Rebuilding the ART-13 Transmitters
Restoration of a $10 (Wasp's Nest Special) USAAF Stewart Warner ART-13A
Operating the ART-13 on a Dynamotor
Using the ART-13 on the Ham Bands Today
Operating the ART-13A with the CU-32 LF Loading Coil on 630M CW
by: Henry Rogers WA7YBS/WHRM
T-47A/ART-13 (sn:2054 from 1944) - Restoration/Rebuild of the $10 ART-13
Those of us "collector-hams" (and especially "vintage military radio collector-hams") can't resist a bargain. When I was offered this rather "rough condition" ART-13A for a ten dollar bill, I wasn't looking for another project but sometimes the "collector" takes over and the combination of "bargain plus project" is too much to resist. This ART-13A presents some different challenges beginning with a thorough cleaning to assess what damage (if any) was caused by the wasp infestation that left many "mud dabber" nests throughout the internal parts of the transmitter. This project covers the basic steps necessary to get an original transmitter that was "storage challenged" up and running along with what cosmetic procedures are necessary to end up with a respectable appearance. - H.Rogers Oct. 12, 2015
||October 12, 2015 -
I picked up this ART-13A today in Sparks, Nevada. It was very reasonably
priced (ten dollars) probably because of the condition of the interior
of the transmitter. It had a lot of "mud dabber" (wasp) nests inside. I
figured that, at the least, it would be a good "parts set." When I got
it home I was then able to inspect it closely. Overall, sn: 2054 was
very complete although very dirty. The only thing missing was the tuning
book and at least the cover of the book was present in the pocket. All
of the knobs were in un-chipped condition. The plastic cover over the
tuning chart was gone but the paper chart is in pretty good condition.
The yellow tag that was tied to the left grab handle was a "ready for
use" tag that indicated that the transmitter had been tested and was
found operational in 1954.
On the downside, the "keying" decal by the KEY input jack is badly deteriorated as is the decal on the lid.
I decided that if the ART-13A passed all of my initial pre-tests, it would probably be a good candidate for a rebuild.
Quick Check - Oct.16, 2015 - I tested the modulation transformer and found it to be the older style with 150 ohms DCR Screen winding. All windings tested fine. Also, both meters tested okay. K-103 and K-104 tested okay. All chokes tested okay. I also discovered that the MIC Switch was "safety wired" in the Carbon position. This is typical for an ART-13 straight from the military.
These tests assured me that the most important parts were okay and the transmitter would probably be fairly easy to get operational.
|photo left: SN 2054 output section doesn't look too bad.
Dust, dirt and a few wasp nests in the front left corner. These nests
wrap around the TEST switch and somewhat surround the RCVR antenna input
photo right: SN 2054 showing the modulator module, frequency calibrator module, 1625 multiplier sections and the 813 output tube. Lots of wasp nests on this side of the transmitter. Note the one nest that surrounds the Second Multiplier tube (center top of photo.)
|Update October 26, 2015
- It looks like ART-13 sn 2054 is going to require substantial
disassembly in order to remove all of the wasp's nests. I checked the
controls to see if they rotated freely and found that Antenna Loading E
was immoveable and High Frequency Tuning Coarse A felt "locked" but it
would "rock" somewhat. First though, I wanted to test my wasp nest
removal process and see if it was going to work. I removed the FCI
module and the Audio module to clean that area of the transmitter
chassis. I found that Glass Plus spray and a small acid brush did a
pretty good job at removing the wasp's nests. It was very messy
but essentially the nest disintegrates and you're left with Glass Plus
and wet dirt. Every so often there would be some insect debris mixed in
with the mud. I had to be careful not to break any components or wires
so my technique was to spray the nest carefully (to result in only the
nest getting wet) and then brush the wet nest with the acid brush. This
was repeated until the nest disintegrated and all that was left was
residual Glass Plus and wet mud. This was removed with paper towel
pieces, Q-tips and dry paint brushes. Tedious work but the results were
worth the effort.
When I removed the ART-13 bottom cover I could see why the Antenna Loading condenser wouldn't move - it was full of wasp's nests. This took about an hour to clean with Glass Plus and brushes finally resulting in an easy to adjust E control. I could also see the Multiplier Section had several nests that were responsible for the "locked" Course A control (more on this in "Problem Two - Serious," below.) I decided at this point it would be necessary to remove all of the sheet metal cabinet pieces to have full chassis access for nest removal. The photos right and below show the permeably tuned coils for the Multipliers that are located just behind the PTO. The wasps built this nest right around the 1st Multiplier coil form (photo right.) It took about an hour to carefully remove the nests with Glass Plus and brushes followed by a clean up of the area with paper towels, Q-tips and dry brushes. Complete access allows for a thorough "clean-up" of the disintegrated nest material and resulted in the transmitter chassis looking really nice. See photo below left.
Update Oct 31, 2015
All of the wasp's nest were now removed (I thought.) I had to substantially disassemble the ART-13 as can be seen in the photo to the right. Only the PA output panel and the power input panels were not removed during nest removal. As can be seen in the photo, the wasp's nests didn't do any damage to any of the components or to the sheet metal and any minor residual nest material can be blown out of the chassis after it dries (since then it's just fine sandy dirt.) The next step was to further clean and lubricate the various contacts and switches that were affected by the wasp infestation.
Testing - Nov 7, 2015 - I moved the transmitter and parts
from the shop to the upstairs repair and test area for reassembly and
testing. Once upstairs with better lighting, I saw there was yet another
wasp nest buried in the Multiplier section. This was removed in the same
manner as the other nests. More detailed cleaning followed until the
transmitter could be turned upside down and no dirt or "mud balls" fell
Body Work and Tube Testing - The reassembly did require some of the panels to be straightened. This was accomplished with oak wooden blocks as cushions and a weighted hammer. Though this sounds like rough treatment, the panels don't loose any paint and end up nice and straight. With all of the panels back in place, now it was time to test all of the tubes. The testing showed all tubes to be in excellent condition except one of the 6V6 tubes had an internal short. Next, was to apply some +28vdc and see how the Autotune worked.
|Problem One - Minor -
Autotune - Before +28vdc was applied, I manually checked
the operation of the Autotune by turning the main shaft by hand. Then I
lubricated all of the bearings with 10W oil and greased the worm gears
with red wheel bearing grease. I also lightly lubed each of the moving
parts in each of the tuning modules.
