[TenTec] [] Running a Century 21 on external 12V power

[Josh Gibbs]

Hi Chris,

The hum is present in the speaker, just not so noticeable because it is
much less sensitive and much further from my ear. I suspect the speaker is
less responsive to frequencies below 120Hz than my headphones, which helps.

I have tried other headphones. I have also listened to the audio directly
at the output of the audio amp board. Even when I short the input to the
audio board to ground, I can still hear the hum. Even when I powered the
pre-amp and audio amp boards via external 12VDC the hum remained. I'm
pretty confident the hum is magnetically coupled. The only time I don't
hear the hum is when I power the radio entirely from external 12 VDC.

Is this normal for this rig, or is there a problem? That is what I'm now
attempting to determine!

The radio works great otherwise, and I likely would never have noticed
anything if I hadn't plugged in my headphones =)

I've learned a lot while working on this radio, which was one of my goals
in buying it. Big thanks to the the awesome amount of knowledge of the
Ten-Tec list, and willingness to share it!

73,

-Josh WA7FPV

On Mon, Jun 5, 2017 at 12:29 PM, Chris Howard w0ep <w0ep at w0ep.us> wrote:

>
>
>
> I'm not an electronics whiz.  But to me, if you're getting
> the noise in headphones but not in the speaker, it sounds
> like the headphone cord is picking it up.
>
> If you have cracked capacitors in the power supply,
> I wonder if you've got a 120 Hz spark gap going there.
> The headphone cord is acting as an antenna and picking it up.
>
> How to test that theory, turn down the audio on the Century 21.
> Use the same headphones on some other piece of electronic
> equipment in close proximity to the C-21 and see if you get the noise.
>
>