All in! Transistors were easy. I was slowed down because when I was putting them in on the EQ board, I spotted a couple of resistors that I'd missed earlier that needed replacing, And, of course, I didn't have any on hand (6.8K 0.5w):
So, new relay in and onto the DBT. Clicks in a few seconds. Phew! Checked a few voltages round the boards and all looking good, so onto "live" testing. Front and knobs are back on and plugged into the "rack of black":
Turn it on, slowly turn up the volume and ...
Absolute silence. Nothing.
Double checked all leads and sources. Changed inputs. Still nothing.
Tried turning a few more knobs and then realised I'd put the Tape Monitor knob on wrong -- so instead of being on Source (as indicated), it was on Dubbing B-A.
Set it correctly and there it is -- sweet, sweet music. Both channels working fine. Everything smooth and silent except for the Mode selector (still had a couple of tiny static noises).
So, I've given that another clean (it's good now), and am about to close this one up. I'll post some audio porn later today and some thoughts on performance when I've had a chance to give it an extended play.
Big thanks to EW for showing the way with the mods on these units and for all the additional helpful info. Cheers.