Check for bad caps. I found this cap list, https://www.dropbox.com/s/7odtzhubjzjndyf/8513.pdf?dl=0

AtomicPlayboy wrote on 2022-12-15, 02:14:

and then after about 3 seconds, there is a buzz/zap sound from the back, and it powers out again - LED goes out, screen goes dark. And then, the sequence repeats.

This is a very clear indication for arcing (technical term for sparks in this case): Some component gets exposed to a voltage that it cannot sustain, and breaks down. Most likely, the component is supposed to sustain the voltage it is exposed to, but degraded due to old age, but there is also a possibility something is broken so a component gets exposed to a higher voltage than permitted. If you can locate the position of the arcing (where the buzz/zap sound comes from), it could be a great help for diagnosing what's specifically wrong. As you most likely know, there are dangerous voltages in the monitor, so I don't recommend trying to put your ear next to the monitor PCBs to locate the zap sound, but if you happen to have PCB tubes or some other kind of insulating tubes with a diameter of 1 to 3 centimeters (0.3 to 1 inch), you can use them like a stethoscope, put your ear to one end and use the other end to probe locations at the monitor. Of course, the easiest way might be operating the monitor with the cover removed in a dark room and just seeing the light that gets emitted from the arc.

99% Flyback transformer is bad.

it used to be fixed as standard. does anyone do that today...

8513 is top on killing flyback transformers. Back in the day (around 1990-94), we had brochures that sells these flys as replacements.

Look everywhere else for suitable monitors that are more reliable.

Cheers,