[TenTec] Century 21 audio hum

[Josh Gibbs]

Greetings Ten-Tec List,

I just recently bought my first Ten-Tec rig - a Century 21/ Digital. I
really like the radio and I'm having a lot of fun with it! The one fault
I've found with it is a loud hum on the audio, which is very annoying when
using headphones.

I replaced the big 13000uF filter capacitor on the power board with a new
one. No change. I've read the manual cover to cover a few times, and have
been studying the schematics. When I run the rig from an external power
supply, there is no hum! This isn't the solution however, as the low pass
filter board uses unregulated 23VDC for the T/R switch. I'm afraid it would
damage the radio if I attempted to transmit while powering it via external
12VDC.

I'd love to hear from other Century 21 owners - do you hear a loud hum when
you plug in headphones? Is this 'normal' for this rig, or do I have a
problem?

Any info or suggestions would be greatly appreciated. I've posted a lot of
pictures of the rig and a high-res scan of the manual here:

https://drive.google.com/open?id=0B_pjAwEfeYcdYmRaTUhRTnN6SFU

Thanks!

-Josh WA7FPV
Near Seattle, WA