Thanks for the very informative reply Gary!
I will try shielding the transformer. I don't have any nickel foil, but
I've got some thin steel plate! Seems like that should have some effect on
the noise if the problem is magnetic in nature. If it works, I will order
some nickel foil.
I will also try putting the external supply (a small switcher) close to the
various boards inside and see if I can find the 'antenna' that way.
The radio very well may be performing the way Ten-Tec intended. The
swamping resistor helps. It may not be possible to make it as quiet as it
is running on an external PSU, but I will keep trying.
I do still intend to get one of those circuit breakers. I'd like to have
the option of external 12V without worrying about killing the finals.
-Josh WA7FPV
> I did not say eliminating the ground pin on the C21 WOULD resolve the hum,
> only that it COULD solve the problem. It seems that it did not so there
> must be another path to ground or another mechanism of 60 Hz coupling that
> is the offending root cause of the hum in the C21 In question. The fact
> that the gentleman was able to worsen the hum by moving his hand near some
> capacitor suggests that there is a high impedance “antenna” for 60 Hz in
> this radio, and a source of significant 60 AC energy within. I say this
> because operating this radio with the ground pin removed is the electrical
> equivalent of operating the radio from an external 12 volt DC supply
> ELECTRICALLY. Something else is coupling 60 Hz AC into the audio line of
> the radio. There is NO Way that he could affect power supply 120 Hz ripple
> by moving his hands around some capacitors inside the radio. He is only
> affecting coupling of 60 Hz energy into the audio line, not power supply
> regulation.
>
> It is even possible that the hum coupling is magnetic, coupling from the
> magnetic transformer core, which is inside the C21 cabinet, directly to
> the audio circuit, and thus the use of an eternal supply would most likely
> eliminate this. This was the exact reason why Counterpoint Audio chose to
> put the power transformer for their esteemed SA-3000 preamplifier in a
> separate case from the main electronics cabinet. Even though Counterpoint
> still sent the AC power from the transformer into the preamplifier cabinet
> to be rectified, filtered and regulated there, the magnetic coupling aspect
> of hum generation (by the transformer) was totally eliminated by 6 feet of
> separation between the transformer and the electronics.It may well be that
> the C21 audio circuit has so much gain (for the dual direct conversion
> design to work) and such high impedance, combined with the possibility that
> the transformer has so much magnetic leakage that this presents itself as
> the hum that is reported. A simple piece of Nickel foil between the
> transformer and the rest of the radio would fix this.
>
> I still contend that there is no reason why operation of a C21 should
> require an external supply. I assume Ten Tec felt that the amount of
> coupling was acceptable for most users and that headphones would only be
> used with an attenuator.
>
>
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