws. I chuck them up in a drill press and file the head until you don't see the slot but there is still material left to shape a rivet head. Don't use philips head screws because the slots are too deep. After you have four good looking "fake rivets" mount the tag and secure the "fake rivets" using 4-40 nuts on the back of the panel. There is plenty of clearance for the nuts but don't use overly long screws. Although you don't have to make "fake rivets" to remount the dial lamp sockets be VERY CAREFUL about the back clearance when remounting them with screws and nuts. Mount the binder head screws on the back side of the panel and use the nuts on the dial lamp socket side. You still might have to file the head of the screws to be sure you have clearance. The distance between the back of the front panel and the tuning dial mask is minimal. Without the clearance, you'll scratch the dial mask the first time you change bands. Make sure you have the clearance.
Next, paint the knobs and the other parts that should be black wrinkle. Use a small scraper made out of a jeweler's file to remove the paint from the "arrows" on the knobs. An Ex-acto knife with the proper blade can also be used to remove the paint from the "arrow." Some versions of the knobs appear that the arrow was stamped into the wrinkle finish paint and shows the base metal but the arrow is not engraved into the metal. Also, some vintage drawings/photos show the knobs without arrows so if you have the stamped type arrows, just leave the knobs black wrinkle without an arrow. Also, paint the handles and the screw heads black satin.
For the cabinet, I've had pretty good luck just shooting over lightly sanded and very clean original paint with the VHT wrinkle finish and then giving it the heat treatment. You might have to paint one side at a time depending on your heat lamps. I usually do one side at a time because I have better control of the wrinkle process that way. I have stripped the cabinets and wrinkle finished and I suppose they look slightly better but it's very hard to tell whether a repainted cabinet has been stripped or just painted over.
What about the Signal Corps acceptance stamps? - Without a doubt, the repainted BC-348 looks incomplete without the orange Signal Corps acceptance stamps. What I do is first make full scale drawings of what the typical acceptance stamp looks like. This will have the square boarder with "SC" at the top, a random number that was the inspector's identification and then an "A" at the bottom. Also, make another scale drawing of the "M1" stamp that is on 90% of the BC-348s. I then take the scale drawings to a stationary office supply type store that makes rubber stamps. It costs about $15 to $20 per stamp. Usually the square boarder on the "SC" stamp costs a little more but you "gotta have it." Once you get the rubber stamps, now you can apply your own acceptance stamps on any of your repainted Army gear. I use Artist's Acrylic to mix up the proper orange color and use water to thin the paint to a very wet mix. I then soak a cloth pad with the orange paint to create an "ink pad." Be sure to practice on some scrap pieces for a while. Also, be sure to have the stamp "runny looking" and be sure to remember - none of the original stampings were straight. After all, the inspectors were just stamping gear they had tested or accepted - it wasn't anything special to them. The "M1" stamp is usually a lighter, almost yellow-orange color ink and done the same way as the "SC" stamp. In fact, the "M1" stamps are usually even "runnier" looking. The "sloppier" the stampings looks, the more original they look.
Take a look a the photo of BC-348-Q SN 11227 (the last photo in Part One.) This BC-348 has been repainted as described in this section (with the exception that I used Krylon BWF paint instead of VHT.) Also, the Signal Corps acceptance stamps are made with reproduction stamps. Overall, the impression of SN 11227 is that it is an excellent condition, original BC-348. This is the effect we want to achieve in our restorations.
1. Various Military Manuals - These are usually the best sources for wiring diagrams, schematics, parts lists and other information that helps to figure out what is "original" for a particular receiver.
2. CQ Surplus Military Radio Conversion Books - Although I knock CQ quite a bit for their conversion and modification publications, their books specifically on the Military Surplus Radio subjects do provide a lot of information on the equipment and may actually provide some insight into how a specific piece of equipment was modified by a former owner.
3. Internet - The Internet is filled with information on BC-348s. Just do a search and see.
Henry Rogers WA7YBS/WHRM © JAN 2010, DEC 2010, re-edited to add the following: Grid Capped Tubes Version (Part Two,) Repainting, AC PS © JAN 2011
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