[R-390] hum problem

[Charles A Taylor]

Group,

I had a serious problem of 60-Hz hum in my Motorola SERNO 4823. One would 
expect this to be a problem in the power supply, and possibly one of the 
two 25Z5 not conducting to the extent that the other one was.

Remember, hum problems tend to be 120 Hz because the power supply is full wave.

Replacing a 5814 in the noise limiter circuit reduced the problem but 
didn't eliminate it.

I tried swapping out all the subassemblies except for the R-F deck (for 
obvious reasons) . That failed to bring down the 60-Hz hum.

Next I went through the audio supply subassembly because it was was a prime 
suspect.

That didn't eliminate the hum problem.

Then I went through the R-F deck replacing tubes pursuing the problem.

It should be remembered that this appeared to perhaps be a case of 
heater/cathode leakage.

Finally, I tuned in a calibrator harmonic and tried adjusting the 17 MHz 
transformer.

THAT fixed the problem, although it's not quite logical this being a 60-Hz 
problem vice a 120-Hz problem.

As one tunes the 17 MHz-transformer through resonance, two points are found 
where the output of the
circuit "squegs," or goes into and out of oscillation at an rate of several 
times a second.

Carefully tune the 17-MHz oscillator with the set tune slightly offset on a 
mechanical filter resonance slope.

Tune for minimum hum, as a hum problem in the 17-MHz oscillator would tend 
to manifest itself as phase
modulation, and tuning a calibrator signal (preferably  on a higher order 
harmonic toward 30 MHz.).

Remember, teardrops can cause temporary shorts in circuitry!

Class dismissed. Pop quiz tomorrow at  0800.

Perfesser Chuck

  Charles A Taylor, WD4INP=.
Greenville, North Carolina