Hallicrafters' Super Pro Receiver
R-274/FRR Receiver - aka: R-274D/FRR, SX-73
History - Circuit Design - Performance - Problems
Hallicrafters vs Hammarlund,...Super Pros, that is,...
Comparison of Features on Both Receivers
Which Super Pro is Best? - Yes! You can vote!
|Hallicrafters built a Super Pro receiver? Well,... not exactly. But, the R-274/FRR has so many similarities to the Hammarlund SP-600 that the former is often referred to as "The Hallicrafters Super Pro." So why did Hallicrafters produce a receiver like the R-274 and then later market it as a high-end commercial-amateur receiver dubbed the SX-73? Is it true that the Signal Corps was the actual designer of the R-274 with Hallicrafters as the contractor? Was the Hallicrafters' R-274 version was actually being produced before the SP-600? This web-article looks at the history of the R-274 along with its circuitry and construction. The section "Hammarlund versus Hallicrafters" lists most of the important similarities and many differences between the R-274 and the SP-600 and allows the reader to decide which company built the best version. I thought it would be interesting to see if any readers-owners would want to submit votes as to their opinion of which Super Pro is best, so I've added an e-mail link for voting. I'll keep the voting tally up-to-date and it should be fun to see what the results will be. - H. Rogers August 2011|
Hallicrafters' Super-Pro Receiver - R-274/FRR
In the late-1940s, the U.S. Army Signal Corps was searching for a manufacturing source for a high-quality receiver that incorporated certain design requirements needed for reliable data (RTTY) reception. Immediately after WWII ended, the Signal Corps started working on increasing the accuracy of RTTY communications by modifying BC-342 receivers, adding voltage regulation and other improvements to the stability of that receiver. By 1947, the Signal Corps was modifying WWII Hammarlund Super Pro receivers (SP-200 Series) to have a three-channel crystal oscillator for increased frequency stability along with a crystal-controlled BFO with the receivers working in diversity to further reduce both signal and frequency drift for reliable RTTY copy. These receivers were all Hammarlund BC-794 receivers that were professionally modified by Wickes Engineering Company and designated as the R-270/FRR to be used with CV-31 Diversity RTTY Converters. Additionally, the Signal Corps also had a modification kit, designated as the MC-531, that could be installed into any of their SP-200 Super Pro receivers that provided the three-channel crystal oscillator for improved frequency stability.
Besides frequency stability, the Signal Corps was looking for several other modernization requirements in their new receiver design. For example, turret band switching was another feature that was required because this allowed for relatively simple repair of the receiver's front-end since each RF coil assembly was easy to remove and replace (or repair.)
Double conversion above around 7.0mc was going to be necessary since much of the post-war communications were now using much higher frequencies than during WWII. Due to the higher frequencies being used, these new receivers had to have a tuning range from that started at the bottom of the AM BC band, .54mc, and went up to the VHF region at 54.0mc. Audio output impedance had to be 600Z ohms. It's likely that the Signal Corps had done most of the initial design work on what they thought their new receiver circuits should include and how it should be built. Whether the Signal Corps had a complete receiver design prepared and prototyped or whether the Signal Corps' receiver consisted of a list of specifications is unknown. >>>
|>>> It's apparent the Signal Corps had been working for some time with
both Hammarlund and Hallicrafters to improve receiver stability and
to improve RTTY accuracy. Hallicrafters was involved with the building
of the CV-31 Series of Diversity RTTY Converters to work with the
Hammarlund/Wickes Engineering R-270/FRR receivers. Hallicrafters was
also still building BC-610 transmitters for the Signal Corps so they
still had a good reputation as a military contractor. Just how much of
the Hallicrafters R-274 is directly from Signal Corps designers and how
much of it is from Hallicrafters' engineers is unknown. What can be
gleaned from a close inspection of the R-274 is that it doesn't look
anything like a typical Hallicrafters
receiver. The layout of the chassis is military in style. The
high-quality components used in construction were
certainly to meet Signal Corps specifications. The mounting hardware is
entirely made up of
machine screws, lock washers and nuts. No self-tapping screws are used.
The front panel doesn't look like any other Hallicrafters
receiver (it looks more like a modernized BC-342.) The wiring
and lead dress is on a much higher level than the typical Hallicrafters
chassis (even when compared to the AN/GRR-2 "military SX-28A.")
The circuit is like nothing Hallicrafters had built before (or after.)
So, was Hallicrafters' R-274 receiver created entirely from a Signal
Corps design with Hallicrafters as the contract builder or was the receiver a combination of both
a Signal Corps design with some input from Hallicrafters engineering?
Hallicrafters collectors like to believe the latter,...but
certainly the significant differences in design, construction and
performance between the R-274 and any other Hallicrafters product leaves
an objective observer believing the former.
photo left: Hallicrafters CV31D Diversity RTTY Converter from 1951 used with R-270/FRR receivers
The SP-600, The R-274, The Contacts and Later Production
SP-600 Super Pro
- In 1948, Hammarlund published an advertisement (see photo
below-left) for the Series 600 Super Pro, the SPC-600-X, receiver that
apparently had all of the features the Signal Corps wanted
and most of the features found on the later SP-600 receiver. The
advertising artwork shows the 1948 concept of the SP-600.
The first actual Signal Corps contract appears to be Contract Order
Number 18566-PHILA-49-7, dating from 1949 and assigned to "The HALLICRAFTERS
Co." (see tag photo lower-right.)
There appear to be two contracts for Hallicrafters R274 and R-274D receivers spanning 1949 up to 1952. On any of these contracts ordering and building could take place over several orders using the same contract number and fulfilling the order might take more than one production run and could take many months to finish (maybe even years.)
Suffixes - When the Signal Corps and the USN began ordering the Hammarlund SP-600 version of the receiver, the R-274 designation had already been assigned to Hallicrafters' production. So, rather than assign an entirely different number to the Hammarlund SP-600, these receivers were given suffix letters of A (Signal Corps,) B (USN) and C (Signal Corps.) Even though the non-suffix designation had been assigned to Hallicrafters, their next new contract receivers would be designated R-274D/FRR. The next known Hallicrafters' contract for the R-274D was Order No. 3464-PHILA-52 from 1952.
