Rebuilding the BC-348 Family of Receivers
Part 1 - BC-348 History, Missing Parts, Dynamotor Service and Retrofit for Q, N & J Versions
Part 2 - Dynamotor Retrofit for BC-348 Grid Cap Versions, Retrofit for BC-224
Part 3 - Expected Performance, Better AC Power Supplies, Painting Wrinkle Finish
Let's face it! Almost every, single BC-348 has fallen victim to the "cut and hack" mania that afflicted many hams back in the fifties and sixties. If you're lucky, the BC-348 you happen to find might only have an AC power supply installed. However, even this simple modification required a lot of changes to the BC-348 circuit and today, if you want to return the BC-348 back to its original configuration, you can't just drop in the DM-28 dynamotor and expect the receiver to function. This article will detail not only what is required to retrofit the DM-28 into a modified BC-348 but also details many other aspects to consider in the restoration of these great receivers. Part One of this article is specifically for the BC-348-Q. The "Q" version was the most popular of the BC-348 series with probably well over 40,000 receivers having been built by Wells Gardner & Co. Of note is that the "Q" version is virtually identical to the "N" and the "J" versions. These three versions use Single-Ended tubes and feature a circuit that is very easy to work on. Dynamotor servicing is covered in this section. Part Two of this article deals specifically with the BC-348-C version that uses the "Grid Cap" tubes. The BC-348-C is very similar to all of the "Grid Cap" tube versions. The earlier versions of the BC-348 receiver are much more difficult to work on. The BC-224 dynamotor retrofit is also covered in this section. Part Three of this article provides expected performance details and addresses the BC-348 owner that wants to keep his AC power supply operation but wants the best possible performance from his BC-348. Although the first two parts of this BC-348 article is about retrofitting a dynamotor for original DC operation, part three provides the detailed information and the schematics for a couple of well-designed AC power supplies can be an asset to the AC-operated BC-348 and its practical use in the mil-rad station.
A Brief History of the BC-348 Series Radio Receivers
Before the BC-348 came about there was an earlier version of this famous Aircraft Receiver. It was the BC-224 and it was designed by RCA around 1935. It was a 12 volt DC operated receiver that provided B+ via an onboard dynamotor. The first BC-224 receivers had the tuning dial at the far left of the front panel giving the receiver a dramatically different appearance. When the BC-224 was first produced most aircraft systems ran on 12-14vdc. However, by the late-thirties, many aircraft systems were beginning to use 24-28vdc which required RCA to develop another receiver to run on 28vdc which was designated as the BC-348. Simultaneous with the release of the BC-348, RCA redesigned the BC-224 to have the same external appearance as the new BC-348. All versions of these early receivers were built by RCA Manufacturing Co., Inc., a subsidiary of RCA that built most of their commercial and military contracts. Both versions, the BC-224 and the BC-348, were built up to 1942, when contacts for the BC-224 versions stop. All BC-224 receivers were built by RCA Manufacturing Co., Inc. with the exception of one contact. The BC-348 contacts continued through WWII with the greatest quantities built during the 1943-44 time period.
The BC-348 operates on 24-28vdc with the high voltage (~+220vdc) provided by an internal dynamotor. Over 200,000 BC-348 receivers were built during WWII by many different contractors building many different versions within that time period. The "Early" circuit used eight tubes with the heaters originally wired in series-parallel for 24vdc operation (two parallel strings of four 6 volt tube heaters in series would operate on 24vdc.) The "early" circuit provided two RF amplifiers, a Mixer, a Local Oscillator, an IF amplifier stage, a combination 2nd IF amp and BFO, a combination 3rd IF amp and Detector/AVC followed by a type 41 audio output stage (this was changed to a 6K6 in some later versions.) Early versions also will have a 991 neon lamp acting as a regulator on the local oscillator and will provide an antenna trim control. A selectable crystal filter was also included in the circuit. The dual dial lamps were adjustable for brightness and were wired in series through a potentiometer and fixed resistor. Frequency coverage was from 200-500kc (not on the B or C version) and 1.5-18mc. The audio output impedance was internally selectable at "low Z" which was around 300 Z ohms or "high Z" which was around 4000 Z ohms. Some BC-348s will have a decal on the front panel indicating if the "low Z" was optioned.During the middle of WWII, the BC-348-J, N and Q "Later Version" of the receiver was introduced. This new version had to be interchangeable with the earlier BC-348 so physically both versions appear almost identical. Inside, however, several changes were made to reduce the cost of building the BC-348 but still maintaining its high level of performance. The "later" version reduced production costs by simplifying the construction and component mounting which additionally made depot repairs of these "later" receivers much easier. The "later" circuit used two RF amplifiers, a converter stage, three IF amplifiers, a duplex diode/triode provided Detector, AVC and BFO functions and a 6K6 provided the audio output.
When the receiver was installed on its FT-154 shock mount and installed in the aircraft, an eight pin Jones plug mated with a receptacle and cable that exited from the rear of the mount containing the 28vdc input, the remote stand-by relay function and an audio output line. The BC-348 was generally interconnected with the transmitter to control boxes allowing the transmitter's control relay to provide antenna switching, receiver stand-by and providing side tone monitoring which allowed for full "break-in" keying.
