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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