Using the BC-375-E Today

Successful operation of the BC-375 family of transmitters on the ham bands is a real challenge. Many of the surviving BC-375 and BC-191 transmitters seem to be in pretty good condition but they will still need a thorough check out and adjustment before actually putting one on the air. The antenna relay seems to always need a lot of attention and since it is in constant operation in CW and controls T/R functions in all modes, its proper cleaning and adjustment is critical. All components need to be checked and the filament voltage adjustment links will probably need to be reset. It's a good idea to monitor the transmitter output with an oscilloscope during test and adjustment since the settings of the audio gain and the modulator bias have a significant effect on audio quality and power output on AM. Generally, operation on 160M and 80M will be trouble-free but operation on 40M pushes the BC-375 frequency high enough for instability and FM'ing to become a real problem. Nowadays, if the BC-375 is operated per the manual and used with its proper dynamotor and a good battery set up for the 24vdc, it is fully capable of military communications-type audio QSOs provided those contacts take place on some of the vintage military AM nets where the participants should know what carbon microphones sound like and "broadcast-quality audio" is not the topic of extended conversations. It is virtually impossible to operate the BC-375 on AM in its original configuration without enduring "critical audio reports" - even on some military nets since many hams are unfamiliar with how carbon mikes sound and of the peculiarities of the "BC-375 sound." Unfortunately, it is routine to endure several comments about poor audio quality and perceived distortion during most QSOs. One has to remember that when the BC-375 was used in the B-17, the radio op really didn't care about his audio quality, after all, he was probably being shot at! - readable communications was the goal. Extensive modifications to the original design concept of the transmitter (with the goal of achieving BC quality audio) seems to go against the very idea of operating vintage WWII military equipment in the first place. However, modifications to the microphone itself can improve the "sound" of BC-375 significantly.

The "weakest link" in the BC-375/BC-191's AM capabilities IS the T-17 or T-32 carbon microphones that were standard equipment. The T-17 was designed to provide intelligible communications from a very noisy environment, so almost all T-17 mikes will be noise-cancelling in design and therefore very limited in audio frequency bandwidth. Also, most of the original carbon elements are practically non-functional nowadays, after all, they are pushing 70 years old! Sometimes the T-17 or T-32 element can be "massaged" into functionability by mechanically coupling an audio driver (like an earphone) to the mike element and injecting a high level 800hz audio sine wave. This seems to effectively break up the "carbon packing" that is the common failure mode (the old story about "hammering the T-17 on the table before using" accomplished the same thing but is sort of hard on the mike's cosmetics today.) About 10 minutes of this treatment will usually get a T-17 working about as good as it can. Consider installing a new modern replacement carbon element like the T-1 (from Audio Sears) or similar replacement carbon telephone mike. These seem to work quite well and deliver audio that is probably similar what a new T-17 sounded like back in 1942. Also, the voice holes in the mike cup can be enlarged or even cut entirely out  to improve the high frequency response - a 1.125" dia. hole in front of the element works fine. This is only recommended on T-17s that are defective and have cosmetic problems - nice condition originals should be preserved. Now, if you want to really improve the sound of the BC-375 or BC-191, read on.

As stated, the "weakest link" in the AM capabilities of the BC-375 or BC-191 is the use of a carbon microphone. No matter what you do, there is only so much improvement that is possible using a T-17 or T-32 mike. It is possible to interface a crystal microphone to the BC-375/BC-191 with some modifications to the microphone itself - not to the transmitter. By using a small in-line amplifier that is designed for Hi-Z crystal input and utilizes a capacitively-coupled output very intelligible audio will result. The amp can be solid-state op-amp technology and can run on a 9vdc battery. The amp should be installed into a small metal project box. PTT can be routed directly through the amp box by using PL-68 type jacks. If you don't want to build such an amp, a suitable unit is available from Radio Shack for about $16.00 (PN 277-1008C.) Even though the input Z is rather low at 5K ohms, it still provides good audio that is highly intelligible because of a significant lack of bass (normal input Z for a crystal mike is about 5meg ohm.) Since the amp's output is through a 100uf 10v capacitor, the transmitter's carbon mike bias voltage is blocked and no damage to the amp occurs. Extremely important information next.

If you plan to modify a T-17 to house a crystal mike element be aware that the mike cable is NOT shielded. It wasn't necessary with the Lo-Z carbon mike but it is crucial with a crystal mike because of its high impedance. You will have to replace the T-17 cable with a regular shielded mike cable that has at least one single conductor for the PTT function. Also, it will be necessary to replace the two wires inside the mike body that run up through the handle to the mike element with a small shielded cable. The rework is somewhat tedious on a T-17-B model and almost impossible on the T-17-D because of its very different construction. Use the Kobitone crystal mike element that is 1.38" in diameter (Mouser PN 25LM022) as this fits into the T-17 head with just enough clearance to fit some foam rubber for support. The cavity behind the mike element should also be filled with foam rubber. Doing a "on the air" comparison, the modified T-17 with the in-line amp provides "hum-free" and very intelligible audio with powerful modulation without over-taxing the dynamotor/battery supply (which would cause "FMing") while switching over to a carbon T-17 resulted in muffled audio and some detectable hum. Thanks to Bob Rosenhahn, KE6LNI for the RS amp idea and suggestions based on his experiments to achieve intelligible audio using a TCS transmitter.

 CW contacts with the BC-375/BC-191can be a lot less problematic, especially on 160M or 80M. However, 40M CW operation will have rapid frequency drift and the CW note will "bloop" quite a bit which usually means nobody will answer your CQ (except to tell you that your signal is "blooping" and drifting.) When operating CW, it's a good idea to keep the earphones on at all times since the "clickity-clack" of the sending relay can get annoying after awhile. The SCR-287 can be a fun station to operate provided you know with whom you are QSOing and it does give a chance for other interested hams to hear what vintage WWII communications transmitters originally sounded like on the air.

Henry Rogers WA7YBS - October 2009