Since there are so many variations, military radio collectors have generally divided the BC-348 into two groups, "early" types (B, C, E, H, K, L, O, P & R) referred to as the "Grid Cap Tubes" version and the "later" versions (J, N & Q) referred to as the "Single-Ended Tubes" version. Many different contractors built BC-348s but Wells Gardner & Co. probably built the greatest quantity of receivers and is the most commonly seen contractor-manufacturer. Estimated production of the entire BC-348 line exceeds 200,000 receivers making it one of the most commonly found WWII aircraft receivers today.
The BC-348 became available as surplus almost immediately after WWII ended. The typical price was around $100 for a receiver that was in its original crate and about $50 for a receiver that was in good-used condition. The dynamotor was still installed in all receivers offered at the time but many surplus dealers were offering to replace the dynamotor with an AC power supply for around $15 extra. The actual AC power supply conversion was fairly simple and the operation of the receiver usually wasn't compromised other than probably adding some hum to the audio output. All of the dealer power supplies were built with the cheapest parts available, used a simple pi-filter and were undoubtedly built and installed as quickly as possible by technicians possessing limited abilities. Since most surplus dealers added an AC power supply to the BC-348 (and every surplus conversion book advocated this modification along with many other "ham-type" mods,) nearly every ham-owner thought they were capable of converting the receiver into anything that was considered necessary for successful operation. The upshot is today it is almost impossible to find a BC-348 that hasn't been modified in some manner.
There were at least two different types of military conversion AC power supplies that show up occasionally. Hallicrafters produced an AC-operated power supply that could be added to the BC-348 by the military. It was designated as EP-298. Very few ever show up so the production quantity must have been very small (uninstalled examples turn up occasionally.) The only installed example I've seen photos of was of a BC-348 used by the Navy. It's likely that the AC-operated BC-348 requirement was post-WWII and was probably for surveillance or some other ground-based application. The Hallicrafters' EP-298 would have required some conversion rework to the BC-348 and the installation would have been performed by depot higher echelon level technicians. The second type of military AC-operated BC-348 is the version used by the Air Force designated as the BC-348-S. The contractor was Belmont Radio who, apparently, rebuilt BC-348 DC-receivers into a very limited number of these AC-operated versions for ground use. The contract was post-WWII.
Many BC-348s owners actually do want to operate their receiver on the AC house line. Nowadays, an all-original, dynamotor equipped BC-348 should NEVER be converted to AC operation. Original receivers are very rare. However, at least 98% of the surviving BC-348 receivers have already been converted to AC,...they just need a little help to achieve the great performance the BC-348 is capable of. Almost always this must involve rebuilding the existing AC power into a unit that filters properly and provides the required negative bias voltage (that many surplus dealers and ham-sters forgot about.) A well-designed AC power supply must have the following,...dual section filtering (two filter chokes) for reducing hum to the imperceptible level. The power transformer CT and negative filter capacitor returns must connect to B- and B- should not be connected to chassis for proper filtering and proper bias voltage. The AC power supply must be turned "ON" by using the AVC-MVC switch (as original.) The proper SO-143 connection must be utilized for the AC input. If possible the PL-103 plug and FT-154 should be utilized. If these suggestions are incorporated in the AC power supply design and the receiver is given a thorough rebuild and a full IF/RF alignment that includes proper adjustment (or repair) of the Crystal Filter operation, the performance of the AC-operated BC-348 will be excellent (better than the original dynamotor operation) and the receiver can become an easy-to-use asset to the mil-rad station. Plans for dual section filtered, AC power supplies are in Part 3 of the article.
Today, with the increasing popularity of hams running completely military radio stations, the BC-348 is often being used as the station receiver. Sometimes this is because the original military configuration requires its use, other times because the operator knows that it's such a great performing receiver - a receiver that "does so much, so well, with so little." Fabulous military looks, excellent sensitivity and good selectivity (when the IF is properly aligned and you actually use the Crystal Filter) have made the BC-348 a favorite among the military radio enthusiasts, knowledgeable hams and collectors of WWII airborne radio equipment.
Finding a Good Candidate for Restoration
|What to Look For
- After you've looked at a lot of
BC-348 receivers, it becomes pretty easy to eliminate the "victims" from
the "survivors." "Victims" will have extra controls, perhaps a "hacked
in" S-meter, generally sloppy workmanship and sometimes lots of extra
holes drilled in the cabinet. Of course, I'm exaggerating a little bit,
but the ideal "survivor" BC-348 will have all the original controls and
no extra ones. Everything will appear correct on the exterior. Pull the
receiver out of the case and check the chassis for obviously missing
components - it is common to find the combination audio output
transformer-filter choke missing. It's also possible that an extra stage
of audio amplification might have been added. What you want to see is
all of the original components present with the exception of an AC power
supply that just fits into the area where the dynamotor originally was
and is mounted to the chassis using the original dynamotor threaded
bosses. This is about as close as you can normally come to finding a
"perfect" BC-348 - at a reasonable price, anyway.