The Hallicrafters R-274 "D" Version - Probably the biggest change in the new Hallicrafters R-274D was how the remote standby worked. The original R-274 remote standby worked in parallel with the front panel Send-Receive switch requiring a NC contact on the remote standby line for receive and a NO for standby. The NO on the remote line (with the panel Send-Receive switch in Send) would disconnect the screen voltage to the two RF amplifiers and the first conversion mixer stage. To disconnect the antenna from the receiver in the Send mode would have to be accomplished with an external T-R relay. The R-274D changed the remote standby to an antenna input protection circuit. Remote standby required that 12 volts DC be connected to the rear terminal strip to operate an antenna relay that was inside the receiver. The relay shorts the antenna input to ground when in the Send position (12vdc applied to remote terminals.) Also, the antenna coil of the receiver is an open circuit to protect it from any RF radiation energy. A neon bulb was also added to protect the receiver antenna input from static discharges. An external T/R relay would still be required. Appearance-wise, some R-274D receivers will have gray-color dial escutcheons. >>>
Production - With examples of both receivers available to the Signal Corps from
1950 up to about 1952 it seemed that the Signal Corps favored the Hammarlund SP-600
over the Hallicrafters R-274. Over the next decade literally tens of thousands of SP-600s were
produced for the military but the Hallicrafters' version was produced in
just a couple of contracts from 1949 into the early 1950s.
Judging by the scant quantity of Hallicrafters R-274 receivers encountered today, the contract quantity was probably fairly low, maybe 1000 receivers per contract. Hallicrafters may have expected more contracts from Signal Corps but it seems that only a couple of contracts account for all R-274/FRR and R-274D/FRR receivers. How popular the R-274 receivers were with the Signal Corps is difficult to establish but few (if any) are ever seen in vintage military photographs.
Since the contracts only accounted for a relatively small quantity of receivers and Hallicrafters had obviously done substantial production line tooling and had set up suppliers for the required parts, it was not going to be profitable if only a couple of contracts resulted from these expenditures. Like other manufacturers that provided the military with radio equipment, e.g. Hammarlund and their SP-600, Hallicrafters decided to offer the R-274D to the commercial and amateur market. To avoid any confusion as to the intended customers, these Hallicrafters receivers were given a new designation of SX-73.
The SX-73 - At $975, the sales of SX-73 receivers must have been incredibly slow. A cabinet is mentioned in the Hallicrafters' ads but usually the artwork shows the SX-73 as a rack mount receiver. Note the B&W artwork in the header photo and see that the data plate was changed to "SX-73." The R-274 (TM11-897) military manual mentions a CY-699/FRR metal case for table use but it is unlikely that this was what Hallicrafters offered (see below for photos of Hallicrafters' CY-1345/GR cabinet for the R-274D.) There was at least one advertisement artwork that showed an SX-73 that was repackaged to look like a civilian Hallicrafters' product with large dial and meter bezels along with stylized grab handles and cabinet. Whether this version was actually produced is unknown.
Today, the SX-73 version is rarely encountered, however the military versions R-274 and R-274D do show up from time to time. These receivers have a dedicated following of enthusiasts who actively use their receivers. From serious SWLs to hams using the R-274 for communications in Vintage Ham Stations, both are users who want a receiver that features impressive sensitivity and great audio reproduction along with a Spartan-like military appearance.
R-274 and SP-600 versus Collins for Signal Corps RTTY Use - Radio Teletype (RTTY) was the initial reason for the Signal Corps' interest in a stable, crystal-controlled receiver that was capable of dependable data reception. By 1950, Collins Radio Company had started to supply the Signal Corps with 51J receivers that had a frequency readout that was accurate "to the kilocycle" and frequency stability that was essentially drift-free. The next year, the Collins R-390 was released and it was also accurate and drift-free (and expensive.) Ultimately, these Collins receivers were what the Signal Corps used for RTTY, installing both the 51J (R-388) and the R-390/R-390A in portable communication huts specifically for RTTY communications. The fate of the Hallicrafters R-274 was no more contracts after 1952. The fate of the Hammarlund SP-600 was greatly reduced numbers produced after the mid-to-late-1950s. Although Hallicrafters tried to market the R-274D as the SX-73, very few were actually produced. It was the end of the design for Hallicrafters and they really never produced a similar receiver afterwards. Hallicrafters must have lost a substantial amount of money with the R-274/SX73 production. However, most enthusiasts maintain that the R-274 was Hallicrafters' best-effort in receiver performance and design (acknowledging the Signal Corps influence, of course.)
NOTE: Throughout the remainder of this article, the Hallicrafters' receiver designation R-274 is used almost exclusively to avoid repetitious listing of both designations. Just remember that the earliest version was the R-274 receiver and the later version was designated as the R-274D receiver. Also, Hammarlund R-274A, B or C receivers are referred to as "SP-600" for the same reason.
R-274 Circuit Details
|The R-274/FRR is a 19 tube, double conversion, superheterodyne
communications receiver that tunes from .54 to 54 mc in six tuning
ranges that are selected by a rotating turret-type band switching design. The receiver is single conversion with double preselection below
7.0 mc and double conversion with double preselection above 7.0 mc. The
first conversion intermediate frequency is 6.000 mc and the second IF is
at 455 kc. Three stages of 455kc IF amplification are used along with a
output stage to drive data devices (RTTY.) A conventional envelope detector is used and two stages of
AF amplification are employed. The audio output is to a triple secondary
output transformer that provides two 150 ohm Z windings that can be
connected in series for 600 ohm Z output or just one winding can be used
if 150 ohms Z is desired. The third winding is 50 ohms Z for
earphone operation and a 2.2K ohm resistor is in series with the 'phone output to
allow either Hi-Z or Lo-Z earphones to be used without changing the 600
ohm output level.
A six-channel crystal oscillator circuit is provided to allow the user the option of six crystal-controlled LO frequencies for better stability. When a channel is selected, the crystal oscillator circuit is capacitive-coupled to the standard LO circuit. The crystal oscillator circuit "swamps" the LO and provides a steady oscillator signal. The crystals installed must take into account where the conversion frequency is in relation to the desired frequency as either 455 kc or 6.000 mc difference will be employed in the calculations. Higher frequencies will require operation on crystal harmonics. Additionally, the receiver must be tuned to the correct frequency for the RF stages and the Mixer to be also in tune. The crystal oscillator allowed for increased frequency stability required for RTTY and other data modes of reception. The six crystals mount in sockets located under a fiber board cover next to V5, the Crystal Oscillator tube.