With only +28vdc connected to the ART-13 and with Channel 1 selected, the power switch was thrown and the Autotune ran to zero (correct) and then proceeded to run all the way the other direction and then stopped (not correct.) Switching channels didn't result in the Autotune running to zero again. Usually most problems with the Autotune are in the forward limit switch or the rear limit switch. In this case, the contacts on the rear limit switch were very dirty and cleaning with DeOxit got the Autotune working correctly.
|Problem Two -
Serious - Nov 10, 2015 - We were now at a point where we
could apply the +HV and +LV and see how the transmitter was going to
work. This test resulted in no grid drive (well, at least we didn't
blow any fuses.) The lack of grid drive was due to either the PTO or
the first Multiplier not working. Further testing was required to
determine the specific problem causing the lack of grid drive. A quick
test showed that the PTO was operating. I measured the plate voltage and
also used a pick up to see the PTO output on the oscilloscope. I did the
same thing for the Multiplier tube and had both plate voltage and output
signal but no grid drive to the 813.
With power off, I attached one lead of a DVM (in ohms) to the input side of the Multiplier's output coupling capacitor and the other DVM lead to the plate of the 1st Multiplier tube. I should have had continuity but it showed open. Upon close inspection I noticed that one of the arms of the 1st Multiplier section switch was twisted and bent up therefore not making contact with the switch contact buttons (see photo right.) I was very careful not to "force" any of the controls during inspection so I'm sure this damage happened in the past when someone "test-turning" the controls forced control A while a wasp nest surrounded the switch arm. Forcing the control to move must have bent the switch arm as it pulled loose from the wasp nest.
The first Multiplier switch is "buried deep" in the chassis and removing any components is next to impossible without removal of the PTO section of the transmitter - a difficult operation. I was pretty sure with a very long and thin needle nose pliers and a couple of long thin screw drivers, I could bend the arm back to its original position. This had to be carefully done since breaking any part of the switch would be a disaster. The operation took about 10 minutes and the switch function was back to normal. A continuity test showed everything was working fine. >>>
- The knobs had most of the white paint fill for the nomenclature
missing. The locking bars and their backing plates needed to be
repainted. The black wrinkle finish needed to be reconditioned and
touched-up. All of the cosmetic restoration was performed as described
in the section titled "Refurbishing the Cosmetics" above in this
As to details,...I used black nitrocellulose lacquer for all touch-ups and for painting the locking bars and their backing plates. At first the paint looks way too dark because the original wrinkle hasn't been reconditioned. Let the lacquer dry for an hour and then rub the wrinkle finish panel with a cotton pad that is lightly saturated with clean 10W machine oil. Wipe the panel down with a clean cotton cloth afterwards. The black wrinkle finish will now match perfectly the black nitrocellulose lacquer. I've found that using Mars Black Artist's Acrylic as a touch up paint is problematic and sometimes the touch-up looks somewhat "brown" rather than black, especially if Armor-All is used to enhance the original wrinkle finish. I'm getting much better and more consistent matching using the black lacquer and reconditioning the wrinkle finish with 10W machine oil.
- November 17, 2015
- At this point, the ART-13 had been completely reassembled and
was looking very nice. I did have to do some body work on the tuning
book pocket since it was bent in several places. I used oak blocks and a
weighted hammer followed by touch-up with black lacquer.
Further testing was necessary before the transmitter could be put "on the air." I wanted to make sure the Autotune was fully functional and that I set up the first several channels to frequencies that I use. I also needed to look at the waveform in AM on the oscilloscope to see that 100% modulation was possible. Finally, I needed to have the transmitter operating into a dummy load and then listen to the signal in a receiver that was not connected to an antenna. By listening to the receiver using a headset, I could get a very good idea of how the ART-13 would sound "on the air." Additionally, I needed to try the CW mode and verify that mode functioned correctly.
- Since I had essentially removed nearly all of the sheet metal panels
during the wasp's nest removal, I had the opportunity to thoroughly
clean in many places that are normally not accessible. The end result
was a chassis that looked very nice in addition to being fully
functional. Here's some cleaning details,...
The final tank coil was pretty dirty between the windings. Fortunately, the coil spacing allows access with a "horse hair" tooth brush with Glass Plus. The dirt can be brushed out going in the same direction as the winds. All chokes were also cleaned with a horse hair brush and Glass Plus. The Glass Plus evaporates leaving no residue.
The area containing the 811s and the 813 tubes was MFP coated. There doesn't appear to be MFP anywhere else in the transmitter. Area cleaned with Glass Plus and various paint brushes.
Tube sockets were cleaned with DeOxit. Switches were also cleaned with DeOxit. The three Jones' plugs were cleaned with DeOxit.
Relay contacts for K-103 and K-104 were cleaned with 600 grit AluOx paper followed by DeOxit.
At this point the ART-13 was functioning using the Dyna-Sim AC power supply with the transmitter output connected to a 50Z ohm dummy load with air variable auxiliary condenser. However, the transmitter was still on the test bench.
||Functional Test - November
19, 2015 - I didn't want to put SN 2054 "on the air"
without first testing how the audio looked on the oscilloscope and how
it sounded in a receiver. Shown to the left is a set-up for testing the
ART-13. The Dyna-Sim power supply is on the floor but the cable can be
seen routed behind the yellow tag at the left side of the ART-13. The
mike used is a Shure 102-C carbon microphone. The metal box on top of
the ART-13 is the auxiliary condenser. To the left of the transmitter is
a Harrison 1KW 50 ohm dummy load. Further to the left is the monitoring
receiver, a Collins R-388 (a 12" pick-up antenna is necessary since the
receiver is well-shielded.) Note that the audio output from the receiver
is to a 600Z ohm headset. This allows me to listen closely to the
character of the audio from the ART-13 without feedback occurring. Not
visible in this shot is the equipment to the right of the transmitter
which consists of a 561 Tektronix oscilloscope, a General Radio Digital
Frequency Counter and other various types of test equipment.
