[TenTec] Century 21 audio hum

[Dukes HiFi]

That’s not a lot of ripple that’s for sure.


The fact that the volume pot does not reduce the hum clearly proves that the direct conversion design of this receiver is not your root cause. of hum.

The fact that you can run the AF stages (including power amp) on an external supply and still get the hum pretty well rules out the power supply regulator.

The fact that putting your fingers near some ceramic disc capacitors (I assume) causes the SAME hum to increase, it sounds more like 60 Hz pickup than 120 Hz ripple to me. 


When you are listening to “normal” signals, is the volume pot set quite far CCW to get the volume low enough to be comfortable? If so, likely your headphones are too sensitive and you need to put a swamping resistor in between the radio and the phones to get the hum to acceptable levels, and then turn the volume op higher to hear the signals.

All circuits pick up some degree of 60 Hz hum. A very sensitive headphone makes this hum annoyingly strong so the use of the swamping resistor to require a higher volume setting for useful listening suppresses the hum and this method is quite common, especially in the tube days.

The value of the swamping resistor is the highest value you can use while still delivering sufficient phone volume with the AF gain set to about 60% of full scale.


Gary

> On May 29, 2017, at 10:49 PM, Josh Gibbs <gibbsjj at gmail.com> wrote:
> 
> 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
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