A six-position selectivity switch provides three bandwidths that are
based on IF transformer characteristics and three bandwidths that are
determined with a Crystal Filter. Unlike many designs that place the
Crystal Filter as part of the First IF amplifier, the R-274 places the
Crystal Filter between the Second and Third IF amplifiers. An IF output is provided to drive RTTY converters that operate at 455
kc input. An Audio Input is provided to allow capacitive-coupled access
to the Audio Gain control and 1st AF amplifier grid input.
The power supply utilizes a pi-filter with 40uf filter capacitors. A
ballast is used in series with a dedicated 12vac power transformer
winding that applies a steady heater voltage to the LO and the Mixer to
improve receiver stability. There is a series load resistor in the
heater supply to the Detector and the First AF Amplifier to reduce hum
level. A pair of 120vac 7W lamps are connected in parallel as load
resistors for the 0C3 voltage regulator.
CY-699/FRR Cabinet - This is the cabinet that is
specified in TM11-897 as the housing for the R-274 receiver. The cabinet
is very light weight although it is made out of sheet steel. The cabinet
consists of two side panels, one front bottom piece, one back bottom
piece, a partial rear cover and a complete top cover. The top cover is
held in place using D-zus fasteners. The cabinet is assembled with 8-32
panhead screws with nuts and lock washers. Four rubber bumpers (feet)
are attached with screws, nuts and lock washers,...two bumpers to the
bottom of each side rail. The receiver is mounted to the cabinet
utilizing the eight, 12-24 front panel rack mounting screws. There is a heavy
paper diagram glued to the underside of the top cover that shows the
assembly particulars for the CY-699 cabinet. The paint is cream-gray
over a red oxide primer. The cabinet identification is stenciled on the
This CY-699/FRR cabinet was complete and in fairly good condition
although it did have a few minor problems. The partial rear cover was
reverse mounted. There were a few bends that needed straightening. All
of the pieces needed cleaning. All four bumpers were missing and several
screws, washers and nuts were also missing. I disassembled the cabinet
into individual pieces. Each piece was cleaned with Glass Plus and 0000
steel wool. Used with lots of Glass Plus and using a "light touch" the
0000 steel wool does a good job of cleaning heavy dirt and contaminates
without damaging the paint. I installed all new hardware for reassembly. A
search through the "used rubber foot box" turned up four matching rubber
bumpers (rubber feet.) Reassembly completed the rework and cleaning
process on this cabinet.
The photo to the right shows R-274 SN: 762 installed in the CY-699/FRR cabinet.
|K6GLH owned this beautiful R-274D receiver installed in the Hallicrafters' CY-1345/GR
cabinet. Note the close-up photograph of the data plate on top of the
cabinet indicating that the CY-1345/GR is definitely a
Hallicrafters' product and is specifically for the Signal Corps. Though
the TM mentions CY-699/FRR as a cabinet, these photos show that the
CY-1345/GR was also an option. Also shown in the rear view are the
various inputs and outputs available on the rear panel of the receiver.
The shield over the remote standby terminals is missing.
photos by Gary Halverson, K6GLH
Estimating the Build-date for a R-274 Receiver
- There are several components used in the R-274 and R-274D receivers
that have date code information that can help in determining when a
particular receiver was built. Since the contract date only indicates
when the order was created and not when a particular receiver was built, the
date codes will provide a general idea as to the when the receiver was
assembled. Hallicrafters would order most of the components specifically
for a production run that would be in planning. Most indications are
that most components were no more than a month or two old when used in
Hallicrafters' assembly line. Therefore, the latest date code that can
be found in a particular receiver would indicate that that receiver was
probably built about one to three months after that date.
MFP Dates - Most R-274 receivers will have a stencil or ink-stamp that indicates the date the receiver was treated with MFP (Moisture & Fungus Proofing.) There will usually be a Signal Corps inspector's stamp indicating the the treatment was inspected and accepted. These dates don't have much of a relationship to the actual build-date of the receiver. In some cases, the MFP treatment could be applied years after the build date. For example, the MFP stencil shown to the right is from the side panel of R-274 SN:762, which is a 1949 contract receiver with latest date-coded parts from March, 1951. Probable build-date is May-June 1951. The receiver was MFP treated about ten months later. Considering that the receiver had to leave the Hallicrafters plant, be shipped and then received at some Signal Corps facility where it would be inspected, tested and then (maybe) MFP-treated, ten months is certainly believable. So, while MFP dates are interesting they aren't reliable for build-date information.
R-274 Performance Today
|Since the Hammalund SP-600 and the
Hallicrafters R-274 were both built to satisfy Signal Corps
specifications, one should
expect similar performance from the respective receivers. What follows
are my observations of the R-274 operation based on actually using the
receiver for two-way communications on the ham bands.
The R-274 is a sensitive receiver and will respond to just about any signal on any of its tuning ranges. For the past few months, I've been using the R-274 receiver on a Vintage Military Radio Net that meets every Sunday morning on 75M. Mode of operation is AM. The R-274 will easily allow copy of all stations in the Net and can cope with any QRN with the ANL provided. As for the R-274's ability to cope with QRM, the crystal filter has been used on a couple of occasions but, normally, the SHARP non-crystal filter setting provides a narrow-enough bandwidth for QRM relief. The crystal filter operation is very good and works like one would expect, this is, narrows the bandwidth and provides some heterodyne relief. I've found the R-274 sensitivity and selectivity to be at the top of the performance expected from an early-1950s vintage receiver.
A common complaint about 1950s vintage receivers, with the obvious exception of those built by Collins, is vague dial accuracy due to the limited resolution of an analog readout that covers wide frequency ranges within each band. The R-274 is no exception to this complaint, even though the dial accuracy specification is 0.2%. That type of accuracy was about the best that could be achieved using variable capacitance tuning with wide frequency coverage in each tuning range. Where the R-274 and most other military receivers compensate for the vague dial readout is by providing a very accurate logging scale. The R-274's logging scale has tremendous resolution and therefore the resetability is very accurate. I use the logging scale for setting Net frequency and it always is quite accurate. Heterodyne frequency meters were always employed by military and commercial users when an accurate receive frequency setup was necessary.
Like all military receivers and most amateur communications receivers of the 1950s, the R-274 uses a standard envelope detector for both AM and CW. To copy SSB signals requires reduction of the RF Gain to a level where the BFO injection has the proper ratio of signal to injection. This is standard operating procedure for all receivers not using a product detector. Even though the receiver has a BFO Injection Level control on the rear panel, the maximum injection level is not high enough for SSB demodulation (it wasn't designed for that anyway.) You'll still have to "ride the RF Gain control."