The photo to the right shows a typical "looks good" candidate. No extra controls, no extra holes. Everything looks pretty good on the exterior with the possible exception of the RCA phone jack for the Antenna input. The photos below show what surprises are lurking inside this particular BC-348.
photo right: BC-348-Q sn 20966 looks like a good restoration candidate.
|Once the receiver is pulled out of the case it becomes apparent that
BC-348-Q sn 20966 was modified by someone lacking the technical ability
and experience to do a competent job. Note that the top of the chassis
looks complete and original except for the AC power supply that was
built onto the old dynamotor chassis but underneath extensive
modifications have removed many of the original parts. Of note, the
complete Jones-type plug assembly is missing along with C-70A/B*, the
dual filter capacitor. Fortunately, the modifier utilized the existing
wiring harness and terminal connections to the AC PS. This particular
BC-348 is certainly a very good candidate for complete
restoration.** Actually, it's in much better condition than most BC-348
receivers you find. But, as with most examples that turn up today, I'll
be collecting parts for this restoration for awhile. In fact, the
plastic bag tied to the front handle contains some of the replacement
parts I've already located.
When you find a BC-348 restoration candidate, you'll want to know "up front" how much work and how many parts you are going to have to find in order to complete the restoration since this is going to affect the final selling price. It is imperative that the prospective BC-348 chassis be inspected thoroughly. If the receiver is being purchased online, be sure to insist on complete photos of the top and bottom of the chassis so you are aware of any missing parts or other problems ahead of purchasing the receiver.
I'm just adding a letter prefix to the original numerical designation to
help in recognizing the type of component.
** Take a look at the "Expected Performance" section and "Rebuilding a Homebrew AC Power Supply for Good Filtering" to see how the restoration of SN 20966 turned out. This BC-348-Q has been restored to original condition with the exception of a well-filter AC Power Supply. When (or if) a DM-28 dynamotor becomes available, SN 20966 can now easily be converted to fully original DC operation.
|What to Look For in Your Preliminary Detailed Inspection
- What you should find upon closer inspection of the modified circuit is
that the original series-parallel wiring of the tube heaters has been
changed to all of the tube heaters connected in parallel. This allowed
the tube heater string to operate on 6.3vac rather than the original
+24vdc. This was the most common AC power supply configuration. You will
find that three resistors have probably been removed R-82, R-84 and
R-85. These resistors limit the surge current on start-up when operating
the (cold) heater string on +24 to +28vdc. You should also find that the
two dial lamps will have been wired in parallel for 6.3vac operation.
Additionally, with the dial lamps there is a series resistor (R-81) and
a potentiometer control (R-111) - "DIAL LIGHTS" - that sometimes have
been removed and a switch substituted for potentiometer and resistor.
It's common to find C-70A/B, the dual filter capacitor, missing. It's a
large square metal box mounted under the chassis.
How little damage is found in how the AC power supply has been fitted into the receiver will depend upon the technical abilities and experience of the original modifier. Most of the time the wiring harness isn't too badly damaged and it will only require a little dressing up, re-routing back to original and re-installation of the spade lugs in order to have the proper connections to the dynamotor terminals. Sometimes you may have to re-install some wiring in order to have the harness wires at the proper place for connection to the dynamotor terminals.
photo right: The underneath of SN 11227 before the DM-28 retrofit. This BC-348-Q AC PS mod had done minimal damage.
The normally found AC power supply modification has some way provided for the AC power cord to exit the receiver. This usually involves removal of the 8 pin Jones-type male plug SO-143 that was how the 24vdc was originally routed into the receiver along with remote stand-by and audio outputs. If SO-143 is missing an original will have to be found. Additionally, the remote stand-by and audio output functions will normally be disconnected. You will usually find the remote stand-by wired to be permanently in "receive."
Originally, the BC-348 was powered up with the AVC-OFF-MVC switch. Normally this is also used by modifiers as the AC power switch. Hopefully, a non-original ON-OFF toggle switch wasn't installed into the front panel. Why this was done on so many BC-348 receivers is a mystery. I think many modifiers thought the rotary switch wouldn't handle switching the AC. I've only encountered one BC-348 that had the rotary switch damaged and that was probably because of a wiring problem in the modification. Normally, the rotary switch handles the AC current fine. When everything is returned to original, the 24vdc will be applied by using the AVC-OFF-MVC switch.
Fortunately, most modifiers seem to mount the AC power supply chassis to the original dynamotor threaded bosses. This does no damage to the chassis. Occasionally, an ambitious modifier will "cut and hack" a power supply into the chassis - both top and bottom - but this is rare.
This should be all of the modifications that will be found in a BC-348 J, N or Q series that has only the AC power supply installation.
Locating Missing Parts
|Parts Sets - Since the BC-348 is a very common
receiver with thousands available it becomes practical and even
desirable to purchase at least one "parts set." Sometimes one receiver
has to be purchased for great external cosmetics and another because the
chassis is perfect. It is very common to have to use at least two and
often three different BC-348 receivers to build-up one complete,
operational and original receiver. It is often much faster to purchase a
"parts set" rather than to spend an eternity looking for a specific part
that never turns up separated from a receiver. Unfortunately, even the
BC-348 receivers that are in a condition that relegates them to "parts
set" status, are nowadays fairly expensive. However, the time that is
saved must be part of the equation in figuring how much to spend on a
Advertising - Sometimes this works but usually it is time consuming with very little success. I've had moderate success with QTH.swap but other advertising avenues seem to not be worth the time involved.
E-Bay - E-Bay works quite well for locating parts. Be prepared to pay absolutely the highest price for any part that happens to turn up for auction. In comparison to time involved in finding parts versus paying e-Bay prices, it's probably worth the higher prices since the items can be located and purchased fairly fast. Who wants their restoration to drag on and on?