With the ART-13 running at approximately 150 watts of carrier output into the dummy load, the waveform was observed on the oscilloscope. Close-talking the mike resulted in obvious "cut off" at 100% negative modulation. Best results were with the mike about 4" from the mouth. Next, the 'phones were put on and the signal tuned in on the R-388. Again, it was noted that "close-talking" the mike resulted in fairly noticeable distortion but with the mike about 4" away from the mouth, the audio sounded undistorted and clear. Of course, the test was using a carbon mike so the typical "carbon sound" was apparent. Inspection of the Audio Module revealed that it had the common modification of changing R203 from 15k to 4.7k. This increases the carbon mike bias for better response (certainly necessary for the typical T-17 mike but a little "hot" for a really responsive carbon mike like the Shure 102-C.)
|The Penultimate Test -
November 25, 2015 - While still connected to the dummy
load, I verified that the ART-13 also sounded fine using a TUG-8
amplified base with the Audio Module switched to DYNAMIC. This position
is used when the particular net being checked into isn't populated by
military radio collectors. The ART-13 can produce excellent sounding AM
when operated in this configuration.
The next step was to connect the ART-13 to the tuned dipole antenna and actually transmit a signal. In this operating position I have other monitoring equipment to check modulation levels and transmitting frequency. I set-up four channels on the Autotune for the "most used" frequencies on 75M. I then telephoned KØDWC, who is located about 3 miles away, to have him listen at his station to my transmitted signal. This provides the penultimate test before actually going "on the air." Chuck's report was "an excellent sounding AM signal - clean, clear audio."
The Final Test - Novemeber 28, 2015 - Of course, the final test is to actually use ART-13 SN: 2054 on the air during a regular AM net. My first net to try was the Saturday Morning AM net on 3870kc. This net is populated by regular AM ops so the reviews were from the perspective of "broadcast quality" audio. I used the TUG-8/DN-50 mike in the DYNAMIC position for hopefully the best quality audio response. Conditions were very good and all reports coming back were positive in both audio quality and signal strength. I was running SN: 2054 with the Dyna-Sim power supply so I was able to load the transmitter to about 150 watts of carrier power output. Antenna was a 135' tuned inverted-vee.
- So, I didn't have to buy anything to get SN:2054 up and running. I did
have to put in about 12 to 15 hours of work to get the transmitter into
functioning condition and looking pretty nice. All in all, it was a
relatively easy project that was completed fairly quickly and the end
result is a great condition, functioning ART-13A. These types of
"projects" workout well for the ART-13 enthusiast that already has
access to all of the accessories that are needed for restoration,
troubleshooting, testing and operation.
Update - September 12, 2016 - I've been using this ART-13A on a fairly regular basis, which would be about once a week. The only problem encountered has been an intermittent contact on one of the finger contacts in the Antenna Loading network (Course C.) The contact clears up with a slight rocking of Course C. This problem occurs rarely and then only when the channel is changed which actuates the Autotune. It's so minor I haven't tried cleaning the finger contacts but I probably will if it starts happening more often. I've set this ART-13A up running on the Dyna-Sim Solid-State power supply and it works with the 1948 National RCR Airway receiver.
Operating the T-47/ART-13 with the DY-12 Dynamotor
|Upon setting up the "$10 ART-13A" as part of an
operating station, I now had no way to operate the USN T-47/ART-13
transmitter. The thought of building another AC power supply didn't
really sound like much fun but doing something different did. Why not
operate this ART-13 on DC?
To set up an ART-13 for DC operation was going to require that I find an operational dynamotor. I found one locally from an old military-collector ham friend of mine. It was a DY-12 which was the correct type for the T-47/ART-13. Though not in the greatest cosmetic condition, it was fully operational and came with the correct DC input connector.
I still needed to find the DC output connector and another U7 connector. Additionally, a suitable cable had to be constructed for connecting the DY-12 to the T-47/ART-13. I bought the connectors off of eBay since this was the quickest way to get them here.
The manual doesn't specify the length of the interconnecting cable since each installation in an airplane was probably slightly different and depended on where the dynamotor was going to be located in relation to the transmitter. I decided that six feet was probably an average length. The cable used two 12 gauge wires, two 16 gauge wires and five 18 gauge wires. Pin 10 is the +HV and I used the center conductor and insulation from a length of RG-58 coax for that wire. The ART-13 manual cable breakdown only specifies the "minimum" wire gauge, so I used the largest gauge wires that would fit into the pin sockets. The wire bundle had to be shielded and wrapped with tape. The shield was made from the braided shield harvested from RG-8 coaxial cable. I connected the shield to the connector shell which provides a chassis connection at the ART-13 and also at the DY-12. >>>
|>>> Once the transmitter cable was complete, the DC input cable had
to be built. The two wires were 8 gauge six feet long and once again the
cable had to be shielded. The connector has a small pin that is
connected internally to the DY-12 chassis. The shield has a drain wire
that is soldered to this small pin on the DY-12 side connector. At the
PP-1104-C side the other shield drain wire has a lug that is connected
to the PP-1104 -28vdc terminal. I've changed this cable to eight feet
long, two 6 gauge wires, shielded (Sept 2018.)
When operating the T-47/ART-13 with the DY-12, the +HV runs around +1100vdc. The resulting RF power output is 110 watts on 75M. There are two DC windings providing two outputs on the DY-12, +400vdc and +700vdc. Normally, the two DC outputs are connected in series for the +1100vdc HV. At high altitudes (>25,000ft.) a barometric switch changes the series connection to reduce the HV to only +700vdc (to prevent arcing.)
To attain full transmitter RF output, the dynamotor must run at its design speed which is achieved at a minimum of +27.5vdc input. This is the voltage at the DY-12 input, not the voltage output of the power supply. There will be a voltage drop across the cable under load. The easiest method of determining the input voltage is to switch to BATT on the meter switch and adjust the power supply so that the BATT indication is at the top the mark on the meter. Alternately, a suitable point can be found inside the ART-13 and the input voltage measured. The lid will have to be off but since the T-47 doesn't have the interlock it's not too difficult to operate the transmitter filaments and see what the voltage drop is and adjust the power supply accordingly. In my installation, the standby voltage is +28vdc and loaded voltage is +27.5vdc at the transmitter. I've switched to a different DY-12. I serviced this one and found it to operate much better than the former DY-12 (Sept 2018.)
|Onboard the aircraft, the typical receiver that was used
with a DC operated ART-13A was the BC-348 (also operated on DC) to
comprise an ARC-8 set-up. This set up would require about 6 amps for
BC-348 dynamotor start-up and about 3 amps when in operation. The BC-348
would have its own power cable that would also attach to the PP-1104-C.
