MORNING STAR OBSERVATORY
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    • ---------------
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BC-348 WWII Shortwave Aircraft Liaison Receiver

Picture
I picked up this WWII and Korean War era radio at a flea market. It's a shortwave receiver tuning 200 to 500Kc and 1.5 to 18Mc, doesn't have a built-in speaker, and runs on 28VDC, the battery charger voltage on American bombers. According to what I've read online, this was the liaison receiver in American heavy bombers such as the B-17, B-24 and B-29 (the B-29 Enola Gay had one of these on board). It was also used on large transport aircraft and by our Allies. Over 100,000 sets were produced with various updates and changes designated by letter codes and some were still in service as late as the early 1970's. This BC-348 is an "N" model.

Un-Hacking The Radio

Picture
A look at the topside of the chassis. Somebody in days past did a lot of poorly wired modifications to the set, including:
- removing the original dynamotor (replaced with an A.C. power supply)
- removing the original audio and RF gain controls
- removing the original AVC-OFF-MVC Switch and knob
- removing the dial lamp control
- removing one of the headphone jacks
- rewiring the tube filaments in parallel
- wiring some kind of hack in the RF amplifier
They also drilled extra holes in the cabinet rear and front panel. These mods were from the old Surplus Radio Conversion handbooks. Personally, I think the handbooks should have been sub-titled "How to hack up a really nice radio".

I restored the receiver to as close to original condition as I could, even installing a replacement dynamotor that I stumbled upon at a hamfest. The only non-original modification retained is a noise limiter circuit since there was already a hole for the switch in the front panel. I also built a separate 28-volt power supply to power the dynamotor and the receiver's tube filaments.

Dynamotors are hard to find, but the original shock mount is really hard to come by, so I made a simple replacement mount for the radio to sit on, as it just looked incomplete without something under it and it needed to be up off the table for the rear power connector to fit.

Picture
The underside of the chassis. Undoing the mods meant finding replacement parts for the dual Audio and RF gain control, the AVC-OFF-MVC rotary switch and knob, and the dial lamp control rheostat and knob. Fair Radio Sales of Lima, OH came to the rescue with some original replacements from a parts set. They also had an original style of power connector (PL P-103) for the set.
Other repairs included:
  • replacing the c.w. oscillator VT-233 tube.
  • replacing several resistors that were out of tolerance, or just plain missing because of the mods.
  • disassembling, cleaning and oiling a non-functioning C.W. Oscillator switch - it would click but not actually turn off so the C.W. oscillator was always on, which may explain why VT-233 was weak.
  • cleaning and lubricating the bearings of the main tuning capacitor - they were not making good electrical contact which caused the frequency to shift slightly when reversing directions while tuning. It was maddening to jump off-frequency every time I tried to fine-tune a station.

I did not find any leaky capacitors (and I checked quite a few of them), so after nearly 70 years the radio is still running on the original caps. I wish all my radios had capacitors this good.

Noise Limiter Mod

Picture
The previous owner drilled a hole in the front panel and mounted a switch for a simple diode noise limiter. Not much I could do about the hole that didn't involve repainting the front panel, so I made a new noise limiter and left the switch in place. I placed the noise limiter circuit on an add-in terminal strip behind the BFO can. This is an audio limiter circuit taken from an old ARRL Radio Amateur's Handbook.

CW Oscillator Repair

Picture
The C.W. Oscillator (BFO). Turning the front panel knob had no effect on the BFO frequency. I found the tuning shaft was sheared off inside the can by a previous owner.

But after the repairs below, and adjusting the frequency per the manual, it works like new.

Picture
The BFO coil after removing the outer can and shaft assembly.

Not seen is a slot in the coil form on the opposite side of the coil for a brass pin. The visible hole in the coil form is for inserting a tool to push the pin out the slot side when disassembling.

The pin in the slot of the coil form limits how far in and out of the coil the ferrite core can travel and also keeps the ferrite core from rotating.

Picture
The threaded screw shaft was sheared where it was screwed and pinned into the ferrite core base (the weakest point due to the hole through the screw for the pin).

The knob shaft and threaded brass screw were seized tight and would not budge, even after a good dose penetrating oil. Apparently someone forcibly trying to turn the knob is what sheared off the brass screw.

​
I drilled out the broken screw from both the knob shaft and ferrite base and re-threaded both for 6-32 threads. Drilling the broken screw end out of the brass base caused the ferrite to detach from the base. I did not drill into the ferrite core itself, just the base. After re-threading the brass base I epoxied the ferrite core back on.
I drilled a 1/16" hole through the new brass screw shaft at the base end to accept the brass pin. The final shaft screw length (~.875") needs to allow the pin to travel forward and back the full length of the slot without the threaded shaft unscrewing from the knob shaft.

Picture
The reassembled components. After reinstalling the rebuilt assembly into the coil form, the brass pin is reinserted through the coil form slot and into the hole in the core base.

When the front panel knob rotates cw it screws the brass screw shaft into the knob shaft, pulling the ferrite core back away from the coil. Likewise, if the knob rotates ccw the threaded brass shaft is unscrewed from the knob shaft pushing the ferrite core into the coil.

To disassemble the BFO coil required bending out the four small tabs on the rear of the can so it could be removed and then drilling out the four press fit indentations on the brass cap. Three of the four drilled out indentations can be seen in this picture.