Commonly Missing Parts
|The following resistors will more than likely be missing:
Number 82 - Resistor - 190 ohms 2W CC - parallel with RF Amplifier
tubes heater string and the Converter tube heater
|The following parts are often missing from modified BC-348 receivers...|
Jones-type male plug, 8 pin, SO-143 - This assembly is made up of several parts that include the contact pins, the bakelite insulators, the two-piece aluminum housing, a bakelite block centering screw and four mounting screws and lock washers. Some parts lists identify this connector as SO-103. The original connections to the plug were +24vdc, -24vdc, AF output (worked against chassis) and Remote Standby. The -24vdc was connected directly to chassis adjacent to SO-143. In the original configuration, the FT-154 shock mount was wired into the aircraft system via P-103 which was mounted to FT-154 and mated with SO-143. When the BC-348 was installed onto the shock mount, then SO-143 engaged P-103 and the receiver was then connected into the aircraft power and control system.
- Pins 1, 5 = Audio Output, parallels front panel phone jacks,
pin 5 is connected to chassis ground
IMPORTANT NOTE: When doing any work on the SO-143 connector where disassembly is required, be sure to carefully examine how it is assembled. You must make notes for correct reassembly! This connector can be assembled several different ways, with different pin lengths, different heights and none of them will fit into the PL-103 receptacle on the shock mount. Only one way of assembly is correct. So, be sure to carefully observe how the unit comes apart, so it goes back together the right way. Be sure to test your BC-348 mounted SO-143 connector for proper fit into the FT-154 mounted PL-103 receptacle before initial disassembly.
Dual Filter Capacitor, Part Number 70 (C-70A/B) - This is a dual filter capacitor with a 6uf capacitor on the B+ line and a 1uf capacitor as a cathode bypass for the 1st AF amplifier tube. This is a large square metal can that is mounted under the chassis and somewhat under the dynamotor bottom cover. Probably because of the proximity to the modification AC power supply, this capacitor is many times removed. Modifiers probably felt that the filtering in their AC power supply was sufficient and just added a replacement capacitor for the cathode bypass unit. It is original to have this unit installed, so if it's missing, an original will have to be found.
Be sure to test the replacement C-70A/B. These were high quality units that rarely, if ever, are bad. Remember, these are oil-filled paper dielectric capacitors,...not electrolytic capacitors. If you are uncomfortable using 75 year old components, then you could rebuild C-70A/B by installing new capacitors inside the can. However, if you replace these oil-filled paper dielectric capacitors with modern electrolytic capacitors you're changing extremely reliable types of capacitors with ones that have a finite life and were probably made in Asia for a few pennies each. I usually just test and install an original unit. Oil-filled paper capacitors usually work fine unless oil is leaking out. Then that C70 should be replaced with a good unit.
Most of the capacitors installed in the BC-348 are high quality metal cased units that rarely fail. I usually only replace defective ones (rare) and then I try to utilize tested-good original components from a parts set. If you feel like all of the capacitors need to be replaced with new components, then you can replace with SBE orange drops if the under chassis appearance is not a concern or the originals can be rebuilt (restuffed) although this is a difficult process.
Audio Output Transformer & Filter Choke, Part Number 115A/B (T-115A/B) - This is the combination audio output transformer and filter choke. It is unique to the BC-348 series. If it is missing an original will have to be found. The audio output transformer on the J, N and Q versions will have two terminals to select either 300 ohms Z (low Z) or 4000 ohms Z (high Z) for the audio output impedance. The two top terminals shown in the photo to the right are marked LO (on left) and HI. Moving the wire from one terminal to the other accomplishes the change in output Z. Some receivers will have a decal on the front panel the shows that "LOW Z" has been wired for the output Z. What Z you select depends on what you intend to operate as your audio reproducer. The LS-3 is popular but its original Z is 8000 ohms. The LS-3 was never originally used with the BC-348. Hi-Z headsets will require the 4000 ohms Hi-Z and the 300 ohms Lo-Z works fine into a 600 ohm Z load - lo-Z 'phones or 600 ohm matching transformer.
Why some modifications removed T-115A/B and then added an extra stage of audio amplification is probably due to the belief that insufficient audio was present for loud speaker operation. This is a myth. The J, N and Q versions produce ample volume to drive a properly matched loud speaker. You can run an original 8K ohm Z LS-3 with the Hi-Z tap selected and the volume will be very loud. If you have modified a LS-3 to have a 600 ohm Z input, it will have plenty of volume using the Lo-Z tap. One has to remember, the original intent was to use earphones, but the LS-3 speaker does work fine with the BC-348 using the original T-115A/B. It's possible that some of the old modifications sought to match the receiver to 8 or 4 ohm Z loud speakers and that is why they went on to add an extra AF output stage with a standard AF output transformer. Totally unnecessary, of course, but that was the "modification craze" - do whatever you can to "undo" what the real radio engineers had designed.
|The Elusive FT-154 Shock Mount - The
FT-154 shock mount is almost never found with a BC-348 receiver. The
shock mounts were probably left on the aircraft when the receivers were
pulled which accounts for why the FT-154 is almost always found
separated from the receiver. Although you can certainly operate the
BC-348 without the shock mount, the FT-154 does provide a proper way to
support the receiver - on cushioned mounts - and safe way to apply power
to the receiver - via the rear connector (if you also have PL-103.)