I've run this set-up and, although the BC-348 is a great little
receiver, it will usually be a challenge to Q-5 reception due to QRM.
The BC-348 has a broad bandwidth so signals would be easy to find since
the WWII radio op was "suited-up" in his electrically-heated flight gear
that included heavy gloves and he was trying to tune the receiver while
probably being shot at. Operating the BC-348 today, adjacent frequency
interference will be heard. If it's bothering copy, switch on the
Crystal Filter and zero the carrier of the received station (if you're
on AM.) This will usually allow for decent copy under difficult Voice
conditions. The BC-348 is an excellent receiver, but I wanted to run
The T-47/ART-13 running on the DY-12 dynamotor has a typical RF output on 75 meters of 110 watts to the antenna. The antenna is two half-waves in-phase (on 75M) and uses two 135ft wires that are center-fed with 77ft of open feed line, matched with a Nye-Viking coupler. I use a fixed 280pf 10kv ceramic capacitor for the external auxiliary condenser on the ART-13. To also add a bit of period flavor to the station, I operate the ART-13 using a Shure 102-C handheld carbon microphone. Between the dynamotor whine, the PP-1104 cooling fan and the carbon mike, the signal reports (and comments) are always interesting (even on vintage military radio nets.) September 2017
This station is now set-up with a Collins R-648 receiver that is also a dynamotor powered rig. The R-648 is also known as the "Airborne R-390A" but it's circuitry borrows a lot from the 51J Series of Collins' receivers. The R-648 is powered by a separate +28vdc 9A power supply. My particular R-648 was built in 1957. Photo of this station coming soon. (Sept 2018.)
Using the ART-13 on the Ham Bands Today
- The ART-13 Audio Module has a "fixed-level" gain setting that was
designed to work with specific WWII vintage military microphones with
the ART-13 operating at its standard voltage levels available from the
dynamotor. There are a couple of carbon microphones specified and a
couple of dynamic microphones also specified in the ART-13 manuals. The
Shure Bros. Co. T-17 was probably the most commonly used carbon
microphone by the military. Many ART-13s will be found with the
CARBON/DYNAMIC switch "safety wired" to the CARBON position.
Carbon Microphones - There are two problems that are normally encountered with carbon microphones. First, the military expected the radio operator to "scream" into the microphone. After all, he was trying to talk over the aircraft noise and was probably being shot at! Second is that many vintage carbon microphones don't have the response they used to. Most military WWII carbon microphones will not allow enough audio for proper modulation due to "carbon packing," "carbon fusing" or other age and use related factors.
Though the ART-13 manuals will specify a couple of microphones to use with the transmitter, nearly always (if you happen to have the particular types) you'll find they don't provide enough response to fully modulate the carrier. For Carbon mikes you'll have to test several examples to find an element that still has proper response and has not been ruined with excessive bias voltage (which causes the carbon particles to fuse together.)
You can buy replacement carbon elements but many times these are of old manufacture that just haven't been used (NOS) but they don't seem to work any better than the well-used older elements. This is probably due to "carbon packing" - another common problem. It's also possible to buy new carbon mikes that do seem to work very well because they haven't been subjected to any abuse and also the carbon particles haven't had time to pack together. Typically, you'll end up testing a lot of carbon mikes before you find an acceptable example.
|IMPORTANT - if you're
having Carbon Mike problems - Check the value of R203 in
the Audio Module of your ART-13. The original value of this resistor was
15K, which resulted in a carbon mike bias that was adequate for the
typical T-17 when it was new. It wasn't long before the value of R203
was changed to 4.7K to give a higher voltage to the mike bias and
therefore better response and gain. In the ART-13B schematics, the
resistor value for R203 is shown as 4.7K. In all of the earlier versions
of the ART-13, the value is 15K. Some carbon mikes will work fine with
R203 at 15K but most won't. If your ART-13's R203 is 15K and
you're fighting low response on every carbon mike you try, change R203
to 4.7K. I've found this upgrade on about half of the Audio Modules I've
inspected, so it was probably incorporated into many ART-13s as they
went through the military repair facilities.
Dynamic Microphones - Many Dynamic mikes usually don't seem to fully modulate either but many times this is due to a mismatch of the impedances. Many of the vintage ham dynamic mikes were fairly Hi-Z, around 10K was common. This mismatch will result in low gain. You can use a matching transformer but that's really getting to be a hassle. Try to find a Dynamic mike that has an output impedance of 250Z ohms which is what the ART-13 is designed for. You might find that some of the newer 500Z ohm dynamic elements are also a good input Z match.
An Easy Microphone Solution - The easiest method to have high output from the microphone, that allows you to run the ART-13 at a higher power output with 100% modulation, is to use a preamplifier between the mike output and the transmitter input. The Astatic TUG-8 mike stand is the easiest way to provide a preamplifier since a battery operated one is in the base of the stand. You can use any of the Astatic mike heads, e.g., 10-D, D-104, T-3, etc., and with the TUG-8 stand, these mikes will all sound fine and provide plenty of output. The preamplifier output Z is a fairly close match to the Dynamic input Z. Be sure to have the switch in DYNAMIC when using the TUG-8. Also, watch your transmitter's output waveform on an oscilloscope to set the TUG-8 "gain" control to proper level of modulation. The "gain" control is adjusted by a slotted shaft potentiometer that is accessed via a hole in the bottom cover of the TUG-8's base.
