[TenTec] Century 21 audio hum

[Josh Gibbs]

Hi Gary,

I attempted to measure the audio output on the scope. I measured it both on
the internal supply and on external. This is with the volume pot turned all
the way down so all you hear is the hum/buzz:

https://www.youtube.com/watch?v=D7mqyUbNhJw
https://goo.gl/photos/DzcaTGDaGWwTSv4u8
https://goo.gl/photos/mGssTuz695hyw8B97

It looks like 120 Hz to me.

Is there some other test that I can do to tell if the voltage regulator is
the problem? I tried to measure the ripple of the regulated output and
there wasn't much.

I tried grounding out the input audio at the audio power amp board: it
silenced the receiver audio, but the hum was unaffected. The hum does not
get louder when I turn up the volume - at a high enough volume level (too
loud to listen to!) you no longer hear the hum over the receiver audio. A
couple of the 'tropical fish' caps on the audio power amp board are very
sensitive to my finger - when I touch them the hum increases. Not sure if
this is relevant, just an observation.

I tried powering just the audio amp board on external 12V. Hum was as loud
as ever.

I'm up for some adventure. This is one of the reasons I bought this rig -
to learn how it works on a relatively deep level. I've already learned
quite a bit and am enjoying the journey. It's a lot of fun to have QSOs on
this old radio (despite the hum!), and the QSK is great compared to my
Kenwood radios.

Greatly appreciate the help!

73,

-Josh WA7FPV