Unlike most military receivers, the R-274 has pretty good audio response. The use of .01uf coupling capacitors in the audio section allows for some bass response provided a matching transformer is used with a good quality, fairly large speaker. The audio response is still basically "communications grade" but it does have notable bass response and sounds better than many other 1950s receivers. Of course, I'm using a floor-mount, bass-reflex speaker with matching transformer for the best bass response and over-all great sound (especially on strong SW-BC stations playing music.)
Remote standby is standard in its configuration that parallels the SEND-RECEIVE panel switch making the R-274 easy to set up in the ham station environment. NOTE: This easy setup is only true for the R-274/FRR. The later R-274D is similar to the Collins R-388 in that it requires a 12vdc input on the remote terminals to actuate an internal relay that grounds the antenna input while disconnecting the receiver antenna coil.
I've also listened to SW BC stations, usually on the 19M and 16M bands, since I mostly listen in the afternoons. Sensitivity is up with the best of the 1950s designs for SWLing. Audio on some SW BC stations is impressive on the R-274 - it isn't "high fidelity" but for a military receiver it sounds pretty good.
R-274 Problems Encountered
|Tuning Gear Box -
Sometimes, when tuning across the band at a fairly
fast rate of speed, the main tuning dial erratically "jitters."
is not really noticeable if the tuning is done slowly. This problem is
caused by wear in the concentric tuning shaft and main dial drive gear
along with the fact that the tuning dial is large and mounted at the end
of the shaft with a small hub.
When the gear box was new, this probably wasn't a problem but correcting
for the wear seems a difficult problem to solve nowadays. This
"jittering" is only noticeable if the tuning knob is "spun"
energetically - something that's really not necessary to do. The only
solution is to lube the gear box as shown in the manual as this will
usually reduce this effect somewhat.
The military TM shows that grease should be coated on the split-gears but, after years of a grease-coating, some of the grease may work its way between the split-gears and cause them to stick together. This shows up as a backlash problem. I had to remove the gear box and thoroughly clean it with a WD-40 flush and that cleared up the backlash. Only light oil should be used on the split-gears, 10W machine oil is fine.
Tuning Condensers - Bearings
- The military TM recommends that the tuning condenser
bearings be lubricated often with grease. These bearings seem to be
unable to retain any lubrication and may be found to be "dry" which
the tuning condenser to "stick" while tuning the receiver. This then
extends the bell-crank coupler springs until the tuning condenser is
pulled by the force of the springs to "snap" to a new position.
If the condenser doesn't "snap" to a new position then when the tuning
knob is released, the receiver will "reverse tune" actually turning the
dial backwards through the gear box. Frequent lubrication of the tuning
condenser bearings will alleviate the problem for awhile but as the TM
says, "lubricate frequently." You might find that the ball
bearings have rusted due to poor storage conditions. The bearing corrosion can be
removed but it does require removal of the tuning condensers from the
chassis. There is a procedure for alleviating this condition in the
"Restoration Comparisons" section further down this page.
Getting the most from the Dial Lamps - The R-274 dial illumination is not dazzling. It's difficult to tell visually if the receiver is turned on in a brightly lighted room. To obtain the most from the two dial lamps requires some adjustment that can only be accomplished with the front dial escutcheon removed. Only four screws mount the escutcheon, so its removal is easy. Each dial lamp socket is mounted with a single screw that can be accessed after the escutcheon is dismounted. Check the bulbs that are installed. Many times, #47s are installed because they draw less current and therefore run cooler. The manual specifies the lamps should be type LM27 6v-8v @ 0.25A which describes a #44 bulb. Since we aren't running the receiver 24 hours a day-7 days a week (like the military did) we really don't have to worry about the slight increase in heat from using the specified #44. #44 lamps have a tendency of darkening the inside of the bulb as they are used and age, so, first install a new pair of #44 lamps. The easiest method to replace the dial lamps is to dismount the socket, install the bulb and then remount the socket. With full visual access to the dial, you can now see the lamp filaments and how their alignment affects the illumination of the dial. Most of the time one or the other of the socket mounts will need a slight bend to have the filament of the bulb exactly align with the dial and provide the greatest illumination. Originally, the lamps had a directional shield over the bulb to eliminate stray light and help direct the light towards the dial. If your R-274 has these shields adjust them for the proper direction of the illumination. Re-install the escutcheon. With the dial lamps aligned with the dial and with #44 lamps installed, your R-274's dial will be about as brightly illuminated as it gets.
Hallicrafters versus Hammarlund,...Super Pros, that is,...
R-274 - Similarities to the SP-600
Hallicrafters' R-274 receiver and Hammarlund's SP-600 receiver were designed to meet the requirements of the Signal Corps. Consequently, there are some specific similarities that would be found in any receiver that had to meet certain pre-design specifications, both electronic and mechanical.
1. Both receivers employ a rotating turret band switching assembly
that places ANT, RF, MIXER and OSC modules into proper engagement
pins that connect the modules to the tuning condenser.
2. Both receivers have a selectable Crystal Oscillator that parallels the standard LO to provide increased stability for data modes of reception. The Hammarlund "X" option is an self-contained unit that mounts on stand-offs and is not used in all versions of the SP-600. Hallicrafters' Crystal Oscillator is chassis mounted and is found on all R-274/D/SX-73 receivers. In both receivers, the main tuning has to at set to the intended frequency to assure that the RF and Mixer stages are tuned for the selected crystal channel frequency.
3. The six-position Selectivity switch is similar in both receivers in that three crystal filter positions are provided along with three non-crystal filter positions. Hammarlund specifies the bandwidth in kilocycles while the Hallicrafters uses general terms like "BROAD," "MEDIUM," "SHARP," etc.
4. The 600 Z ohm, balanced audio output transformer is similar to that used in the SP-600. Impedances are slightly different but the overall operation is the same.
5. An IF output is provided on both receivers. Both receivers were intended to be able to drive IF input type devices for RTTY and other data modes of reception.
6. Both receivers have adjustable BFO injection on the rear panel.
7. During alignment, all adjustments are accessible from the top of the receiver.
8. Both receivers are the same overall physical size. If the ballast tube is counted as a "tube" in the R-274, then both receivers use 20 tubes.
