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BC-348-H Liaison Receiver
Many older ham radio operators are familiar with the legendary BC-348 receiver.  These sets were available on the postwar surplus market and often heavily modified to make a really nice communications receiver.  The 348 was originally designed as the liaison receiver in larger multi-engined Army Air Corps aircraft in the 1930s.  These sets were paired up with the BC-375 liaison transmitter to make up the SCR-287 system.  Later in the war the archaic BC-375 was replaced by the Collins T-47 but the BC-348 remained as part of the ART-13 system.

There were many suffixes attached to the basic BC-348 model number.  Mostly this suffix designated the manufacturer and a specific contract but it also denoted some differences in circuitry.  Companies like RCA, Stromberg Carlson, Wells-Gardner, and Belmont cranked out these sets by the thousands during W.W.II.  Basically these sets used eight tubes and covered 200Kc-500Kc and 1.5Mc-18Mc in six bands.  The sets were designed to operate from the 28VDC aircraft power with a built-in dynamotor (DM-28) that provided 220VDC for the tube plates.  The 6.3V tube filaments and dial lamps were connected in a series and parallel combination to be powered from the 28VDC supply.

I have acquired two versions of this set: a BC-348-H manufactured by Belmont Radio Crop., and a BC-348-M by Stromberg Carlson.  I wanted to restore one set back to its original configuration to be powered from 24VDC using a dynamotor and installed in an FT-154 shock mount.  The H model I have had some of the usual ham modifications including the dynamotor being replaced with an AC power supply, the filaments being rewired for 6V, and the 8-pin pseudo Jones connector being removed.  There were a few other minor circuit changes made as well.  However the front panel had not been hacked up to install an S-meter, extra switches and controls, or replacing the antenna and ground terminals with an SO-239 coax connector.  The M also had the usual circuit changes but still retained the 8-pin SO-104 connector.  The dial light control had been replaced with a switch and the antenna and ground terminals were replaced with an SO-239.  I could probably replace the light control but the SO-239 installation would be difficult to restore.  I decided to go ahead and restore the H model using parts from the M to replace anything that was missing.

I found a complete manual at James Moorer's web site.
Cosmetic Restoration


Front of the BC-348-H before I
began restoration.
The rear of the cabinet.  Notice the
numerous ventilation holes that had
been drilled.
The rear of the chassis showing the
AC power supply built on the
original dynamotor mount.

A previous owner of the BC-348-H pasted a circular logging scale behind the main tuning knob and appeared to have slopped on some black paint or other coating over the entire front panel.  Trying to clean this coating off also took off the original black wrinkle finish down to bare metal.  I decided that I would have to strip and repaint the entire front panel.  The first thing I did was to remove all of the knobs so I could remove the panel.  There are nine screws holding the panel to the chassis that need to be removed.  The thumb screw rods that hold the chassis into the cabinet must be removed as well as the handles.  The dial bezel is held on with five screws from inside the panel.  The antenna and ground terminals as well as any other components attached to the panel were removed.


The front panel after removal
from the chassis.

I stripped the panel down to bare metal with your run-of-the-mill paint stripper.  To be sure that I had good paint adhesion I washed the entire panel down with acetone and then scrubbed it with hot soapy water.  I wore rubber gloves when handling the clean panel to keep it clean.  I found a web site with some hints on how to get good results when applying wrinkle paint.  I purchased some Plasti-Kote black wrinkle paint from a local auto parts store.  When it came time to paint I turned the kitchen oven up to 250oF and took the panel outside to paint.  I laid the panel on a piece of plastic and spaced it above the ground with a couple of wooden blocks.  The directions on the can of paint called for applying a coat then waiting ten minutes before recoating.  It recommended using three heavy coats to get good results.  When the last coat was completed I placed the panel in the oven, closed the door, and turned it off.  I allowed the paint to cure in the oven for a few hours.  The wrinkle turned out pretty well on the front of the panel but the sides did not turn out as well.  I attribute this to the heating elements in my oven being on the top and bottom and I had placed the panel horizontally to cure.  If I had to do another one I would probably line a box with aluminum foil to place the panel in.  Hopefully that would radiate heat to the sides.  The results can somewhat be seen in the photos below.


This is the panel after it was stripped,
cleaned, and masked.
The panel after being repainted.

The lettering on the panel is raised from the rest of the panel but not by much.  The thickness of two or three coats of paint is about even with the thickness of the lettering.  To expose the lettering I carefully scraped the paint away with the blade of a small screwdriver.  I also stripped and repainted all of the knobs except for the tuning knob with the same black wrinkle paint I used on the panel.


The panel after being repainted.
The panel after exposing the lettering
and installation onto the chassis.

A previous owner had drilled a bunch of ventilation holes in the cabinet.  These would have to be filled.  I stripped the original paint using the same stripper that had been used on the front panel.  I used a couple pieces of fiberglass cloth coated with epoxy resin to act as a backer on the inside of the cabinet where the holes were located.  Once the epoxy had cured, I could begin filling the holes with body putty.  With the body putty cured it was sanded flush with the surface.  The cabinet was then masked and primed with some galvanizing primer.  After that was done it was ready for wrinkle paint.
Electrical Restoration

As mentioned before, the goal of this restoration was to put this receiver back into its original military configuration.  Any electrical modifications had to be undone so it would function the way it was intended.  Most of these sets had the tube filaments rewired to run from 6V and this set was no different.  I re-rewired the filaments to run from 28V and removed jumpers across the dial light dropping resistors so that the lights would also run from 28V.  There had been a couple of resistors added around the 6B8 detector that I removed as well.

