[TenTec] Century 21 audio hum

[Stuart Rohre]

Yes, the symptoms of an open diode are no 0.6 volt across it on diode 
test function of a DMM, and much higher ripple or strange ripple 
waveform on scope test. That, on output of the bridge, (+to -)  No 0.6 
volt could mean a shorted section in a bridge as well.

Maybe the back of the board where bridge mounts; can be made handy to 
test. IF mounted on a PC board.

It may be a function of the headphones sensitivity and the Normal hum 
from the earlier direct conversion models.  The capacitors someone 
mentioned to bypass RF hum, are disk ceramics, and anything you might 
have would be OK for a test (0.1 to 0.01 mf_)  The caps would go across 
the plus to minus of the electrolytics that filter the power supply output.

If you have a scope and a handful of disk values, you can hook across 
the DC buss and see hum, then tack solder the disks into place across 
each electrolytic and evaluate if they did some good.

That network of resistors TT came up with may make the hum reduce to 
manageable levels.

There probably are not all that many electrolytics and if the Radio is 
10 years or more old, (and maybe even 5), changing all the electrolytics
might be an expected service step at this age of the radio.   But some 
electolytics last a long time, just they dry out and hum increases.

One troubleshooting step is to have some spare electrolytics of half the 
marked ones, and parallel temporarily with clip leads or tack solder, 
across the rig electrolytics to see if hum reduces. Observe keeping plus 
and negtive correctly hooked up.

-Stuart Rohre
K5KVH