[TenTec] Century 21 audio hum

Josh Gibbs gibbsjj at gmail.com
Tue May 30 22:32:22 EDT 2017

Hi Jim,

I'd say buzz. I tried to record the sound:


I also tried to look at it on the scope:


Right now, the C21 isn't hooked up to anything else... so no problems there
regarding bonding between equipment.

The green wire (safety ground) is connected to the chassis, as is the
headphone jack. I tried running the radio with the green wire disconnected
- didn't make any difference.

Something I just measured that I had not measured before - the AC ripple
(measured at the big filter cap) goes from about ~300 mV P-P at receive to
over 1 V P-P when I transmit (into a 50 Ohm dummy load). There is no spec
in the manual for this, so it may be normal. The regulated DC is holding
steady at ~13 V even when transmitting.


-Josh WA7FPV

On Tue, May 30, 2017 at 5:43 PM, Jim Brown <k9yc at audiosystemsgroup.com>

> On Tue,5/30/2017 1:57 PM, Carl Moreschi wrote:
>> I don't think 8 ohm headphones were correct.  I think much higher
>> impedance phones forces you to turn the volume up and drown out the hum.
> 8 ohm phones are fairly rare. Most are in the range of 50-600 ohms.
> Question to the original poster -- is what you hear HUM (pure 60 Hz) or
> BUZZ (harmonics of 60 Hz)?
> Hum is most often stray magnetic field from a big power transformer. Buzz
> is most often related to leakage current from the AC power system. Leakage
> current is best controlled by proper bonding -- every chassis in the shack
> bonded to every other chassis, that combo bonded to all the other grounds
> in your home, all gear plugged into outlets that use the same green wire
> back to the panel (or from outlets whose green wires are bonded together at
> the outlet).
> One wiring problem within equipment I've run into is related to connection
> of the green wire from the power plug, or to the return of the power
> transformer center tap to the chassis. If either of those connections is to
> the shell of headphone jack, it could cause what you're hearing.
> 73, Jim K9YC

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