Also, the FT-154 does somewhat change the total overall appearance of
the BC-348 in a positive way.
I have encountered two different BC-348 receiver cases that have had the bottom plate that has the pointed engagement pins removed. This was done by drilling out the pop-rivets and removing the entire bottom plate. This leaves the case with out anything between its bottom and the table it's setting on. Both cases had glued-on rubber feet on the bottom of the case. Of course, this totally ruins the receiver case for future mounting on the correct FT-154. Fortunately, a BC-348 case is not too difficult to find. Don't worry that the inside paper tag might not match your version receiver. Most receivers were indiscriminately mounted in any case handy while in the repair depot so today most BC-348 receivers are not in matching cases.
The current prices for the shock mount reflects its rarity. An FT-154 might sell for $150 to $200+ but sometimes you'll find them reasonably priced (even on eBay.)
|PL-103 - This receptacle is mounted to
back of the FT-154 Shock Mount and is what the SO-143 plugs into when
the receiver is mounted to FT-154. There are two versions that were
made, the PL-P-103, which is straight back exit type and the PL-Q-103,
which is a right-angle exit. The +/-28vdc input, the audio output and
the remote standby line are all connected to the BC-348 via PL-103. The
photo to the right shows a close up of the PL-P-103. As can be seen, this
connector assembly has several parts to it. You can also reference the
photo of the FT-154 shock mount to see where and how the PL-103 mounts.
Be aware that the original mounting screws were "shouldered" and center
PL-103 for proper alignment with the receiver SO-143 plug. Be careful
using lock washers with these screws since some washers will not allow
the screw "shoulder" to center the mounting.
The cables exit out the back of the connector and then are routed down
through a metal tube that either points directly to the rear or points
to the side. The wrinkle finish metal cover goes over the entire
connector and only the metal tube can be seen exiting from under the
There are some vertical exit plugs for the BC-348, the PL-P-103 (same identification as the straight back exit which is confusing,) and these plugs do work fine. It's tight clearance from the bottom of the outlet tube to the table surface but you'll have about a half an inch which is plenty of room for the cable. Originally, these plugs were used when the BC-348 receiver was going to be mounted in a manner other than directly on a table top or the table had a hole for routing the cables underneath. At present these are the only easily available PL-103 and they show up regularly on eBay or can sometimes be purchased from Fair Radio Sales.
If You Can't Find a PL-103 - The correct PL-103 receptacle is very difficult to find and usually very expensive. While you are searching for this part, you'll probably want to run your BC-348. Buy a packet of "Push-On Connectors - Female .187" x .032" - these crimp onto the wire ends and then will push onto the flat finger contacts of the SO-143 plug on the BC-348. They are a fairly "tight fit" and won't pull off easily. You can make up a harness that has 14 ga. wires for the 28vdc input (LV+ and LV-) and 20 ga. wires for the Standby Relay function and the Audio Output (if you want to use this output rather than the phone jacks on the front of the receiver.) This set-up will look totally original unless you look at the rear of the receiver and the Push-On Connectors work great. When a PL-103 receptacle is found, you can use the same harness by removing the Push-On Connectors and then soldering the harness wires to the proper pins of the PL-103.
DM-28 Dynamotor Details
|Finding the DM-28 and What to Look For -
The correct dynamotor for the BC-348 is the DM-28 which operates on
24vdc to 28vdc input, originally supplied by the aircraft
battery-charger system. The dynamotor provides an average of
220vdc output. It takes about 1.8A under full load to run the dynamotor.
But the initial start current, that is, the current required to actually
get the armature moving, is closer to 8A or 9A. This current is only
required for an instant and as soon as the armature begins to move the
current drops rapidly. Additionally, the tube heaters are run from the
28vdc buss and they add to the total current necessary to power-up the
receiver. The DM-28 assembly consists of the dynamotor, the
filter assembly, the terminal strip and the chassis. You'll need to find
the entire DM-28 assembly, as shown in the photo to the right.
DM-28 prices vary from source to source. E-Bay prices were normally
around $125 for the complete assembly, although lately (10-2010) the
prices have risen to over $200 for the complete unit. Swap meet prices
might be less, if you are lucky enough to find a DM-28 at a swap meet.
Sometimes just the dynamotor turns up but, without all of the other
parts, it will only be useful as a spare unit. Most restorers feel that
the time saved in an eBay purchase makes up for the higher prices - at
least you will have the DM-28 and can proceed with the project.
