[ARC5] Transmitter repair

[Jack Antonio]

Just spent some time this weekend doing work on some
SCR-274-N transmitters that have come into the shack.

Just thought I'd share what I found, maybe it will
help someone else doing some troubleshooting.

BC-696
Found this at a swap a couple of years ago, it had
been cabled up to a modulator, and used in a stock
configuration. No mods whatsoever.

Initial checks, with a bench supply closely approximating
a dynamotor, showed 8 intermittent, unstable Watts.

Turns out the tubes (except the 1629) were utterly flat.

The 1626 would drop out of oscillation when the roller
inductor approached resonance. I've seen flat 1625s
before, but rarely a bad 1626. The 1626 filament even
looked dim, causing me to check the filament voltage,
which was where it was supposed to be.

With newer tubes, 30 solid Watts.

BC-458

This transmitter had been slightly modified,with
a key jack and the cathodes rewired, but the relays
were all there, and no other mods. Since it is
a black wrinkle set, and in nice shape, I'll be
patching the hole for the key jack.

Worked right off the bat, 35 stable Watts, sounded clean,
but the eye tube showed a resonance indication all
the time, eye wide open.

Problem was C-58, the 3 section can on the back. C-58b
was bad, but the other two sections worked. When I replaced
it with a rebuilt, all OK. I then checked the pulled
capacitor by applying voltage, the other two sections quickly
failed with 300 volts applied.

Another BC-458, a Colonial

Intermittent power out. Partially caused by the roller
on the inductor, but the main cause was that the front
panel was bent just enough to cause the contact pin on
the back of the antenna connector to make intermittent
contact with the spring leaf.

Dial calibration is off, but the tuning adjustment is
frozen. This transmitter is going to need a major disassembly
to fix the mechanical issues.

One other thing. Some years ago, I was bringing a transmitter
back to life, I think it was a BC-696. I fixed the electrical
problems, it was putting out nice stable clean power, but I
then went back to correct a couple of minor mechanical problems.
After reassembly, low power,dial calibration was off and the note
unstable. Turns out when I put the tubes back in, I had swapped
the 1629 eye tube, and the 1626 oscillator. The 1629 makes
a lousy oscillator. It was late at night, and I should have
gone to bed earlier.

Lastly, a question. It may have been asked before, if so
I can't remember if I ever saw an answer.

Did Colonial make complete sets of SCR-274-N, or just
transmitters and receivers? I don't think
I've ever seen a Colonial BC-456 modulator or rack.

Jack Antonio
WA7DIA