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Originally Posted by MigaTech
Yes J300 is Tick generation and I am sure OP has to change setting for use with ATX PSU. Also PIN 14 on the current PSU is one to check.
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Yep, though without some sort of logic probe or oscilloscope to hand, it's very difficult to check a tick signal - it's too fast for a multimeter to register. Setting it to internal should eliminate any issues with the PSU's own tick signal, but again, I wonder about the presence of the jumper on the A2000, which is a totally different design to the much more common B2000. I also have a feeling that there should at least be some boot attempt without it - power LED should come up, since the signal has to be checked by the system to see if it's valid. I don't know the ins and outs of it though, so that's just speculation.
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What about resistors do you think any of them might be down?
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Yep, there could be some issue with pull-ups, though typically that would introduce bus errors and give green or yellow screens. I can't imagine so many resistors all failing simultaneously to completely disable the bus in one go. That's way down the list of things to look at in my mind anyway. My approach would be:
- Check the PSU
- Check the reset lines
- Check the clocks
And if all of them are ok
- Check the ROM overlay signal from the CIA.