[TenTec] Century 21 audio hum

[Stuart Rohre]

Actually, loss of one diode can happen more commonly than we think.

The loss of part of the full wave bridge first affects the ripple, and 
could induce higher hum levels.

We have seen it happen in Astron power supplies which did not have the 
mod that adds an MOV across the AC input.  A lightning event nearby put 
a hefty potential on neutral and played havoc with power supply in DC 
source and in a rig that was connected for 24/7 monitoring.

With the radio being direct conversion, it is going to have some amount 
of hum due to the nature of the detection.

Sometimes the HF bypass with ceramic caps helps the electrolytics filter 
the supply to the receiver, and are worth a try.

Equipping the power supply that had a bridge fail and some other 
similarly affected in that lightning surge, has helped, as several 
severe storms have been sucessfully weathered. This is an EOC that keeps 
some radios on for automatic reception from the potential response areas.

-Stuart Rohre
K5KVH