Sorry for the delay!
You can do a basic check by measuring the power supply voltage to the LM324. That's the 14-pin IC near the top of the board. Here's a diagram of it:
This shows it viewed from above. Measure the DC voltage between pin 4 (V+) and pin 11 (GND) on the IC. I assume you have a multimeter?
Making this measurement is potentially VERY DANGEROUS. You need to measure it with the power ON, and there are mains voltages all around inside that cabinet, and in parts of the board. You should treat EVERYTHING in the cabinet, and on the board, as live.
You should work with one hand in your pocket - usually the left hand in your back pocket. This makes it difficult to hold two probes, so you should get a friend to help. Hold one probe each. Rest your hand on some part of the cabinet near to the board, so you have good control over your hand and the meter probe. Touch the end of the probe to the outside of the pin, where it enters the socket. Don't push hard, and don't push sideways, or the probe may slide off and bridge two pins together.
The main thing I need is to trace out the circuit diagram of that board.
You'll need to remove the board and take photos of both sides. I need to see them in good illumination conditions (see
https://www.electronicspoint.com/threads/how-to-take-photos-of-circuit-boards.267883/) and the pictures need to be from directly above the middle of the board, with the board square (i.e. not rotated) in the photo. That's so I can match the two sides up into a single image that shows the components and the tracks. This makes it a lot quicker and more accurate to trace out the circuit.
Also can you tell me what experience you have working with electronic devices.