I've not seen one of those in person, but have noticed the occasional one on ebay.
Ya, a better picture of the inside would be helpful.
When I saw this thread I googled for a schem.... not much out there.
Depending on the circuit board(s) it could be easy or "hard" to sus out a schem.
It's not actually hard to reverse engineer the circuit, but it can be very labor intense and VERY easy to make a mistake.
I've done that on almost every board I've traced.
Sometimes the circuit topology is very confusing and it only comes to light once you have the schem complete....
and THEN you notice the mistake and how obvious it is.
The average amp like this takes me a week or 2 or 3, to get a schem drawn up.
That's cuz I have a regular job and only work on the amp in spare time.
Take lots of breaks, especially if you find the going tough and get frustrated with how long and slow it's taking.
I start to get a little cross-eyed after staring at the board and then computer screen for too long when doing this.
Take pics of all connections and label wires and such BEFORE taking stuff apart or un-soldering things,
and you WILL need to un-solder stuff to get the board out so you can trace it.
My best (favorite) tool is an electric vacuum solder sucker.
I could NOT do this without it!