[ARC5] Taming the BC-375

[Todd, KA1KAQ]

Thanks, Dave -

This time I printed it out for future use. Never can have too much
help when it comes to this old stuff.

~ Todd,  KA1KAQ

On 10/18/07, David Stinson <arc5 at ix.netcom.com> wrote:
> 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 :).
>
> ------------------------------
>
> 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
>
>
>
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