Differences between the R-274 and the SP-600
There are certainly more differences between the receivers than similarities. Here are some of the more important differences.
|1. The R-274 Bandswitch Turret is made up of modules that are built
on heavy plastic chassis. The SP-600 uses ceramic chassis. The fixed
spring contacts are very delicate in the R-274 with the turret pins
just "press-contacting" the fixed spring contacts. The SP-600 fixed
contacts are spring split-halves that tightly contact the turret pin sides when
2. The tuning condenser is not shielded in the R-274 unless the top cover is installed. The SP-600 tuning unit is entirely shielded.
3. The R-274 first conversion oscillator frequency is 6.455mc (6.0mc conversion frequency.) The SP-600 uses 3.955mc for a first conversion oscillator frequency (3.5mc conversion frequency.)
4. The R-274 switchover to double conversion occurs at 7.0mc. The SP-600 is at 7.4mc. This is interesting in that Hallicrafters (or the Signal Corps) obviously deliberately chose 7.0mc to allow double conversion to start at the 40 meter ham band while Hammarlund (or the Signal Corps) deliberately chose to place the switchover at 7.4mc, or above the 40 meter ham band.
5. The tuning ranges on the R-274 are set up to allow Range 2 to cover the 160 meter ham band, Range 3 to cover the 80 meter ham band, Range 4 to cover the 40 meter ham band and Range 5 to cover 20, 15 and 10 meters. This set up is much more convenient for ham operation in the 160, 80 and 40 meter ham bands. The SP-600 has Range 3 covering 80 meters at the low end and 40 meters at the high end. This requires spanning the entire Range 3 tuning to move from 80 meters to 40 meters.
6. The gear box of the R-274 uses a very simple approach with anti-backlash gears and an oddball spring-coupled bell-crank condenser coupling. The SP-600 uses a spring-loaded rim drive system that is very smooth. Problems with the R-274 gear box are roughness in the feel and erratic tuning dial movement. Problems with the SP-600 are drive-wheel spring-load fatigue and dirt contamination resulting in dial drive slippage.
7. Nearly all coupling and decoupling capacitors used in the R-274 are ceramic disk capacitors. The early SP-600s used molded tubular capacitors that over time developed chronic leakage current that resulted in damage to associated resistors and other circuit problems. Later SP-600s were built with almost all ceramic disk capacitors - started about 1955.
8. The R-274 doesn't have a dual scale meter and circuitry to provide either RF input or AF output measurements. The SP-600 has this capability.
1. The R-274 uses only a Pi-filter for the B+ filtering. The SP-600 uses a dual section filter (two chokes and three filter capacitors.)
2. The R-274 uses a ballast tube to regulate the LO and Mixer filaments for improved stability. The SP-600 does not use any filament regulation.
3. The R-274 uses two 120vac 7W lamps in parallel as series load resistors for the 0C3 voltage regulator. The SP-600 uses a standard resistor series load for the 0A2 voltage regulator.
4. The Detector tube and the First AF Amplifier in the R-274 have a series load resistor in the heater supply to reduce hum - not used on the SP-600.
Views of the top of the chassis of the SP-600 JX-21 (left) and the R-274 (right) show that the receivers are similar only in performance and specifications.
The SP-600 has the power supply mounted on the right side of the chassis, the tuner and bandswitching mounted in the center and the IF-DET-AF on the left side of the chassis. Note that the R-274 only utilizes one filter choke. The SP-600 uses two chokes (one is under the "X" option chassis.)
The R-274 has the tuner-bandswitching mounted on the right side of the chassis, the power supply mounted on the far left-rear of the left side and the IF-DET-AF on the right side of the left side chassis. The tuning capacitor is only shielded when the top cover is installed.
The SP-600 photo (left) shows that under the chassis is wired with the various components mounted at the sockets and components terminals with no use of component boards. The turret is enclosed in the large shielded box and the first conversion crystal oscillator is in the smaller shielded box. Note that all of the original molded capacitors have been replaced (SP-600-25C)
The R-274 photo (right) shows that under the chassis is wired with a combination of some components mounted at the sockets and terminals while most of the components are actually mounted on component boards and connected to the various circuit points via harnesses. While the turret is within its own compartment, full shielding is accomplished only when the bottom cover is installed.
Decoupling Capacitors - The rebuilding efforts necessary on early
versions of the Hammarlund SP-600 are legend. Over 50 molded tubular,
capacitors must be replaced and that requires extensive disassembly of
the receiver. The RF Platform alone has 20 molded capacitors that must
be replaced and you can't access the RF Platform unless it is removed
from the receiver. Additionally, it's common to find several burned resistors due to the
excessive leakage current of the defective capacitors. No doubt, you
wouldn't be operating an all-original early SP-600 without first doing
extensive rework. (for details on the rework necessary on early
SP-600 receivers, go to our article "Rebuilding the Hammarlund SP-600.")
When the receivers were new, the major reliability problem with the molded capacitors used in the early Hammarlunds wasn't yet apparent. Therefore, our view from 50+ years later gives us the advantage of knowing that all early Hammarlund SP-600 receivers will be fraught with chronic problems because of the molded paper-dielectric capacitors used in their construction. Certainly an important factor to consider since the rework involved is so tedious. In 1950, use of ceramic disk capacitors was something that most manufacturers hadn't moved to yet, even though the leakage current problem of paper-dielectric capacitors was well-known. The fact that Hallicrafters (well,...the Signal Corps) chose to use ceramic disks in the R-274 is admirable. The fact that Hammarlund changed over to ceramics a few years later does verify that the reliability problem with the molded capacitors was rapidly becoming obvious to all manufacturers by the mid-to-late 1950s.
Probably one of the most important considerations in a comparison with the SP-600 is that the R-274 uses almost entirely ceramic disk capacitors which do not exhibit leakage current due to their construction. This eliminates the most time consuming and difficult part of the rebuilding process when electronically restoring the R-274.
Carrier Level Meters
- The Carrier Level meter on the R-274 is sometimes found with an open coil
and non-operational. This must have been a common problem as the Signal
Corps had special replacement meters available for the R-274. These
replacement meters have a black scale with white nomenclature while the
originals have the reverse. The SP-600 meter is often found with a
moisture stained scale since it is essentially a paper scale mounted on
a metal support scale. Later SP-600 meters made by Marion Electric were much higher quality with
standard painted scales that are always in excellent condition.
Early-style SP-600 meters often have
severely worn bearings that will cause erratic needle behavior. A
"sticking" needle is common but it is caused by worn bearings,
not the needle rubbing against the scale or the inside of the glass.
