[TenTec] Century 21 audio hum

[Curt Benjamin]

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