"Have you checked the health of the bridge rectifier? I wonder if a partially
blown diode assembly could be the culprit.”
Not likely.
Diodes have two failure modes, open and shorted. If they get leaky momentarily,
they then go on to short VERY quickly due to immense heath brought about by
driving the filter capacitor(s).
The direct conversion design does leave the radio susceptible to 60 Hz hum.
However, the fact that you were able to get hum-free operation with an external
12 volt supply suggests that this is not your problem.
I assume the hum is there regardless of volume pot setting. If so, then this
also vindicates the direct conversion design as the source of the hum.
It would seem more likely that the regulator for the internal 12 volt line is
not working properly. This would produce 120 Hz hum, not 60 Hz and is easily
curable by fixing the regulator. This would be associated with Q1 (MJE52D) and
IC 1 (uA 723). The output there is that which should concern you as this would
present itself as 120 Hz hum on the audio.
The uA723 regulator is notorious in terms of its relatively poor reliability,
although it HAS managed to survive for 40 years so not too bad.
If you are adventurous, you can replace the uA723 and the associated transistor
with a 7812 regulator. This modernizes the radio and is a big improvement.
If you are interested, I can tell you how to do it.
Gary
W0DVN
> On May 29, 2017, at 6:19 PM, Curt Benjamin via TenTec <tentec@xxxxxxxxxxxxxx>
> wrote:
>
> Josh, direct conversion receivers such as the one in the C21 are inherently
> susceptible to AC hum from the power supply since all the system gain comes
> from the AF stages. The 23 VDC is only used in the T/R switch to bias the PIN
> diode open so the voltages coming from the transmit output power doesn't bias
> the TR switch closed during transmit, so yes, it really needs to be there -
> unless you transmit low power when powering it off external +12VDC.
>
> Have you checked the health of the bridge rectifier? I wonder if a partially
> blown diode assembly could be the culprit.
>
> Curt, K8AI
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