Spare SP-600 meters are fairly easy to find. A spare R-274 meter is
almost impossible to find.
IF Transformers - The IF transformers in the SP-600 are physically large and easy to access which is helpful since there is a tubular molded bypass capacitor located inside each can. The adjustments are easily accomplished using a standard small blade screwdriver. The R-274 uses what may be higher quality IF transformers and no bypass capacitor is located inside the can. The adjustments require a special tool to adjust the secondary of each transformer. The primary can be adjusted with a small blade screwdriver.
Front Panels - The front panel of the R-274 has silk-screened nomenclature which makes any repainting difficult. The nomenclature is very simple with the exception of the arrows for the dial lock operation. A silk-screen job probably wouldn't be too difficult or expensive, if a total repainting of the front panel was necessary. The front panel nomenclature on the SP-600 is engraved which makes the restoration of the panel relatively easy. Repaint the front panel with automotive-quality paint and then refill the engraving with light beige color Artist's Acrylic paint (pure white will look too bright.)
If "body work" is necessary, like straightening a bent front panel, the R-274 panel is fairly soft aluminum that is easy to straighten. The SP-600 aluminum panels are heat-treated and, if bent, are difficult to straighten.
Cabinets - The R-274 is designed so the receiver can be operated without a cabinet either as a table receiver or as a rack mount receiver. Though there were a couple of military cabinets available they are rarely encountered. The normal shielding for the chassis construction essentially becomes a useable cabinet. The SP-600 is rarely found with a cabinet either. The rack mount SP-600 originally had a bottom cover and a top dust cover but these are rarely found installed on any receivers nowadays. The rear pylons of each side plate of the SP-600 give it an odd look when not installed in a rack or a cabinet. Dust ingression is a potential problem for the SP-600 gear box and the chassis top. This would especially be a problem where the humidity is high. Both receivers are dimensionally deep and require special cabinets.
|Tuning Gear Boxes and Tuning Condenser Bearings - If you have to work on the gear box, the R-274 gear box is very easy to remove and work on. The SP-600 is a major job to remove but it is so robustly built, it is seldom necessary to do anything other than a general clean and lube. The R-274 gear box is prone to have wear in the concentric shafts that drive the main tuning dial and the logging scale. This wear ends up causing an erratic "jittering" movement of the main tuning dial. Cleaning and lubrication will help smooth the operation. The SP-600 dial drive often will exhibit some slippage. This is almost always due to spring fatigue on the "S" spring that applies the force for the drive wheel against the logging dial rim. The R-274 tuning condenser has bearings that require constant lubrication, even the TM mentions this. If lubrication is not sufficient, the tuning condenser rotor will "stick" and then extend the spring couplers on the bell-crank which then forces the rotor to jump to a new position. Sometimes the tuning will actually "reverse tune" when the knob is released. Needless to say, this makes tuning in any signals problematic.||R-274 Tuning Condenser Bearing Fix - I encountered this sticking problem on the first R-274 I owned. This receiver is now owned by KØDWC who reports that after this "fix" the tuning condenser is not sticking and the tuning is reliable. Unless you have severe sticking, don't bother with this fix. You can probably cure minor sticking with multiple lubrications and working the grease/oil mix into the bearings. For severe sticking,...read on. The only way to fix the tuning condenser bearing problem is to remove both of the condensers from the receiver and modify the bearings for better lubrication. The problem comes from the larger condenser use of two sealed ball bearings to support the rotor and the smaller condenser use of a sealed ball bearing in the front and a thrust ball bearing in the rear. These bearings were manufactured using steel with no protection from corrosion and, consequently, if the receiver is in a humid environment for several decades, these bearings will rust. Apparently the rust is able to work its way inside the sealed bearing. Since the bearing seems to be incapable of remaining sealed, it leaks out its lubricant over time and then the rust sets in causing roughness in rotation and eventually grabbing and sticking. To fix this problem, first remove the two tuning condensers. This can be accomplished from the top of the chassis with only removing the top and rear panels. When the condensers are out of the receiver, drill two small 3/32" holes in the metal seal between all three of the ball bearings. This allows easy application of a spray tube (the little red pipette) from a can of WD-40. Flush out the bearing until the WD-40 runs out clean. Next, I use a fairly light-weight, modern grease and work this grease into each of the bearings through the two access holes. Stop working the grease into the bearing when you see it coming out around the shaft at the rear of the bearing. Now, pack some more grease to seal the holes and remount the bearing cover. This fix seems to have lasted for quite a long time and is about as close as you can come to installing new bearings, which by the way, is literally impossible without destroying the tuning condenser. Use the procedure shown in TM11-897 to remove and reinstall the tuning condensers.|
Pro & Con Comparisons - Which Super Pro is Better?
SP-600 - Pros
Domination of the Ham Station Landscape - There's no doubt that the SP-600 will fill up the desk top if it's installed in an original cabinet. The receiver is huge. The knobs are massive. The twin viewing ports that surround the tuning dial and logging scale are reminiscent of ship's "port holes." The receiver does become a focal point due to its impressive appearance.
Super-Smooth Tuning - The action of a brass wheel driving the brass rim of a large dial imparts a certain ultra-smooth feel to the tuning of the SP-600. This effect is magnified by the very large knobs used for tuning and band switching functions. Every user is impressed with the tuning "feel" of the SP-600.
Sensitivity - No doubt the SP-600 is a very sensitive receiver. It will respond to everything that can be received with band conditions being the only limitation.
Ease of Alignment - The entire alignment procedure can be accomplished with the receiver setting on the bench. No alignment adjustments are under the chassis. One caveat though is the engagement of the alignment tool through the holes in the RF Platform. This operation requires actually looking into the holes with a flashlight to assure the proper engagement of the alignment tool into the adjustment slot.
Front Panel Nomenclature - The nomenclature is engraved making any repaint of the front panel easy to accomplish.
High Production Levels - This means that since so many SP-600s were built over the years, replacement parts (or even entire "parts sets") are very easy to find which will shorten the total restoration time.
SP-600 - Cons
Molded Capacitors - Every early SP-600 will require replacement of all of the molded capacitors before they can be operated safely. Burned resistors are common due to excessive leakage current. Later SP-600 receivers have ceramic disk capacitors installed that eliminated this problem. Molded caps were used from 1950 up to about 1955.