Servicing the DM-24 (BC-224 Dynamotor) or DM-28 (BC-348 Dynamotor)
The earlier version receiver, the BC-224, uses a DM-24 dynamotor that is almost identical to the DM-28. I recently (2015) had to service a DM-24 and remembered to take photos along the way. The following section has photos and other details that can be applied to either the DM-24 or the DM-28 dynamotors.
|Servicing the DM-24 or DM-28 - More than likely, your newly acquired DM-24 or 28 will not have had anyone checking the lubrication of the bearings in the last several decades, if ever. You are going to have to pull the end-bells of the dynamotor to check the bearings. The end-bells are mounted with two screws that are usually "safety-wired" to prevent them from loosening over time. You'll have to remove the safety wire and then remove the screws then the end-bells should easily come off. Once the end-bells are off, you can inspect the commutators. There normally will be darkening of copper where the brushes were riding on the commutator but there should be very little wear. Remove the brushes but be very careful to observe and mark their orientation because you have to re-install them exactly the same way. I usually use a small paint "dot" to indicate location and orientation. Next, the bearing/brush end caps need to be removed in order to remove the armature. First remove the two ball bearing covers to access the grease-side of the ball bearings. There is a thin gasket that will very likely be damaged upon removal. Be careful and probably only the screw holes will suffer and the gasket can still be reused. Remove the three long screws and nuts that hold the end caps to the main housing. The end caps are usually a tight fit into the main housing but don't try to "wedge" them off using a screw driver or you'll mar the metal. Instead use a small drift to tap the end cap from the opposite side through the clearance area between the field coils at the top of the main housing. Use a small hammer and tap lightly until you see the end cap start to show a gap in the fit to the main housing. Then you can lightly pry the end cap loose and not damage the metal. If the ball bearings are stuck in the end cap just use a socket wrench (about 9/16" size) as a drift to tap the ball bearing loose from the end cap. Be sure the socket size works against the ball bearing outer housing. Next, remove the screws that mount the field coil winding wires to the brush barrels. Now you should be able to remove the end caps.||
|At this point the armature can be removed from the main housing. The
ball bearings are a tight press fit and difficult to remove but it
shouldn't be necessary unless there is severe damage to the commutator
or the ball bearings. At this point we're going to clean the commutators
using 600 grit aluminum oxide paper. Wipe the bright copper segments
with denatured alcohol to clean and clean between the segments with a
stiff paint brush. Next, clean the ball bearings. You'll probably
see old, brownish-yellow grease that is not really in the ball bearings
anymore. Clean the ball bearings with WD-40 and a small stiff paint
brush to remove the dried grease. Then use denatured alcohol as a solvent to remove the WD-40.
Blow out with compressed air, if you have it available, otherwise just
let the bearings air dry (it only takes a few minutes.) Repack the ball
bearings with new wheel bearing grease. The newer wheel bearing greases
are usually red in color and good for very high temperatures. Rotate the
armature as you work in the grease. Use your thumb to force the grease
into the bearing. When the bearing is full, it should look like the
photo below left.
You can now reassembly the dynamotor. The commutator with the smaller more numerous segments is the HV side. The end caps are marked with embossed lettering to indicate whether it is the HV side or the LV side. Also, the field coil wires are usually color-coded so be sure you have the armature installed correctly. Be sure you've cleaned the end caps around the brush barrels and where the ball bearings mount. Cleaning the bearing area will assure that the end cap reassembly just slides into place. Once both end caps are in place, install the long screws and nuts and evenly tighten them. This will bring the end caps into alignment and proper mounting. There should be a spacer in each of the ball bearing to end cover area. Be sure to check that they are the proper ones for the particular side. You might have to slightly "tap" the ball bearing (using the socket) to get the alignment correct for the proper spacer. This places the commutators in the proper alignment with the brush barrels.
Now you can re-install the brushes. Apply 27vdc to LV+ and LV- terminals (or 13.8vdc for the DM-24) and the dynamotor should start running very smoothly and very quietly. Remove the voltage and replace the end-caps. Check the condition of the rubber mounts - they should be elastic and not all dried out and hard. If you find hardened motor mounts they will have to be replaced otherwise the dynamotor will run noisily.
The photo above right shows a DM-24 about halfway through a servicing. Note that the parts are on the workbench just above the dynamotor. The brushes and brush barrels are marked with colored dots to assure correct orientation and location of the brushes when reassembled.
The photo left shows the LV ball bearing repacked with modern wheel bearing grease.
The photo right shows the DM-24 reassembled and ready to have the end bells remounted.
Note that this particular DM-24-H is built by Wincharger Corporation. Wincharger was a popular "farm radio" battery charging system (6vdc battery) powered by a relatively small airplane propeller (~ 4 ft dia) that drove a generator. The Wincharger was mounted on a small tower that was mounted to the roof of rural farm houses.