|Operating AM (VOICE) - Initially, an oscilloscope must be used to determine if the microphone chosen is really a suitable one for the ART-13. The oscilloscope will almost instantly show you whether or not you are achieving 100% modulation. The easiest way to use the oscilloscope as a "modulation monitor" is to connect a "pick-up wire" to one of the vertical amplifier inputs. The length of the "pick-up wire" will be depend on your power output, location of the antenna and how much vertical amplification you use on the 'scope. Most of the time a three foot wire works fine, sometimes you might have to use a 10' wire - experiment with what gives the best 'scope pattern. While transmitting in VOICE, adjust the 'scope for a modulation envelope pattern and you will almost instantly see if you have full modulation. Just a slight "touch" at negative 100% is seen as either a slightly brighter "dot" at zero or a short "line" at zero. Both are indications that the ART-13 is modulating to 100% negative and you don't want to go any further than that with your audio drive level. Note the positive excursion also. This should be symmetrical and at 100% positive. If you find that you are not achieving 100% positive but are at 100% negative modulation you might have weak 811 modulator tubes. Also, it's possible that there is some excessive current being drawn on the +HV which limits the positive peaks of modulation. You can reduce carrier power somewhat and see if that helps with the modulation symmetry.||Operating CW - Many ART-13 owners don't operate the transmitter in the CW mode. This really isn't because of a lack of CW experience but mainly because there are some limitations to "comfortable" operation in the CW mode. The ART-13, like most military transmitters, uses a Keying Relay to provide many functions inside and outside the transmitter. Outside the transmitter would be the Antenna switching and Standby function between the ART-13 and the receiver that provides "break-in" operation in CW. However, the Keying Relay is noisy - no doubt, on the airplane it was probably barely noticeable, but the typical ham shack will be a cacophony of racket when the ART-13 is transmitting CW. Also, if you're a fast CW operator, the Keying Relay will limit your speed. Although the ART-13 manual says that 30 wpm operation is possible, 15 wpm is a more realistic "speed limit." There are some modifications to eliminate the Keying Relay operation in CW and key the various circuits directly but one would have to decide whether or not the CW mode on an ART-13 was going to be a major facet of operation that would justify "hacking up" the transmitter. Besides, operating the ART-13 in CW in the original configuration is authentic and provides the user with some of the experiences that the airborne radio ops had to endure.|
Auxiliary Condenser and SWR - The ART-13 is not a
ham transmitter, so it's not surprising that it doesn't readily operate
into a dipole or quarter-wave vertical antenna. One of the
idiosyncrasies of the ART-13 is that it was designed to operate into
antennas found on aircraft. This was usually either a 20-60 foot end fed
wire from the front of the plane to the tail, or a 200 foot long
trailing wire antenna that was also end fed. Sometimes short whips were
also used. Anyway, it's likely that no ART-13 ever originally operated
into the 50 to 75 Z ohm unbalanced, low reactance (mostly resistive)
type load that most ham antennas provide. The common solution is to add
approximately 200pf to 800pf of capacitance from the COND terminal to
the GND terminal which then provides a Pi-Network of sorts in Tuning
positions 1 to 6. In positions 7 to 13, a Pi-network is already provided
and usually above 7.2mc an auxiliary condenser shouldn't be required.
Most amateurs operate the ART-13 in the 80-75M and this will require
If you operate on just one frequency in the 80-75M band, then a fixed value capacitor can be used. It's also possible to provide several fixed value capacitors to provide optimum loading on several frequencies. It's also possible to use a transmitting air variable capacitor which will allow optimum setting of the antenna loading and any frequency or any tuning range. Most transmitting variables are about 350pf maximum capacitance and will require a capacitive shunt to increase the maximum value to around 800pf depending on exactly where you want the antenna loading to be set. Most users find that around 500pf works quite well on 80M. It will depend on exactly where you want to load up the ART-13 considering the power supply's current availability provided for the +HV and the ART-13 power output desired. (See section below "Air Variable Auxiliary Capacitor" for a description and photos of a method of mounting an suitable variable capacitor for this purpose.)
SWR - Be sure when loading the ART-13 into an antenna that you use the lowest setting of C that allows tuning the transmitter to the antenna load. Although you can find higher settings of C that might appear to tune to the antenna, you might actually be tuning the transmitter to a harmonic instead. If you have an SWR bridge in the antenna line, look at the SWR and, if the antenna is matched for say 80M and you show a low SWR, then the setting of C is okay. 40M operation is very prone to tuning to a harmonic - watch your SWR and monitor your transmitting frequency.
|Frequency Monitoring and
Why Zero Beating Doesn't Work - The easiest way to
monitor the frequency of the ART-13 is with a digital frequency counter.
The Counter can have a short "pick-up" wire connected to the counter
input and that should provide sufficient signal for it to operate on.
Normal AM net operations are on specific frequencies so it's rather easy
to set the ART-13 up before hand.
If you have to do a minor adjustment to the frequency merely unlock the locking bar on Control B and fine tune the transmitter to the correct frequency. Be sure to re-tighten the locking bar afterwards. If you have the transmitter in MANUAL you can just adjust B to the correct frequency (leaving the locking bar tight.)
If you try to use the CALIBRATE position to zero beat the ART-13 tuned frequency to a received signal, you'll find that it doesn't work. The reason is the CALIBRATE position was to allow the radio op to use the FCI module's 50kc calibrate signal and to listen on earphones in SIDETONE for a heterodyne on the nearest 50kc cal point to the desired frequency for transmit. Only the PTO is in operation and it tunes 1000kc to 1510kc (in two ranges.) CALIBRATE was never intended for zero beating a signal in the receiver.
BC-348 Receiver Operating with ART-13 - ARC-8 was the designation for one of the most common set-ups for aircraft use of the ART-13 and combined it with the popular BC-348 receiver. Today, many hams run the ARC-8 configuration as it provides an aura of authenticity to a vintage military radio station (and the BC-348 is a great performing receiver.) The BC-348 can be wired into the U-8/U socket on the ART-13 to provide full break-in operation. Use pins 23 and 24 on the ART-13 U-8/U socket to provide a NC connection to pins 3 and 4 on the PL-103 connector on the BC-348 which will allow the ART-13 to control the stand-by function. Pins 23, 24 and 25 on the U-8/U socket access the internal sending relay in the ART-13. Pin 23 is Common (arm,) pin 24 is NC and pin 25 is NO.
Using the R-390A with the ART-13 - The R-390 and R-390A receivers require a NO set of contacts for receive and going to NC during transmit for proper BREAK IN operation of the receivers. You can use pin 23 (arm) and pin 25 (NO) to provide a proper "Break-in" for the R-390 or R-390A receivers.
Physical Location and Shock Mounts - There are a couple of different shock mounts that were used with the ART-13 transmitters. All of them are beyond difficult to locate. Probably when the transmitters were removed from service, the shock mount was left behind and scrapped with the airplane. Today, ART-13 transmitters are rarely offered or found with an original shock mount.