Capacitor & Resistor Replacement

Most of the capacitors in this radio are Micamold types.  These are paper capacitors that are formed to look like mica capacitors.  After 60+ years they fail just like the tubular paper caps.  In fact, I tested one of the capacitors from this set for leakage on my Eico 950B and it failed at 25V.  The cap was rated for 500V.

There are four resistor boards under the chassis that contain both resistors and capacitors.  There are a bunch of Micamolds on these boards in hard to reach positions.

Micamold capacitors mounted on either side of the
RF amplifier deck.
 
Underside of IF deck before restoration.

I replaced all of the Micamolds and checked all of the resistors.  Any resistors that were out of tolerance were replaced.  There is a 0.05uF capacitor inside the output transformer can that is used as the AVC timing capacitor when the BFO is turned on.  This capacitor turned out to be leaky.  Instead of opening the can to replace it I just snipped the lead where it comes into the chassis and installed a new 0.05uF capacitor on a terminal strip.  A screw on the chassis served to mount the strip and the replacement cap can be seen just above the SO-104 connector in the photo on the right below.


 Underside of chassis before restoration.
Underside of chassis after restoration.  You can see
the yellow replacement Mallory 150 series capacitors.

The seven boxes in the lower left corner of the chassis in the above photos each contain two 0.5uF capacitors.  I decided to gut and restuff these boxes.  The best way to open these boxes is with a propane torch to melt the sealing solder and then to pry the lid off with a screwdriver.  The guts will usually come out with the lid but if they don't they are not difficult to remove while the can is still hot.

RF Units

The BC-348 uses separate units that house the antenna, RF, 1st detector, and oscillator sections.  These are the four boxes shown in the photos below.  Steps on how to remove these from the chassis are detailed in the instruction manual.  These modules contain a few mica capacitors and some resistors.  I checked all of the resistors and replaced any that were out of tolerance.  I also sprayed the switch contacts with cleaner.  Be careful to align the switch back to the correct position to allow the shaft to be reinserted.


RF modules:
Oscillator, Detector,
RF, and Antenna.
Antenna unit.
RF unit.
Detector unit.
Oscillator unit.

IF Section

One thing that I noticed was a green wire coming from one of the IF transformer cans.  I knew it was not original so I opened up the can to remove it.  That's when I discovered another modification that I had never seen before.  A previous owner had wrapped two turns of wire around the IF transformer form and soldered the ends together.  This was in an attempt to decrease the coupling between the primary and secondary windings to narrow the bandwidth.  This was done to all of the IF transformers.  I opened each can up and removed the windings.  I also replaced the rubber grommets to protect the tube grid leads coming from some of these cans.

Dynamotor

I purchased a DM-28-Q dynamotor to use to power the receiver.  I began restoring the dynamotor by cleaning and lubing the bearings and armature and replacing the wiring.  To break it in I powered it up with my 24V power supply.  I connected a meter across the output leads to check the output.  With 24V in I was getting about 24V out.  I checked the current being pulled and found that it was more than twice what it should be when fully loaded.  It appeared that the armature was shorted.  What to do?

The DM-28 is obviously shot unless I can find a replacement armature.  I had noticed that the DM-36 dynamotor has almost identical characteristics to the DM-28: 28V input and 220V output at 80mA.  I purchased a DM-36 and decided to run the receiver from it for the time being.  I removed the dynamotor from its original base and fabricated a new base patterned after the original DM-28 base.  I used the terminal strip from the dynamotor base already in the receiver that had been used as a base for an AC power supply.  The whole thing was built on a scrap panel from a 2U 19" rack mount cabinet kit.


The DM-28-Q I had purchased
that turned out having a shorted
armature.
Dimensions and layout of a new
base plate for the DM-36 dynamotor.


DM-36 dynamotor chassis.


General

I removed the SO-104 connector from the BC-348-M to install in the H.  I hit all of the tube sockets and switch contacts with contact cleaner and lubed the tuning mechanism with a few drops of silicon oil.  The dynamotor was installed and the set was ready to have power applied.  Pins 2 and 6 need to be jumpered together for the set to work.  Using alligator clip leads I connected the power pins to my 24V power supply and jumpered pin 2 to pin 6.  I plugged in a pair of headphones and, with the mode switch in the AVC position, applied power.  The dynamotor began spinning, the regulator tube glowed, and after a few seconds I was able to get sound through the headphones.

Alignment

The set still needed to be aligned to be at peak performance.  I followed the alignment chart in the manual to align the IF at 915Kc and then align each band.  The chart is a little confusing when trying to identify the appropriate trimmers to adjust.  The chart gives the reference number of the trimmer.  The trimmers are located on the rear of each RF unit and generally are numbered 1 through 6.  Trimmers labeled 1 are adjusted for the 200-500Kc band, trimmers labeled 2 are adjusted for the 1.5-3.0Mc band, etc.  It is a good idea to turn power on and let the set sit and warm up for half an hour so that everything stabilizes before attempting to perform an alignment.

Done

After a few months I finally was able to finish the restoration of this radio.  I had to wait for warmer weather to finish painting the cabinet.  That warmer weather finally came in April 2005.  Below are some photos of the completed BC-348-H.  I still need to get a DM-28 fixed up to complete the set but the DM-36 is fine for now.






Tips

Here are a few tips when restoring one of these sets.

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This site created and maintained by Sean Barton.

Last updated 5/1/2005.