BC-375E Operational Considerations
The government publication "Instruction book for Operation and Maintenance of Radio Transmitter BC-375E and associated equipment”,
Army Air Forces Technical Order No. 08-10-139 dated Jan 5, 1943 is an excellent guide for operating and maintaining the BC-375E and is very easy to read.
In the antenna tuner section, it has one of the best explained and written passages on antenna reactance theory, that I have ever seen.
For operating and maintaining the BC-191, please refer to government publication “Technical Manual, Radio Transmitter BC-191-A,
BC-191-B, BC-191-C, BC-191-D, BC-191-E and BC-AA-191”, War Department, TM 11-800, dated Nov 12, 1942.
There is two main ways of operating the BC-375E: AC or DC operation. A few issues unique to equipment designed to operate off of DC systems must be considered.
dynamotor is rated for 26 volts 20 amps and the BD-77M dynamotor is
rated for 12 volts 52 amps for steady state operation. The problem
to consider is upon startup, the peak starting current required to bring
the dynamotor from 0 to 5000 rpm in a few seconds is many times the
rated steady state value.
microphones require almost pure DC to operate. Any ripple present on the
supply line will be introduced into the microphone transformer.
relay also needs DC to operate (but the ripple requirements are less
4. All five tubes being directly heated types, need DC current unless they are rewired and driven by a center tapped transformer. I would avoid this, besides altering a historical artifact, it very difficult to do as the BC-375 is densely wired.
The simplest way to address all the issues above, is to run the dynamotor and tubes off of a battery system like on the original aircraft and vehicular installation. The can be done with a pair of 12 volt car batteries (in series for the BC-375E, in parallel for the BC-191) hooked up to a pair (only one for BC-191) of automotive battery chargers.
If you are
contemplating dynamotor operation, read the dynamotor section of the
manual very carefully. It is a machine handling 2/3 horsepower and spinning
at 5000 rpm. Be careful not to damage the armature in any way, it does
not take much to unbalance something spinning at 5000 rpm. After 60 years,
the lubrication most certainly needs to be cleaned out and redone. Both
pair of brushes should be checked to ensure they are within specifications. Also ensure the
dynamotor is firmly secured to a stable surface.
Using an AC high voltage power supply solves problem 1) "the dynamotor start-up problem", since the dynamotor is not used . A simple power supply
(rated at 1000 volts 350 ma) is described in the “Surplus Radio Conversion Manual, Volume 2” by R.C. Evenson & O.R. Beach, 1948, Editors & Engineers Ltd.
It uses a pair of 816 rectifiers that make nice modulation indicators where the ionized mercury vapor becomes darker blue during modulation peaks. Of course, the
mercury vapor rectifiers can be swapped with solid state rectifier strings.
If the dynamotor is not used as in the original installation, I would suggest adding a relay in place of dynamotor solenoid to cut-off the high voltage supply
(i.e. 1000 volts) when a tuning unit drawer is removed, or else the deadly high voltage will be present at the male tuning unit contacts on the BC-375E.
A 26 volt 8 ampere DC supply (13 volt 16 ampere for the BC-191) is needed to operate the low voltage circuits: filaments, antenna relay and carbon
microphone. I would stick to a nice simple linear power supply. Switching power supplies do not handle in-rush currents very well (like that of cold tube
filaments) and are RFI generators.
One very important thing that is not mentioned in the manuals is what 60 years will do to a radio and the special attention needed in addition to the
recommended operating procedures. Jack Antonio WA7DIA posted some excellent suggestions on the MilSurplus Reflector on December 15, 2002 about
the "gotchas" to watch out for before firing up your transmitter. His posting is reprinted below with his kind permission.
"Before starting at all, I would highly recommend getting the manual and studying it closely so you know how the transmitter is supposed to work, how
the transmit/standby switching is accomplished, and do ohmmeter checks to make sure the relay contacts are clean etc. Also, remember that the transmitter runs
1000 volts of plate voltage, and in the BC-375 at least, the dynamotor runs continuously, even when the transmitter is not keyed, so be careful, think about what you
are doing and stay safe.
1) Make sure all the power resistors in the back have continuity. One of the resistor in mine had opened up, probably due to corrosion. Also check the tube
sockets, the contacts in mine had developed a sticky grunge and had to be cleaned with silver polish.
2) Check all your fuses. They have a history of opening up with age.
3) If you have a dud 211 in either the oscillator or PA position, The transmitter will draw lots of plate current, as the biases for all the 211 tubes are derived from
the grid of the PA, with no drive from a dead oscillator there is no bias and hence lots of plate current.
4) The 3 section can type capacitor has a history of going leaky and burning up. Check it in yours. The capacitor in question is the 3 x 1 mfd 3000 volt unit, C34,
Reference number 1197A-B-C, GE No. K-7877630.
5) I ran mine at about 90 watts out on 80 and 75 with no problems. I ran the transmitter into a coax fed resonant dipole through a 100 pf transmitting mica. Remember these transmitters were designed to feed capacitive antennas, so give it what it wants to see and it will be happy. I haven't tried mine out on 40 or 160 yet.
6) The neutralization adjustment is very critical for good AM and minimum CW chirp. I adjusted mine looking at an FM service monitor and adjusting the neutralization
cap for minimum FM. This was a two handed operation-one hand to adjust, and the other to keep the transmitter on the test frequency. However I think you could
adjust the cap by listening to the transmitter on an AM receiver set to a narrow bandwidth, say 3 kc or so, and adjusting for best sounding AM."
For more BC-375 information,
click on the following links.
Suggestions or comments: [email protected]