N Cook wrote:

Cured the main problem but the owner seems to live with what to me seems an
excessive amount of stray mains interference, without any signal leads and
input contacts shorted to ground.
This amp is 110V running off a 240/110 V autotransformer with the chassis
having an earth to mains earth (UK)
Other than probably something to do with the awkward cross-linking of the 2
sections of 2 7025, of normal and vibrato channels . It is possible to
minimise the noise using one "normal" signal input and nothing in the
"vibrato" inputs but the vibrato volume set on about 3 of the scale, not 0.
Is there a recognised fix/amelioration of this effect. ?

What do you consider excessive mains interference? They are quite noisy
amps at the best of times with the volume up.
When you say interference, I presume you dont mean mains hum. If it IS
mains hum, check the smoothing caps and make sure that the bias is
correct and both output valves are ok. Some Fenders have both a bias and
a hum bucking adjustment.

If it`s more of a 'fizzle' it might be fairly typical of am amp of that
age. Are the screening cans still present on the preamp and reverb drive
tubes? Are the shorting jacks actually shorting the input when there`s
no plug in the socket? You'll probably have to trace it back with an
earpiece type probe.

They do tend to be noisier out of the cabinet, if there`s a foil screen
above the chassis, make sure that it`s making contact with chassis earth
or that will make it worse.

You might want to change all the 100k resistors in the preamp section,
that gets rid of a lot of rustling/frying noises.

hth

Ron(UK)