Hmm,
Looks like an entertaining evening to me :grin:
When presented with a mystery board, look for little clues:
1. Identify the 0V rail- often this will be the largest/fattest trace on the board, and the shields of any screened cables may connect to it.
2. PSU rails will have decoupling caps- look for caps connecting from a rail to 0V and from another rail to 0V with its polarity reversed. Another good way of finding the PSU rails is look at the IC's- if you've got NE5534/5532/071/072's etc etc you can easily find the PSU rails and trace them back to the edge connector with eye/DMM.
3. Trace wiring of transformers to edge connector- finding how many taps there are and measuring their DC resistance can often tell you a lot about the unit.
4. Input and outputs...a little clue here is the screened (looks like single pair) cables running from edge connector to the front of the unit- this kinda yells "Input" to me (I may be wrong!), but if it is a linedriver with-gain, the input would need to be screened from the output, and the board looks like a low-level-at-front-high-level-at-back setup.....
5. Connection of input or output transformers...in this case, check to see where the circuit-side of the transformer windings go- if they connect to the power transistors with no rectifier(s) inbetween, then they are probably the output transformers, and the power transistors the output stages.
6. Divide and conquer! Draw out all the pins on the edge connector and start tracing them into the board. Just crossing out the ones that aren't use can be a good psychological start to a board-tracing mission.
7. There can only be so many connections to a line-driver:
Power +/0 or +/0/-
Signal Input Bal or Unbal
Signal Output Bal or Unbal
Remote Gain Control (digital or analogue)
Remote monitoring/shutdown etc
So with a bit of pondering, you should be able to get somewhere!
Mark