Hi,

My HRD has developed a fault and I'm pretty good with electronics,
safety (discharging caps etc) and soldering, but I don't have the
circuit knowledge to work out which components are faulty. Could
someone here offer any help please ?

The symptoms are that it's making a horrible buzz on the 'drive' &
'more drive' channels, as well as letting some sound through on the
clean channel when the volume is at zero.

Here's the full schematic.... http://www.fender.com/support/amp_sc..._Schematic.pdf

And here's just the preamp section: http://studentweb.eku.edu/justin_hol...HRDxpreamp.gif

Here's a Wav file of the noise it's making...
http://www.fileden.com/files/2006/6/...9/Amp_Buzz.wav
Be careful when listening to it, it starts off very quietly then
gradually gets loud - so don't turn up your speakers because you can't
hear it very well when it starts !

The increase in oscillation frequency is caused by me turning the
DRIVE knob from 12 down to 0. Turning the BASS knob from 12 to 0 also
has a similar effect. Adjusting any of the other controls does not
cause a change in frequency - only in volume / tone of the buzz.

This is happening *only* on the Drive channel, and gets nastier with
the MORE DRIVE button pushed.

The recording was done with nothing plugged into the amp.

With the amp on the clean channel, you can hear a little output from
the speaker with the VOLUME set to zero - which it never used to do.
Apart from this anomaly, the amp behaves fine on the clean channel and
I used it for practice this week.

I've tried swapping positions of the preamp valves and also a
different set of power amp valves - no difference. I've also tried
resoldering R78 & R79, which apparently have a tendency to develop dry
joints, and all the big electrolytic caps.

I'm more than happy to give test point readings to anyone who can
help... the schematic gives TP voltages so hopefully that might be of
use.

Anyone ? Many thanks in advance - I really can't afford to take this
anywhere to be repaired.