[ARC5] Taming the BC-375

[David Stinson]

One of our members recently asked me about "taming" his BC-375.
I sent this reply, without benefit of my notes, since work is pressing
hard lately and I don't get home at a decent hour lately.

I post it here, both for information and other opinions/tips
that might be of help to he and others (and me, of course :).

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The first thing to do is adjust the keying relay.  
It looks very intimidating, but is actually fairly simple. 
The idea is to make the contacts in the order: antenna, oscillator, PA.
This is an essential adjustment.  The manual calls for special tools
and a test set, but I did mine with nothing more special than
a few hand tools and "eyeballing."  The relay rotates, rather
than pulling-in vertically as most do.  

That you are running the dynamotor is certainly a "leg up"
on many other operators.  "Gutless" power supplies are
the #1 reason the 375 got a bad name post-war.

Warm the transmitter for at least one hour before proceeding.

Turn the microphone gain all the way down.

Biasing adjustments "by the book" are important
and should be done next.

The VT4/211 tubes tend to be variable in quality.
Many lack emission or become "gassy-"  even NOS tubes.
Those with no emisson at all will usually recover
if your resolder the pins on the bottom.
Gassy tubes will cause all kinds of troubles.
The first step is to test all you tubes.
To do so, put on some oven mits (the tubes are HOT)
and exchange each tube into the "PA" socket.
If you can't get rated plate current, the tube is suspect.
If you see a blue glow, the tube is certainly gassy.
Once you are confident of your tubes, re-check the bias settings.

Oscillator stability will be enhanced by "hand picking" 
the 211 tube for the oscillator.  If you have a 40-meter drawer,
plug it in and note the "key down" drift.  Substitute all
the good 211s you have until you find the one that
drifts the least with keying.  While all 375s drift with 
keying on 40-meters and above, mine drifts only a
few hundred Hertz, which is good for a 375 on 40 meters ;-).
Key-down drift and "chirp" are minor on 160 and 80 meters.

On neutralizing the final: the "book" procedure is not adequate.
You must have an FM monitor covering the frequency of operation.
I use my ICOM-735 for this.  Tune the transmitter to rated output.
Switch to MCW and turn up the audio until you can hear the tone
in your FM receiver.  Carefully adjust the neutralizing cap to minimize
the FM audio.  This is tedious, because the operation frequency will
also change.  The adjustment is "touchy-" but once found, the adjustment
will "stick" until you change PA tubes (usually a minor "tweak"), or 
until some componant in the tuning drawer "ages in."  I usually check
my neutralization each week or two, just to be safe, or if I hear 
the rare complaint (once, that I remember).  The neut. adjustment
"drifted" a little the first few days I operated the rig- perhaps
I was driving out accumulated moisture- and since then has
not needed much attention, unless I change a PA tube.
Some require a significant change, but most do not.

Setting modulation "by the book" won't work.
You will certainly overmodulate and FM.
It's commonly believed that one should turn the audio gain
on these old transmitters way up in order to compensate
for carbon mikes.  This is not true and will spoil all your 
efforts to stabilize the rig.  Every percent of overmodulation
will be matched by "FMing."  "7" is much too high.
Mine runs at 3-4, IIRC.  A good mike element is key.
Turn the audio gain all the way back down again.
Place the rig in 'phone operation.
If you have an old T-17 microphone in original condition,
forget it.  Were it capable of driving the rig, it will sound awful.
I have a of dozen and none are suitable.  The elements 
have not aged well, and were intended to be used with 
interphone amplification before being applied to a rig in any case.
I use the carbon element out of a telephone and it works well.

Modulate the transmitter while watching the RF waveform
with your 'scope.  If you don't have a scope, use a monitor
receiver with SSB bandwidth (if you FM from overmodulation,
it will be evident sooner with a narrow bandwidth).
Turn up the microphone gain slowly
until you see or hear good modulation and leave it there.
The Type-10 mike amp does a good job, assuming the tube is good.

Give these things a go and let me know how you "come out."
GL OM ES 73 DE Dave AB5S