Phew, that was fun!! I enjoy tracing out circuits, although this was a lot easier than a PCB. I should probably get out more though
It's usually what I do when I get a new piece of equipment to rebuild. It is very instructive and I find it really familiarises me with the circuit and also the layout. As you have found, components are often not where you expect them to be!
I've had this amp for well over 20 years and never even looked at it to find out what it was
I now feel, after the last 2 1/2 hours, well acquainted with it and am now keen to give it an overhaul and a new set of output valves!
I also discovered that the tone controls have been rewired which I will have to attend to.
As regards to your actual phono preamp fault, I would suggest the following checks which will prove the switching and wiring. We'll concentrate on the right hand channel which uses V2. This is more accessible as it doesn't have as many components crammed over it!
The following is done with the amp UNPLUGGED
1) Insert a lead with a phono plug on the end into the RH 'phono' input jack.
2) Set selector on front panel to 'phono'.
3) Work out which is pin 7 on V2.
4) Measure continuity (0 ohms on your meter) between the input lead (inserted in 1 above) and pin 7.
This will prove that signals will get from the input jack to the input (pin7 grid) of the preamp via the selector switch.
The next test will verify the wiring & switching between the output of the preamp and V3.
1) In picture 2, one end of C104 (a 0.1 mfd wax covered cap) goes to a piece of tagstrip and a yellow wire. Point 1 in picture 2.
2) On picture 3, look at the wiring on the balance control and find point 'A'.
3)Set front panel selector to 'Phono'.
4) Set 'tape monitor' switch to 'off'.
5) Set 'mono/ stereo' switch to 'stereo'.
You should now get continuity between point '1' and point 'A'.
This will verify the wiring/ switching between the output of the phono preamp and V3.
I hope you can make some sense of my drawings but if you need any clarification, please ask!
I also note that your amp looks much more shiny and non rusty than mine: but then it probably hasn't been stored in my damp loft for 20 years!!
All the best
Nick