Communications Grade Audio - The SP-600 audio response is rolled off 3db at 125 hz, resulting in a very obvious lack of bass response. This was to assure that weak signal copy was possible and that CW and RTTY were not affected by an over-emphasis of bass response. The SP-600's stock audio really is communications grade.
600 ohm Z Audio Output - A 600 ohm Z to 8 ohm Z matching transformer is generally required for matching the stock audio output to a standard 8 ohm Z speaker.
No Antenna Trimmer Control - The alignment of the Antenna Input stage usually will set the input impedance to around 100 ohms. There is no other compensation in the receiver for various antenna loads thus requiring the use of a "tuned" antenna.
Dubious Crystal Filter Action - The SP-600 Crystal Filter is very subtle in its operation. Many operators find that it doesn't do much at all. It can narrow the bandwidth but the Phasing is not very effective.
Double Conversion & Tuning Ranges - The changeover to double conversion is set at 7.4mc, just above the 40M ham band. Tuning Range 3 has 80M at the low end and 40M at the high end requiring spanning the entire band to switch from 80M to 40M.
Dial Accuracy - All 1950s vintage receivers (not built by Collins) have vague dial accuracy. The accuracy is in the logging scale on the SP-600.
SP-600 JX-17 - This popular diversity version of the SP-600 does not have the Remote Standby option. All other versions have a standard Remote Standby function.
Cabinet - The stock Hammarlund cabinet is usually not found with the receivers as most receivers were rack mounted. Most rack mounted versions will be missing their dust cover and most of the time the bottom cover is also missing. The Hammarlund cabinet is relatively easy to find but it can sometimes be fairly expensive.
R-274, R-274D - Pros
Unique Appearance - The R-274 always is an "attention getter" because of its very Spartan-like military appearance.
Sensitivity - The R-274 is very sensitive and will respond to any signals on the band. Limitations would be due to band conditions.
Audio Response - The R-274 uses .01uf coupling capacitors in the audio section that allows a better lower-end to the audio response. Some might consider it communications grade audio but communications grade audio never sounded so good.
Double Conversion & Tuning Ranges - The changeover to double conversion is set to 7.0mc allowing double conversion to be used on 40M. The tuning ranges allow 160M, 80M and 40M to be covered on separate bands.
Crystal Filter - The R-274 Crystal Filter is located between the second and third IF amplifier. It functions quite nicely with good Phasing action that narrows bandwidth dramatically and can reduce heterodynes.
Antenna Trimmer - The R-274 has an Antenna Trimmer control to allow adjustment of the antenna input circuit impedance to compensate for various antenna loads.
Ceramic Disk Capacitors - The majority of coupling and decoupling capacitors used are ceramic disk capacitors which eliminates leakage current problems and eases the rebuilding process.
Ease of Alignment - All alignment adjustments are on top of the chassis. The turret adjustments are easy to see (no flashlight required.) The exception to "ease of alignment" might be that the IF transformers requires a sort-of-special tool for adjusting the secondary. A broad but thin blade screw driver will work (either make the tool or modify an existing tool.)
Remote Standby - This function is easy to access and use on the early version, the R-274/FRR. 12vdc required for remote standby in the later version R-274D.
R-274, R-274D - Cons
First Conversion Frequency IF Leakage - 6.000mc is used for the dual conversion frequency. It's possible that an extremely strong SW BC station on 6.000mc could "leak" into the IF of the receiver causing heterodynes or even dominating the first IF on Bands 4, 5 or 6. Radio Havana is a particularly strong SW BC station that "beams its signal" into the Eastern USA. Radio Havana operates on 6.000mc (along with several other frequencies) and can present a formidable signal if conditions are favorable possibly resulting in signal leakage into the IF of the R-274. More information below including testing results.*
Gear Box and Tuning Condenser - The R-274 gear box is not very robust and is subject to a lot of wear in the concentric shafts that operate the tuning dial and the logging scale which causes "jittering" when "fast tuned." The tuning condenser itself requires a lot of attention to proper lubrication if smooth tuning is to result (even the TM mentions that the tuning condenser must be lubricated "often.")
Dial Illumination - The
dial illumination is borderline feeble. It's difficult to tell if the
receiver is on in a well-lighted room since the only indication of
"power on" is the illuminated dial. (see "Getting the Most
from the Dial Lamps" in the "R-274 Problems Encountered" section above.)
600 ohm Z Audio Output - A 600 ohm Z to 8 ohm Z matching transformer is generally required for matching the stock audio output to a standard 8 ohm Z speaker.
Front Panel Nomenclature - The nomenclature is silk-screened making any repainting of the front panel difficult.
Knobs - The Hallicrafters knobs are very easy to chip around the edges. The good news is that they are stock Hallicrafters knobs and easy to find replacements for.
Cabinets - The CY-699/FRR cabinet is next to impossible to find. The Hallicrafters CY-1345/GR is also a rare item. Luckily, unlike the SP-600 which looks pretty bad without a cabinet, the R-274 looks nice since it's a fully shielded box when the covers are installed.
Low Production Level - The R-274 or R-274D are not a common receivers. Therefore replacement parts can be difficult or impossible to find. Many parts are either standard components or stock Hallicrafters' parts but some items are in the "unobtainium" category. It's unlikely that any examples of either of these receivers are going to be scraped for parts. NOTE: Some parts and parts sets have turned up on eBay from time to time (May 2017.)
