Atari 520ST Repair, part 1

I’ve recently gotten my hands on an Atari 520ST. A faulty one, of course. I’ve had some fun attempting to learn my way around it’s design, and have come to some minor conclusions of what it’s diagnosis is; But first, the actual problem: The CPU is being halted almost immediately after reset.

I didn’t have a thumb drive on hand, i swear i know how to screenshot.

The scope capture here shows the DTACK signal (top) in reference to the system clock (bottom). The DTACK input of the MC68000 microprocessor signals to the CPU that it is ready to commence the data transfer. DTACK oscillating with different pulse widths as seen above is exactly i would expect, and tells me that the CPU is definitely running.

It doesn’t run for very long though; In fact it’s only a ~1000 cycles total before DTACK stops oscillating. Running might not be the best term here either, as it turns out. While the CPU is definitely trying to execute something, the is absolutely 0 activity on it’s data bus. This could explain the crash, however when the MC68000 encounters an error like this, it is supposed to pull its BERR pin low so signal to the rest of the system that its given up it’s futile attempt to work. Here though, it stays constantly high.

That’s a lot of contradicting information, so what do i know works: All 6 ROMS, plus the SHIFTER (The ST’s video controller).

Not a very impressive list, I’ll admit. I’m still trying to figure my way around this board; as of now I suspect the CPU is busted. the original MC68000 dies very quickly when exposed to bus contention, I’ve found, so i wouldn’t be surprised.