When placing the transmitter in its location in the station you will have to simulate what the shock mount provided. Not the vibration dampening but the elevation of the transmitter up off of its bottom plate. This elevation was how the ART-13 cools itself - by convection. Use spacers made of wood or rubber to raise the transmitter up so there is at least one inch of clearance between the table and the bottom cover. Two inches is better. This is usually enough to allow air flow and cooling.
|Construction of a Variable Auxiliary Capacitor Set-up - By far the best method of providing the additional capacitance required for dipole antenna operation below 7.0MC is to use an air variable. The variable capacitor allows very careful adjustment of the loading and resulting power output of the ART-13 when it is used with an antenna that has low reactance. For months, I just had the variable capacitor setting next to the ART-13 with some wires connecting it to the COND and GND terminals. No problems were encountered with this method but it certainly looked non-military and somewhat dangerous. An aluminum housing was drawn up and we had the local sheet metal shop built four metal enclosures. These would house the air variable capacitors for my two ART-13 transmitters and two of KØDWC's ART-13s. The air variables were fairly large types that I had in the junk box with each having a maximum capacitance of 1000pf and a minimum value of 100pf. About mid-range works very well with the ART-13 when operating on 75M. >>>||
>>> Make sure that the capacitor rotor is connected to both the housing and to the GND terminal on the ART-13. Since the rotor is connected to the capacitor's frame, mounting of the capacitor to the housing accomplishes the ground connection. I used an SO-239 UHF connector mounted on the rear of the housing that also connects to the capacitor frame and the capacitor stator connects to the SO-239 center terminal. A piece of RG-58/U coax with a PL-259 on one end and the other end having the shield connected to the ART-13 GND terminal and the center conductor connected to the ART-13 COND terminal is how the auxiliary capacitor is connected to the ART-13. The bottom cover is a flat piece of aluminum that the sheet metal shop cut for me. The bottom covers mount to 1/4" aluminum square bar that is mounted in the housing. Tapped holes allow mounting the bottom covers and small rubber feet are attached to the bottom plate. I painted one of my boxes black wrinkle finish and then used a spare ART-13 knob (D control knob is 0 to 100, 180º scale.) The finished Auxiliary Capacitor box looks very military and like something that should go with the ART-13 transmitter.
|Loading up the
ART-13 on 75 Meters - If you read the loading
instructions in the ART-13 manuals you are sure to be confused since
these are written for operation with aircraft antennae. Hams generally
are using a dipole antenna which will tune differently. First, make sure
you have an auxiliary capacitor connected to the COND terminal to the
GROUND terminal. This can be a fixed ceramic HV cap of around 500pf or a
transmitting air variable that provides adjustable capacitance from 30pf
up to 800pf. Depending on your air variable, you may have to use shunt
capacitors to get up to 800pf.
1. Adjust Control E to 200
- minimum capacitance
These are the steps to load the ART-13 up to a tuned dipole antenna using a variable air capacitor as the Auxiliary Capacitor. When using a fixed capacitor, you won't be able to vary the loading as much. Use Control E to see how much variability in loading you have. You can only increase capacitance with Control E if it was set for 200. This will increase the plate current in the configuration used on 80M. Normally, the value of the "fixed" Auxiliary capacitor is chosen to give some adjustability on Control E for the desired loading for the frequency and antenna used.
NOTE: The ART-13 loading will vary somewhat depending on the level of +HV used. This description applies for +HV levels around +1400vdc. Higher or lower levels of +HV will result in slightly different resonant points for Control D.
40 Meter Operation - Operation on 40M is basically the same as 75M except that Control C will be advanced up to triangle #8 or higher. At this point, the output network is actually a Pi-network and although the auxiliary capacitor probably isn't required, it does make loading much easier if it is used. Interestingly, the 40M band seems to be split with Control A referencing 7.2MC as the point of change. Usually, when operating in triangle #8 on Control A the lowest frequency tuned is about 7.15MC so you won't be able to tune into the CW portion of the band. Switch to triangle #7 on Control A for operation in the CW portion of the 40M band and you'll find that entire 40M band can be tuned. You might find that the Grid Drive Current is down a bit on 40M, since the alignment procedure allows for reduced Grid Drive at the band edges.
20 Meter Operation - 20 Meter operation is similar to 40M operation. You'll find that it is very near the top frequency that the ART-13 is capable of but usually no problems will be encountered. You'll probably have to have Control C at triangle #12 for loading up to full power. It's normal for the power output to be down slightly on 20M. I can get about 100 watts output on 20M with +1400vdc +HV.
Again, if you encounter very low Grid Drive you'll probably have to align the Multiplier Section of the transmitter. Use the procedure in the manual and be patient because it is a tedious, time-consuming job.
If you experience arcing inside the transmitter on 40M or 20M but not on 80M, it's probably a loading problem. Reset the position of control C first. If that doesn't help, try reducing the loading for lower total plate current. Most arcing problems are due to mismatched antenna loads.
|30 Meter CW Operation - Since the ART-13 is continuous coverage from 2.0MC up to 18.0MC, it's possible to do some CW operating on the 30M band. This is a small segment from 10.100MC up to 10.150MC available for ham operation in CW/Digital modes only. Naturally, a general coverage receiver will have to be used in conjunction with the ART-13. Since most hams are using tuned dipoles for an antenna, be aware that these normally will work fine on harmonically related frequencies but may not allow loading on 30M. I just use a separate small dipole specifically cut for 30M. Dimensions are 23 feet per leg and I feed the antenna with coax and a 1:1 balun. Be sure to check the "30M Band Plan" PDF file because each segment of the 30M band is allotted to specific functions and modes. Also, 200 watts is the maximum power.|
Operating the ART-13 on LF - 630 Meters (472kc to 479kc)
|630 Meter Basic
Info - This new Medium Frequency band is 472kc up to
479kc,...only 7kc wide. There's a band plan that allocates CW to the
lower few kcs although CW is legal for the entire 7kc. Data
transmissions are supposed to be in the top few kc. Operation is limited
to 5 watts EIRP but since most antennae are so inefficient at 472kc, it
could take perhaps 100 watts antenna input power or more to
achieve the 5 watts EIRP. You will have to do some antenna modeling
(calculations) to see the efficiency of your proposed antenna at 472kc
and calculate what input power results in 5 watts EIRP. Additionally,
you must submit your physical location to the Utilities Technology
Commission for approval of operation on 630M. They will confirm that you
are located more than 1km from the nearest power transmission line that
might use controlled-carrier troubleshooting data on it.
