Try grounding ground side of speaker to ground from power supply.
If hum is still there there is a 4.7 ohm resistor that might be shorted
Or a 270 uf cap open both feed the audio amp
Byron Cordes
> On Jun 5, 2017, at 3:42 PM, Josh Gibbs <gibbsjj@gmail.com> wrote:
>
> 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@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.
>>
>>
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