Other Useful Information
VT Designation to Standard Tube Identification for
the BC-348 J, N and Q Versions
VT-116 = 6SJ7
VT-150 = 6SA7
VT-233 = 6SR7
VT-117 = 6SK7 VT-152 = 6K6GT
|Schematics - These are available from several sources. I'm using the schematic that is in "Surplus Schematic Handbook" by Kenneth B. Grayson W2HDM, which was part of the CQ Technical Series, published by Cowan Publishing Corporation in 1960. My references to various components are from this schematic except that I have added a letter prefix to identify what kind of component it is. So, R-84 is listed on the schematic and parts list as 84. Manuals and schematics are available free from many online sources. BAMA has available the correct manual for the BC-348 J, N and Q versions. The schematic available on BAMA for the J, N and Q is actually for the earlier versions and is not usable for the J, N and Q versions. I'm including this link to my scan of the correct schematic for the BC-348 J, N and Q from the "Surplus Schematic Handbook." (1.7M) BC348Q Schematic|
Retrofitting the Parts and Testing the BC-348-Q, N & J
|Before you Begin the Retrofit - If the modified BC-348 you've selected for the retrofit is in great condition and appears mostly complete, it's probably a good idea to get the receiver operational on its AC power supply. It probably did work at one time, especially if it looks like a nicely done job, so getting the receiver operational before the retrofit of the dynamotor will reduce the amount of work you have to do later.|
|How to Change the Tube Heaters Parallel Wiring
Back to Series Parallel - Look at the two drawings that are
shown below. One shows the typical Parallel hook-up that is found in
most AC modifications. The other shows the correct, original
Series-Parallel connections for the tube heater strings. Most of the
time, the conversion to Parallel only has a few jumps and a couple of
extra wires to remove. Once you know what to look for by looking at the
drawings below, it becomes fairly easy to remove the tube heater
modifications. You'll have to install R-84 and R-85 as shown in the
photo left. The terminal board (pin 3) is the +24vdc tube heaters
connection (sometimes pin 1 in some receivers - see note below with
drawing.) The resistor from pin 3 to pin 1 is R-85, 5 ohms 1W and
connects +24vdc to the tube heaters for the AF output, 1 & 2 RF amps and
the Converter tubes. Also connected to pin 3 is R-84, 7 ohms 1W that
also connects to pin 7 of the IF amplifier tube and then on through the
other IF amp tubes and the Det-AVC tube. There is a shield that mounts
over the IF amp tube socket so R-84 leads must be sleeved and mounted
close to the chassis with the one lead coming up vertically to the
terminal board. This provides good clearance for the shield.
Underneath the RF/AF chassis (remove the front panel access cover to access) you will have to remove the Parallel wiring modifications and return the wiring to Series-Parallel. You will also have to add R-82, 190 ohms 2W from pin 2 of the 1RF amp tube to ground. There is a ground lug on the tube socket mounting that was originally used to mount R-85. Note that the wire jump from the AF output tube socket to the Converter tube has to exit through a chassis eyelet and then connects to a terminal on the metal housing for the Converter tube. Usually no changes were done here since the return connection to chassis is from pin 7 of this tube. Reference the wiring diagram shown below for how the original series-parallel is accomplished. Also shown for reference is the typical modified parallel wiring diagram.
Once the tube heater wiring is returned to original, "ohm out" the connections to verify it is correct. Then install the tubes and apply +24vdc to the tube heater terminal and verify the correct voltage on each tube heater, approximately 6vdc. You will probably see about 5.8vdc or so if you use +24.0vdc but remember that a fully charged battery set-up will provide about 27.5vdc. The design allows for wide variation from a low of 24vdc up to about 28vdc.
Original +24vdc Series-Parallel Wiring - This wiring diagram shows how the tube heater wiring is accomplished for Series-Parallel operation on +24vdc. The wire from pin 7 of the 2nd RF amp tube socket passes through an eyelet in the chassis and is then connected to a terminal on the outside of the Converter tube housing. Sometimes the connections to the terminal strip (where the resistors are mounted) are inverted - that is - connections going to pin 1 go to pin 3 and vice versa.
Typical Modified Tube Heater Wiring for 6.3vac Operation - This wiring diagram will show what is usually found in BC-348 receivers that have been modified for operation on an AC power supply that provided 6.3vac for the tube heaters. This is how most of the AC modified BC-348 receivers seem to be wired but there are other variations that accomplish the same thing. As mentioned in the Series-Parallel description about the terminal strip - it is common to find pin 1 and pin 3 connections inverted from what is shown here.
|Wiring the Dial Lights Back to Series - Once the heaters are in the proper configuration, the next step is to change the dial lamps wiring back to series. Remove the dial cover to access the dial lamp sockets. There will be two wires installed between each lamp socket. Remove the non-original wire which should leave one original wire connecting the two sockets to each other. The left side lamp socket should also be connected to the wire that runs through the panel on the left side. Then install the wire that comes through the panel on the right side to the right side lamp socket. This wire should then connect to R-111, the "DIAL LIGHTS" potentiometer. >>>||>>> From the pot a non-original wire is usually connected to the chassis lug. Remove that wire and install R-81. R-81 is a 60 ohm 2W wire wound resistor but it isn't too critical, (I installed 100 ohms because that was all I could find and it works fine.) R-81 connects between R-111 and chassis. Install #47 lamps and test with +24vdc applied to the +LV input. The dial lamp circuit is connected directly to the switched +24vdc input and jumped to the tube heater wiring inside the dynamotor chassis. Usually, in an AC conversion, you'll find that the dial lamps are connected to the tube heaters in some way. For dynamotor operation you'll have to disconnect the lamps from there and connect to the +24vdc "on" position at the AVC-MVC switch.|
|Rebuilding the Wiring Harness Back to Original
Identify each of the wires from the harness that are going to have to
connect to the dynamotor terminals. There should be five wires.
is the B+ line (consists of two white with red tracer wires.)
is the B- and bias line.
Blank or "not ID'd" is the connection for the tube heaters
- it is internally jumped to LV+
inside the dynamotor chassis. Sometimes, since these two wires are
jumped inside the dynamotor, you might find these two wires reversed in
position in the harness. Check that the plain white wire goes to the
AVC-MVC switch and the wire that is white with a black tracer goes to
the tube sockets. When operating the BC-348 on a dynamotor, it doesn't
matter the order of these two wires but if you're utilizing an original
harness for operation with an AC power supply you'll have to verify
where these two wires are connected in the receiver circuitry.