|* 6.000mc leakage into Dual Conversion IF - In order for this to happen the receiver must be operating on Band 4, 5 or 6 so the 6.0mc conversion is taking place. To test the level at which 6.0mc IF leakage occurs, I connected a RF signal generator directly to the antenna input of the R-274. I used a HP 606B generator that has the capability of measuring the RF level output. I connected the generator to the receiver using RG-58U with the entire output shielded from generator to receiver. I set the receiver to 23 mc as this is about midway in the double-conversion coverage (and not a harmonic of 6.0mc.) The BFO was turned on to aid in detection of the leakage signal. At 5000uv rms the leakage at 6.0mc is easily detectable and would be easily noticeable under certain conditions. At 2000uv rms the leakage is barely detectable and only if the signal was interrupted (keying the signal on and off.) At 1000uv rms no leakage was detectable. The minimum signal level where the leakage was detectable was 1750uv rms. This test is 100% of a 6mc signal applied to the antenna input and shows the level of 6.0mc that is detectable at 23mc with no other interference from other signals at 23mc. How this translates into a SW-BC station operating on 6.0mc "leaking" into the dual conversion IF when the receiver is tuned above 7.0mc would be dependent on where in the spectrum (from 7.0mc to 54mc) the receiver is tuned, whether a strong station is being received on the tuned frequency (or just an empty frequency,) how strong the interfering 6.0mc signal is and the receiving antenna system used. The larger the antenna system or, if the antenna is specifically tuned for 6.0mc, would result in the strongest signals at 6.0mc and the strongest resultant leakage into the IF. Also, 6.0mc propagation conditions and time of day would be a factor. Suffice it to say, 6.0mc leakage in the dual conversion IF is certainly possible but the conditions under which it would happen would be rare and would depend on your QTH, your antenna and your listening habits (if you only use the R-274 on 75M, you'll never have to worry.) If you happen to be in one of the locations where a 6.0mc signal is extremely strong and you are experiencing leakage into the dual conversion IF an easy solution is to install a fairly high-Q 6.0mc wave-trap in the antenna feed line. Any parallel LC that can resonate at 6.0mc will then present a high-impedance and reduce the level of the offending 6.0mc signal. If you wanted to receive 6.0mc, then you would have to "short" the wave-trap (or remove it from the feed line.) Another solution would be to use an antenna coupler that allows "tuning" the antenna to a specific resonance. This would somewhat reduce the level of the 6.0mc signal since the antenna would be "tuned" to a different frequency. A combination of the two might be required in severe cases.|
|Current Selling Prices - The comparisons above don't address the current selling prices of the respective receivers - certainly an important consideration. Just looking at the asking prices ignores the fact that most equipment sells for somewhat less than the "asking price." However, all things being equal, "asking price" versus "selling price" will generally affect either type of receiver when it comes to the actual transaction. It appears that if you are looking specifically for the R-274 or R-274D, you'll end up paying about two to five times what you would pay for a comparable condition SP-600. There will be exceptions to this, of course. But, be prepared to pay substantially money more for the R-274 or R-274D receiver and you'll probably have to really "dig deep" for the SX-73 version.|
- Hopefully I've provided enough information to help you make a decision
as to which company built the best version of the "Super Pro." Since Hallicrafters only built a couple of military contracts and the R-274D/SX73 production lasted from
late-1951 up to early-1954 while Hammarlund built over 40 different
versions of the SP-600 over a period lasting two decades, it seems that the
market obviously favored Hammarlund.
Since the actual performance of the SP-600 and the R-274 are so similar, maybe it will be the non-performance related issues that will ultimately be the deciding factor. These might include the tuning range set up that is advantageous in the R-274. Or the fabulous, velvet-smooth tuning of the SP-600. The restoration work required for each receiver should be heavily weighed in your decision. Ultimately, it might come down to the actual selling prices today. It's all subjective,... so, let's vote on it.
You can place your vote here - send me an e-mail that indicates which Super-Pro is the best, the Hammarlund SP-600 or the Hallicrafters R-274 aka: SX-73 and briefly why you chose your particular favorite. Please let me know in the e-mail if you own one or the other receiver or if you own (or have owned) examples of both. To vote, just "Click" on "Super Pro Vote" and send your choice:
Voting Comments - May 2017 - Wow! I can't believe that the Hallicrafters R-274 actually has more negative votes than positive votes! It appears that most of the positive comments on the SP-600 have to do with its ease of restoration while the positives for the R-274 are mostly performance and construction related. The only SP-600 negative comment is that it's ugly,...well,...okay. The negatives for the R-274 are lack of parts, three mechanical issues and two performance issues. Keep the votes coming and thanks to those who have voted so far.
X = positive vote O = negative vote NC = No Comment
Read voting results across from left to right, SP-600 then R-274 then name or ham call of voter
Come on,...we need more votes for the best Super Pro.
+10 & -7 votes SP-600,... +8 & -9 votes R-274,... SP-600 ahead by 2 positive and has 2 less negative votes - May 2017
- Well, now it's time for my opinion,...as can be seen in the voting
above, originally I voted for the Hammarlund SP-600 only because it's easy
to find parts for during a restoration. At that time, I voted against the Hallicrafters R-274 only because of the tuning condenser bearing
problem I had with the first R-274 I owned. I did come up with a fix for
the bearing problem but, eventually, I traded that R-274 off. My second
R-274 was acquired recently (April 2017.) I must have liked the first
receiver since I did buy another one. This second R-274 has good
bearings in the tuning condenser with no sticking or erratic tuning
behavior. It makes me wonder if I based a major complaint on experiences
with just one particular example. Hmmmm. Maybe I should vote again! (and
If you like a receiver with a military, Spartan-like appearance that also has great sensitivity and the ability to cope with almost any type of QRM. If you want a military receiver that produces "non-military" audio, that is, really "enjoyable-to-listen-to" audio. If you want a Super Pro that's different from the commonly-seen SP-600 Super Pro, then you might want to consider the Signal Corps/Hallicrafters R-274/FRR. Hallicrafters really did go "all out" on this receiver and it ISN'T your typical Hallicrafters in construction or design. In my opinion, the Signal Corps was the primary designer of the R-274/FRR, but this receiver certainly was Hallicrafters' best-effort at a "cost no object" build that unfortunately wasn't favored by its creator, the Signal Corps. The reasons are obvious to R-274 owners. The receiver was "old technology" that worked very well but was still not up to what was becoming available from Collins in the form of the R-388 or R-390. By the last R-274 contract, in 1952, the Signal Corps couldn't justify buying any more of their late-forties creation. Hallicrafters began offering them as the SX-73 but the receiver ended-up being too expensive for the commercial and ham market. The R-274 is a wonderful receiver that admirably reflects the design thoughts for a specific function (accurate RTTY reception) and the production capabilities that were available in the late-forties. Luckily, enough of the R-274 receivers were built and have survived that nowadays it isn't too difficult to find good-condition examples for sale from time to time.
1. Military manual, TM11-897 - Technical data, circuit description, schematic, parts list. Version May 1952.
2. Shortwave Receivers Past & Present by Fred Osterman - details on SP-600 contracts based on Les Locklear's data. A very complete reference book on a wide variety of different shortwave receivers. Especially good for more modern types and foreign-made examples.
3. Thanks to all of the R-274 owners and SP-600 owners that have supplied reports, information and votes. Much of this information has been incorporated into this updated and expanded version of this article - May 2017
|Henry Rogers - Western Historic Radio Museum © 2011 History re-written with additional information, new photos added, minor corrections, more restoration info, voting comments © May 2017|
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