I got a Ibanez IBZ-20 guitar amplifier, that had some issues that needed to be fixed. All potentiometers were dead, and jacks were badly corroded, but in addition amplification was not working correctly. I could not find any schematics for this old amplifier, so I decided to reverse-engineer the whole circuit board.
Here is the schematic that I made:IBZ-20-Schematic-BW
It turned out that problem with the amplifier was that one of the two 2SD313 power transistors was gone open. This in turn caused smaller transistors on PCB to act as power stage, and whole speaker current was passed trough small 270ohm resistor that could not handle that kind of dissipation.
I also decided to replace all capacitors, and removed that brown glue, which turns into corrosive and conductive mess when it ages and attracts moisture from air.