Re-capped power supply and brought an SX-28 back to life. Everything was working while radio was on its side. But, as soon as I sat it flat and switched it back on after a few hours, lo and behold, no audio.
Plenty of lights, tube filaments, etc.

All this time I have been feeding 110 volts AC via a variac (with voltage and current meters). When radio played, it was pulling about .800 amps. Now it's only pulling .400 amps.

I suspected the grounds for one of the electrolytics (C41 and C44) might have become ungrounded. Sure enough, one did and I jumpered it to its own ground. Still no sound.

1-Checked the B+ supply on the filter caps. About 340 volts at the input filter cap and a bit down from that after the choke and at the output filter cap.

2-Checked the 6V6 tubes on a tube checker. They passed. Measured correct voltages at the 6V6 tubes--300 plus on the plates. Checked all electrolytic grounds again. Don't see anything shorting to ground, etc.

Any ideas about why my radio is playing games with me? Would would be very grateful for any help I can get. I was flying high all day after fixing the radio, then decided to listen to it one more time before bedtime.

A little trouble shooting might find it.
Does the audio section work? If you touch the vol control CT do you hear a hum?

If so the problem is earlier.. just keep feeding in signals back to the 1st stage.

Probably just a loose wire though.... maybe a wire-tail clipping shorting something or such.

Is the send/rcv switch in the rcv position? Silly, but it happens. It can also go open by itself, as it's being asked to switch much more voltage than it's designed for.

About the only other way this could happen is if the big resistor on the cathodes of the 6V6s has gone open. More than half the power drain on the B+ is those two tubes.

This should be moved to the Communications Receivers forum, lots of SX-28 experience there.

Carl

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