Equipment Required - Your ART-13 must have the plug-in LF Oscillator module installed. There are two types, the early "Navy" version with six ranges (O-16/ART-13 - 200kc up to 1500kc) and the later "USAAF" version with three ranges (O-17/ART-13A - 200kc up to 600kc.) Both types tune 630 meters (472-479kc.)
Once you have the LFO module, then you'll notice (when looking at the ART-13 schematic) that there's no RF output antenna coupling stage for LF inside the ART-13. The LF output from the PA is coupled through the plate blocking capacitor directly to the terminal marked "LOADING COIL" (J-117.) Onboard the airplane, an "ANTENNA LOADING COIL" (antenna tuner) was installed if LF operation was going to be required. CU-32/ART-13A Antenna Loading Coil was the largest version with the most elaborate circuits and it was built specifically for the USAAF. It allowed tuning the normal type of aircraft antennae for operation with the ART-13 operating from 200kc up to 600kc and included a RF amp meter. There was also the smaller USN CU-25 that used a much simpler circuit but still allowed tuning from 200kc up to 600kc. There was also the small USN CU-26 that was used for tuning 500kc up to 1500kc. The CU-32 allowed switching from either a trailing wire or a fixed antenna. The CU-25 and CU-26 were for trailing wire only. The CU-32 is shown in the photo to the right on top of the ART-13. It should be noted that the CU-32 is about the same size as the ART-13. However, it does weigh considerably less than the ART-13 (note the photo below to see why the CU-32 is a "light weight.")
|BC-348 Operation on LF with
the CU-32 Loading Coil Tuner - The BC-348 was always
connected to the ART-13's RECEIVER (J110) terminal. With LF selected on
the ART-13, the vacuum switch inside the CU-32 toggles back to "HF" when
the PTT line is deactivated (key up) and that then allows the BC-348 to
connect to the aircraft antenna via the ART-13's "HF" routing through
the CU-32. When PTT is actuated (key down,) the CU-32 vacuum switch
toggles to LF which connects the ART-13 J-117 terminal to the CU-32 LF
input. The ART-13 output is routed through the CU-32 to the selected
aircraft antenna. The BC-348 is further isolated during "key down" via
the ART-13's vacuum T-R switch. With PTT deactivated (key up,) the
ART-13 and CU-32 return to "HF - Receive" mode. Since the BC-348 isn't
routed thru the CU-32, the receiver's match to the particular aircraft
antenna used would be dependent on its LF alignment on the Q, N and J
versions. The other BC-348 versions have a front panel antenna trimmer
for antenna matching although its effectiveness might be limited
somewhat on LF. The BC-348 does tune 630 meters and it can provide
decent performance if aligned correctly on its 200kc to 500kc band. Of
course, your LF receiver doesn't have to be a BC-348. Any LF receiver
can be connected to the ART-13 RECEIVER terminal and utilize the ART-13
remote standby function (U-8/U connector) for fully integrated operation
on LF with the ART-13. Or, you can use an entirely separate receive
antenna such as a tuned loop. On 472kc, the use of a tuned loop receive
antenna will greatly reduce the noise level and boost received signals.
Directivity of the loop is also an advantage.
Operation of the ART-13 with the CU-32 on 630 Meters - Operation must be on CW. This mode of operation utilizes a keying relay in the ART-13 that essentially is "keying" the PTT line. The keying of the ART-13 also provides keying to the vacuum switch in the CU-32 via the U-11/U connector/cable on the ART-13 (U-11/U is only operational if LF is selected on the ART-13.) Since there are three relays involved, the ART-13 keying relay, the ART-13 vacuum T-R switch and the CU-32 relay/vacuum switch, all working with the radio operator's keying, the maximum CW speed will probably be limited to around 15 WPM. I tried a bug and even at slow speeds (for a bug,) it's too fast for the relays to follow and allow the ART-13 to key "dits" properly. A hand key must be used and top speed seems to be around 15WPM.
Be sure to do your initial testing on LF with the ART-13 in the "TUNE" mode. This limits the power output to prevent excessive current flow when the PA is out of resonance. With a short end-fed wire of about 100 feet in length, you'll probably find that the two LOADING controls will be at maximum and the "plate dip" is fairly sharp. A 200 feet long wire will perform much better and will load up easier.
The actual CW signal on 473.5kc from my ART-13/CU-32 combo running into a 163ft end-fed wire sounded loud, crisp and clean as received by KØDWC located a few miles away. I've also had a two-way CW contact with K6KBE from Ione, California (located just over 100 miles away.) I've heard WØYSE on 630M from Washington state but haven't worked him,...yet.
Operating CW on the ART-13 with CU-32 is a very noisy experience. There are three relays "clacking along" with your sending. While the airplane was aloft there was so much other noise going on the sending relays were seldom heard, especially since the radio op normally was using 'phones. Not much can be done about the noise if you want to run the ART-13 and CU-32 as they were originally operated. Modifications to change any of the method of operation seems to go against the entire idea of collecting, restoring and operating WWII vintage military radio gear in the first place. Sure, it's noisy, but if you use a set of 'phones, as the old WWII ops did, it's not too bad.
For Sidetone to monitor your sending, the ART-13 provides two jacks to access the sidetone circuit. There is a volume control that is located under the tuning chart. Use a 600Z ohm load, either phones or loudspeaker. You can also listen to the receiver if you're using a separate antenna. Since I'm using a remotely tuned loop on the receiver, I just turn the RF gain down and use the SP-600VLF receiver as the CW monitor. Actually, with this method, I'm actually listening to the transmitted signal which is probably better since any problems would become quickly apparent.
