"Don Bowey" wrote in message
...
On 2/5/08 6:15 PM, in article ,
"Arfa
Daily" wrote:

(snip)

If it is a multi-winding transformer, then I agree that there is a
distinct
possibility that there may be hum just on the supply rails to the
preamps.


Arfa



That wasn't my point.

My point was that even if there is a faulty DC supply common to both the
amplifier and the preamp (typical low-end), changes in the volume control
setting will still be heard as an amplitude or character sound change in
the
hum from the speaker.

Maybe. But by no means guaranteed. Low(er) rails derived from the main
output rails, will usually have their own decoupling caps, and if they are
derived via monolithic regs of the 78 / 79xx series, then the typical ripple
rejection of some 80dB that these devices have, will pretty much knock any
ripple on the head, before you even start counting in downstream decoupling
caps. Thus, my point, that you could have a DC fault giving rise to dirty
'raw' rails fed directly to the power amps, whilst still maintaining clean
low rails to the preamps, in which case, the volume control will have little
or no effect on the hum.

Anyway, this is just becoming a worthless discussion of perhaps's, maybe's
and diagnostic experience, as Jak suggests below. I can only contribute
suggestions based on what 30 odd years of repairing this stuff on a daily
basis, has taught me about faults like this.

Arfa