In article , "starman"
wrote:

The purpose of bypassing (in parallel) the power supply diodes
(rectifiers) with capacitors (.010-mfd) is to remove RF switching
transients (caused by the diodes) from the DC output. This can often
eliminate the kind of hum which is heard when the radio is tuned to a
strong station but the hum goes away when the volume is turned down all
the way. Regular AC hum caused by a problem with the power supply filter
capacitor(s) can still be heard when the volume is down.

There's a third pssibility: the hum may be coming from the AC line itself
(as 60 or 120 Hz modulated RF in common mode). The design of the power
supply has little influence on this. Common mode chokes are sometimes
helpful, but not terribly effective. Measures to reduce common mode
coupling to the receiver's antenna input (balanced antenna, grounded coax
feed) are more effective in this case.

--
| John Doty "You can't confuse me, that's my job."
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