LV+ is the switched +24vdc and dial
lamps line. LV- line is connected to chassis. Normally, modifiers
didn't change any of the wiring placement in the harness so, hopefully,
all that will be required is to install spade lugs on the wire sets. You
may have to add wire if the length of the original wire has been cut to
fit into the modified AC power supply. Every modified BC-348 is somewhat
different but try to keep to original wiring routing as much as
possible. If the either LV line needs to be lengthened be sure to use
16ga. wire on these lines. Once you have the order and length correct,
then install the spade lugs. These have to be soldered to the wire ends,
as original. Remove any plastic insulation on the spade lug ends and
then solder to the wire ends. To make the harness look original
install cable lace made of waxed string. Connect wire harness to the
dynamotor terminal board to complete the installation.
|Powering the BC-348 with 24vdc to 28vdc - The original power source for the BC-348 was the aircraft battery-charger buss that provided around 24vdc when the airplane was non-operational and on the ground. This was usually when all the equipment was pre-flight tested and the BC-375 transmitter set-up for the frequencies to be used by the radioman. Once the aircraft was in flight the battery-charger system provided +28vdc average to the radio equipment. If you are going to operate the BC-348 on batteries expect the voltage to vary from fully charged at about 27vdc down to 23.5vdc (when the batteries need to be charged.) If you are going to operate the receiver on a DC power supply, provide about +27.5vdc for good operation of the tube heaters and the dynamotor. The DC power supply will have to be capable of providing around 6A for the surge current to start the dynamotor. After the dynamotor is up to speed the dynamotor's current draw is usually less than 2A. Remember though, you'll have to add the tube heaters to the current draw since they also run on the +28vdc buss. >>>||>>> For initial testing connect +27.5vdc to pin 3 or 4 on SO-143 and connect the negative lead to pin 8 or 7. Be sure to use a test jump between pin 2 and 6 to take the receiver out of stand-by. With the +27.5vdc on, switch the AVC-OFF-MVC switch to AVC. The dynamotor will come on along with the dial lights. After about 30 or 40 seconds the receiver should come to life. Use earphones (of the proper Z for what you have T-115A/B wired for) and tune in some stations on SW BC. Check all of the controls and see that they function. Tune in stations on all bands to test. Switch to MVC and turn on the CW OSC. You can now tune in some SSB or CW stations on the ham bands. Note that in AVC, the VOLUME will be set rather low since the AVC is controlling the receiver sensitivity and VOLUME controls the audio gain. In MVC you'll have to advance the VOLUME higher since VOLUME now controls sensitivity and the audio gain is at maximum. This gives the proper ratio of RF to BFO injection for good CW or SSB demodulation.|
|IF Alignment - The BC-348 uses an Intermediate Frequency of 915kc. The passband selectivity is directly related to how the IF is aligned. There are other factors, such as the amount of coupling in the IF transformers and other factors of design, but an accurate alignment of the IF system will result in the best selectivity that the BC-348 is capable of. You will have to determine the exact crystal frequency of the Crystal Filter first. This can be determined by switching in the Crystal Filter and then sweeping an RF Signal Generator coupled to the converter grid. The crystal should be very close to 915kc. You'll hear a "peaking" of the background noise as you sweep through the crystal frequency. At the exact peak of the response is the exact crystal frequency. Whatever the crystal frequency is, that is where the RF Signal Generator should be set for the IF alignment.|
|To assure that the Crystal Filter will operate correctly and be a
useable asset to operation, you should perform the IF alignment with the
Crystal Filter ON. This will narrow the IF passband to about 1KC. The
end result is you'll have a Crystal Filter that works great when the alignment is
adjust all of the IF transformers, both top and bottom adjustments, for
the peak response to the RF Signal Generator input frequency. Monitor
the audio output using an AC voltmeter. You'll have to use 1000~
modulation on your RF Signal Generator for the meter to read and operate
the receiver in MVC for the best results.
You can also use an unmodulated signal and monitor the AVC line, with the receiver in AVC. If you have a digital frequency counter monitoring the RF Signal Generator frequency, check it often as you proceed with the IF adjustments. Go through all of the IF adjustments twice to assure accurate adjustment. This method of alignment will result in the best IF selectivity for the BC-348. Also, the Crystal Filter will operate correctly since the IF is adjusted to exactly the crystal frequency with the Crystal Filter in the circuit.
RF Alignment - The accuracy of the RF alignment affects how well the BC-348 tracks - that is, how accurate the dial readout is. Since the receiver was adjusted at the factory with more or less fixed components, there are no padder adjustments for the tracking except for the LF band. The tracking is adjusted with trimmer capacitors on each of the tuning ranges. These are adjusted at the top of each of the bands. How accurately you adjust the RF Signal Generator frequency input, the receiver's tuning dial and each of the trimmers, results in how accurate the final tracking is. Most BC-348 receivers adjust up to be very accurate - within the limits of an analog readout and a tuning range that spans several megacycles.
Western Historic Radio Museum
Vintage Radio Communication Equipment Rebuilding & Restoration Articles,
Vintage Radio History and WHRM Radio Photo Galleries
1909 - 1969
- 60 years of Radio Technology -
This website created and maintained by: Henry Rogers - Radio Boulevard, Western Historic Radio Museum © 1997/2023