- What is wrong with the impedance? Or is it fine the way it is?
I cannot see, if the OT is wired with its black wire to ground, too.
- speakerjacks are grounded to the chassis
Ok, but the OT needs to be grounded, too.
- what fx switch are we talking about? The jack switch? Is the picture clear enough?
The jack switch, yes - but as Peter said, it is pretty much corroded and may not give any proper connection - I think the same could be the case with the send jack.
- 'CT' is what is written on the notice with the output transformer (see attached)
As the layout I am used to use is different in various points I was not aware that CT wire is running that way through the chassis.
As I said - at the moment I have no idea and from my point of view I would change all the things which have been mentioned in order to track the fault.
You could change tone stacks caps as Peter mentioned some posts earlier, as they might be faulty - voltages should be stable at all points after the amp has been warmed up for around 1 or 2 minutes and not "floating" for a few volts anywhere (if the multimeter toggles between - lets say - 191 and 192 it is ok, but it should not show 172 changing to 184 changing to 177 or so ... just an example).
And the VTL have been unchecked until now, I think, which is also a possibility to give no sound (or less).
Regards, Stone