Utilizing "Fixed Ant" and "Trailing Ant" for HF and LF Antenna Selection - The original intent of this switch was to select between a trailing wire antenna or a short vertical antenna, with both antennae being utilized for HF or LF operation. A more useable purpose for this switch is to use the "Fixed" position for the HF antenna. Then the "Trailing Ant" can be used for the LF antenna. When the ART-13 is NOT in LF, the CU-32 is bypassed except for the Antenna Selector switch. So, when operating HF switching to "Fixed Ant" will allow the HF antenna to be connected to the "Fixed" output terminal with the coax shield connected to chassis. Selecting LF on the ART-13 activates the PTT switching of the CU-32 and the antenna selector can be switched to "Trailing Ant" and a LF antenna used. I'm assuming that most ops are going to have a separate LF antenna such as a 200 ft end-fed wire but will use their regular HF antenna for 80M or 40M operation.
Effective Isotropic Radiated Power - The EIRP limit of 5 watts is somewhat difficult to determine with any accuracy. This regulation seems to be in place mainly to limit the size of the antenna versus the amount of RF power used and thus to control the effectiveness of the transmitted signal. For example, a full-size 630 meter half-wave antenna is over 1000 feet long. This type of "full-size" antenna would actually exhibit a slight gain when compared to an isotropic radiator (+1.6db.) Therefore, the power input would have to be < 5 watts to stay within the regulations (assuming there were no other losses.) With a full-size antenna, even just 5 watts input would result in a formidable signal on CW. However, small, "city-lot-size" antennae will be very inefficient at 630 meters and therefore would exhibit considerable loss rather than gain. That's how the RF power input to a "small" antenna can be very high and yet only result in 5 watts EIRP. Think of the small, inefficient antenna as acting like a "dummy load." You can input lots of RF watts and it does radiate somewhat (a few feet) but it isn't an efficient radiator. So, to stay within the regulations, one has to know the efficiency of their antenna which is determined by the antenna's input resistance and its size. From efficiency and power input you can then calculate EIRP. Antenna resistance usually has to be estimated based on several physical factors with antenna size being the most important. The best information for answering the EIRP question can be found at www.472.org in their section on"630M Antennas" and "EIRP." Their formulae and examples indicate that the "average" ham antenna takes several hundred watts input to achieve 5 watt EIRP.
Propagation and Successfully Receiving LF Signals - A lot of the conversations about prospective 630 meter operation seem to be concerned about the transmitted signal. Very little is written about how to successfully receive DX signals in "real time" on the lower frequencies. Propagation on 630 meters isn't groundwave only. At night, skywave propagation dominates this region of the spectrum. It's easy to receive medium wave Airport NDBs that run 25 watts out to distances of 1500 miles. Regional NDBs that run 100 to 400 watts can be received out to the east coast. However, these great nighttime conditions don't last long. Long wave enthusiasts consider the "season" to be from the Autumnal Equinox to the Vernal Equinox, or about mid-September to about mid-March. Actually, the best part of the "long wave season" is from late-November to about mid-January. This is when the conditions are the least noisy allowing very weak signals to be heard. So, fall and winter nights or very early mornings work best on 630M. Daytime is good for testing or for local QSOs.
Your physical location is another important part of successful 630 meter operation. In noisy urban areas the RFI is so intense that virtually nothing can be heard below the AM BC band. In these locations, a shielded, magnetic loop is about the only type of antenna that might allow for some reception. These loops are shielded with a non-ferrous metal tube and, generally, the antenna only responds to the magnetic portion of the electromagnetic signal. A tunable amplifier is necessary with these loops (usually part of the loop design.) Since most RFI is electrical in nature, the shielded, magnetic loop provides some relief in noisy areas. Less noisy areas can usually receive DX using a remote tuned loop. These are just a fairly large coil of wire that's tuned with varactor diodes in parallel with the loop. A pick up loop is mounted within the loop field and that connects to the receiver. Usually, an amplifier isn't required on a remotely tuned loop. In quiet rural areas, it might be possible to use a wire antenna with good results. I use both a large wire antenna and a remotely tuned loop. I find that I can always use the loop antenna. Most of the time, only winter conditions are quiet enough to allow DX reception on the large wire antenna.
For a complete write-up on how to successfully receive DX longwave signals using vintage radio equipment, go to my article "Vintage Long Wave Receivers" - go to the Home Index.
|Conclusion - Today, the ART-13 is one of the "hottest" military transmitters around. It seems like everyone that has an interest in older gear, especially military gear, is looking for one or maybe even has one (or two,...or more) stored somewhere for future restoration. However, this isn't really a new phenomena,...the ART-13 has been a popular transmitter with hams for almost seventy years. Articles about converting and using the transmitter go back to the late-forties. As AM operation was replaced by SSB, the ART-13's popularity did drop but the resurgence of AM operation during the early 1990s (and the increasing interest in WWII gear) has brought the ART-13 back to prominence. Though not intended as a ham transmitter, it is interesting that so many of the WWII design approaches for this transmitter's circuit are found in later, post-war, Collins gear. The ART-13 has had a long history with hams as a versatile, potent and good sounding transmitter and today there are certainly more ART-13s "on the air" in the ham bands than ever before.||References:
1. Various ART-13 Manuals - there are several available from BAMA
2. USAF - Radio Mechanics VOL. 1 - Extension Course 3012 - specifically on the ART-13A and the BC-348. Theory, operation, maintenance, course Q&As, informative about post-war uses and maintenance.
3. Electric Radio - N6PY Bill Feldman, article on ART-13 basics. Tells how to perform an initial checkout of an ART-13. Also, a mod on removing the "locking" current off of the Autotune motor when in standby. W6MIT John Svoboda wrote a couple of articles on ART-13 power supply design and on exactly how the loading network operates.
4. Various "Radio News" and other radio magazines from 1946 to 1950 - surplus advertising, conversion articles, prices and other information.
5. Thanks to Mike Everette, W4DSE, for many details on ART-13 history, use, repair and restoration
6. Online - MilSurplus Group, specifics on ART-13 variations by KK5F and WA5CAB
7. Thanks to W6MIT John Svoboda for the heat gun suggestion on the Multiplier trimmer capacitors
8. Thanks to the many conversations with other ART-13 users and enthusiasts that have provided so much information over the years.
|Henry Rogers WA7YBS © May, 2011, additions on dynamotors Nov. 2015, addition on restoration ART-13A sn:2054, various corrections and other additions Nov 2015, new header photo & surplus market availability info Feb 2016, Correct knob sync info Dec 2017, added 630M info Jan 2018,|
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