I only pressed the brass end back on the coil form. The snug fit of the brass cap and bending the tabs back over on the coil can seem to hold everything together just fine.

Crystal Filter

Picture
The crystal filter wasn't working so with my frequency meter and o'scope I did a rough check of the frequency response of the crystal filter in-circuit which is illustrated in the drawing. This gave me an approximation of the center frequency which had drifted a bit from the design frequency of 915Kc. Then I checked the IF alignment and discovered the IF transformers were peaked on ~908Kc, so I realigned the IF to ~918Kc and the filter came to life - just needed to get everyone operating on the same center frequency.

Dynamotor

Picture
The DM-28 Dynamotor. It is a motor-generator that produces the high voltage for the tubes. Routine maintenance for the dynamotor includes re-greasing the bearings every 1000 hours or about 6 months of normal operation.

Picture
The end cover slips off after removing the two end screws. Inside the cover are the re-greasing instructions.

Picture
One of the two sets of commutators and brushes. The bearing is under the end plate. Both ends of the dynamotor look the same.

Picture
The cover plate, gasket and stack of spacers.

Last edit 5/16/2022
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  • Home
    • About
    • Contact
  • Astrophotography
  • The Observatory
    • The New Observatory
    • - About the ExploraDome
    • - About the Lancaster Dome
    • - Building the Lancaster Dome
  • Equipment
    • CCD Cameras >
      • Orion Starshoot Pro
      • QSI 683WS
      • ZWO ASI-178MM
    • Eyepieces
    • Filters
    • Mounts >
      • Orion HDX-110 (EQ-8) >
        • New HDX-110 Setup
        • Orion HDX110 EQ-8
    • Observatory Computers
    • Telescopes >
      • Astro-Tech AT6RC
      • Celestron C-11 XLT
      • Zhumell 152mm Refractor
  • Projects
    • Arduino Focuser
    • C11 XLT Mods
    • Orion SSPRO Camera >
      • Disassembled Views
      • Noise Reduction Mod
      • Temp Controller Mod
    • Ultima 8 Mods
    • Zhumell Refractor Mods
  • Non-Astronomy
    • Amateur Radio >
      • Boat Anchor Radios >
        • BC-348N Receiver
        • Collins 51J4 HF Receiver
        • R-392 HF Receiver
      • DIY >
        • Station Boom Microphone
      • Ameco >
        • Ameco AC-1
        • Ameco OCM
        • Ameco PT-2
        • Ameco R5A
      • Heathkit >
        • Heathkit SB-102 Station
      • Knight-Kit >
        • Knight Kit Span Master II
      • Yaesu FT-817 >
        • Yaesu FT-817ND
        • LDG Z-817 Autotuner
        • Windcamp LiPo Battery
    • Antique Radios >
      • Building A Radio Workbench
      • Radio Dial Neon Clock
      • Radios >
        • 1938 Zenith 6-S-254
        • Echophone EC-1
        • Eltz Radione 2
        • Panasonic RF-5000A
        • Silvertone Model 4465
    • Geiger Counters >
      • AN/PDR-27G Radiac Set
      • Lionel CD V-700 Geiger Counter
      • RCA WF-12A Geiger Counter
    • Test Equipment >
      • DIY Test Equipment >
        • DIY Curve Tracer
        • DIY Signature Tracer
      • Military Test Equipment >
        • BC-221-AF Frequency Meter
        • LM-21 Frequency Meter
        • TS-323/UR Frequency Meter
        • TS-465 C/U Sweep Generator
        • TS-505D/U VTVM
        • TV-3C/U Tube Tester
      • Tube Testers >
        • Alternating Current Sink
        • Drug Store Tester
        • EMC Model 200 Tube Tester
        • Hickok Model 6000A Tube Tester
        • Hickok OZ-1 (550X)
        • Precise Model 111 Tube Tester
        • Precision Model 10-12
        • Supreme Model 385
        • TV-3C/U Tube Tester
      • VTVMs and Multimeters >
        • B&K Model 290 Hi-Low FET Multimeter
        • Hickok 225K VTVM
        • McMurdo Silver VOMAX 900
        • Micronta 22-220 FET Multimeter
        • Simpson 360-2 Digital FET DMM
        • TS-505D/U VTVM
      • Hickok >
        • Hickok 19X RF Generator
        • Hickok 225K VTVM
        • Hickok 288X RF Generator
        • Hickok OZ-1 (550X)
        • Hickok 6000A Tube Tester
      • Simpson >
        • Simpson 360 DMM
        • Simpson 360-2 DMM
        • Simpson 480 Genescope
      • Supreme Instruments >
        • Supreme 385 Automatic
        • Supreme 560 Vedolyzer
        • Supreme 560A Vedolyzer
        • Supreme Model 561 AF/RF Generator
        • Supreme's Model 562 Audolyzer
        • Supreme Model 562 Audolyzer circa 1941
        • Supreme Model 562 Audolyzer circa 1946
        • Supreme Model 563 Audio Generator
        • Supreme Model 576 RF Generator
      • Other Test Equipment >
        • Heathkit IT-28 Capacitor Tester
        • Rider 162 Chanalyst
        • Tektronix 326 O'scope
        • VU-Data Model 941B Oscilloscope
    • ---------------
    • Fossils
    • Rocks and Minerals
    • Star Trek >
      • Star Trek Rock Mood Lamp
      • Star Trek Communicator