Ok, removed the levco 2mb board and the machine powered up right away! Floppy works perfect and boots into finder 1.1. So, does anyone have experience with these boards? It clamps right over the 68000, I thought those had to be removed? Anyways, if anyone has a levco 2mb board jnstallled jn a 128 or 512 I’d love to hear from you as there is very little info out there on these...
Have you seen this video before? The design uses a vacuum and HEPA filter to remove dust from a small, filtered chamber. It probably is not perfectly dust-free, but I'm sure it would be a vast improvement over working out in the open. I'm not totally sure if this is really true, but from what I have read, it sounds like older hard drives may be more resilient to dust than newer ones anyway, so this design may be a good way to safely resurrect some broken disks.
Thanks. I hope to try some of that tomorrow. In the meantime, I looked back through the Power Central and Power Demo code, and both of them show the following flow:
A condition is tested. One branch writes $520F 0000, then $5206 0000 -- this must be the "cache off" condition.
The other branch writes $5207 0000, then tests a different condition and writes either $5205 0000 or $5204 0000.
I can post code listings if that's useful, but basically it appears there are two more functional addresses in the GALs ($5207 and $5204). Could this have anything to do with enabling or disabling the FPU? There is a switch for that in Power Central, but I don't know if that needs any bits flipped in hardware.
Out of curiosity, what is happening when one removes jumpers C17 and B23, and why would it affect the floppy drive? Interested to know what's going on with the clocks and cache.
One final issue I didn't mention in the first post is that the bad P33, once it heats up for a half hour or so, becomes unreliable and generates random crashes. There's no obvious pattern I can see in MacsBug, but it looks like it might be loading instructions incorrectly from memory (one or more bits randomly flipped). I think this problem might be on the P33 logic rather than in the CPU itself, but I'm not sure.