The daemon Mac IIci: Can it be fixed?

[stephen_usher]

The story so far...

This machine has been through a few hands before it reached me.

It bongs with memory but there's no display. It also doesn't access the floppy or the SCSI (suggesting a VIA problem amongst other things).

As the PSU sits directly on the motherboard and the chips I need to test are, as sod's law states, directly underneath I created a power extension cable this morning. But it doesn't work. The PSU just keeps turning on and then turning off. At first I thought it was a short even though I'd quadruple checked the wiring. However, i think it's the power-on signal is being attenuated to much and I need to shorten the cable, as I made it 2m for flexibility.

[rubber_jonnie]

The story so far...

This machine has been through a few hands before it reached me.

It bongs with memory but there's no display. It also doesn't access the floppy or the SCSI (suggesting a VIA problem amongst other things).

As the PSU sits directly on the motherboard and the chips I need to test are, as sod's law states, directly underneath I created a power extension cable this morning. But it doesn't work. The PSU just keeps turning on and then turning off. At first I thought it was a short even though I'd quadruple checked the wiring. However, i think it's the power-on signal is being attenuated to much and I need to shorten the cable, as I made it 2m for flexibility.

I'm eagerly awaiting the restoration of this machine and wishing you luck!!

[rubber_jonnie]

Just watching the video you linked to and it's giving me A5000 flashbacks, very similar sorts of damage from the battery and caps.

It was also quite strange watching use almost identical techniques to my own.

[stephen_usher]

Well, A9 going to the RVB chip doesn't look right...




It appears correctly at the ASC so there's an issue somewhere.

[stephen_usher]

Bother, it's not A9. Those horrid Bomarc schematics strike again. It's actually RAM data bit 25.

[stephen_usher]

Right, I've got video out by setting the VGA adapter to the correct DIP switch settings. (If the IIci's video chip doesn't see the right sort of monitor it turns the video circuit off!.)

IMG_3137.jpeg

IMG_3136.jpeg

From the looks of things the memory management unit isn't giving the video chip access to RAM, or at least when it does it's not halting the processor. Given that the MPU is where the battery damage was this does seem like it's a likely problem area.

[stephen_usher]

Well, it looks like the machine is crashing during the video sub-system initialisation, as can be seen from this RAM data bus "capture":





You can here a click, which is when the video sync briefly changes as seen on the OSSC and screen (not shown in the video). This would be the first time that the video sub-system would access the RAM and the CPU supposedly kept from accessing it.

[rubber_jonnie]

So is that just a white/grey screen, or is it the dithered one where the disk icon appears when booting with no OS?

[stephen_usher]

It's the default grey/white screen before the CPU initialises anything. If it chimes then the display goes black.

About 80% of the time it crashes before attempting to fully initialise the display (and then crash) so I'm wondering if there's an address bus issue or some other signal which is not getting through to the CPU. The problem is that everything's surface mount so it's difficult to set up instrumentation.

[rubber_jonnie]

It's the default grey/white screen before the CPU initialises anything. If it chimes then the display goes black.

About 80% of the time it crashes before attempting to fully initialise the display (and then crash) so I'm wondering if there's an address bus issue or some other signal which is not getting through to the CPU. The problem is that everything's surface mount so it's difficult to set up instrumentation.

So you get the grey screen, but then no mouse pointer of floppy icon. Nasty.

Surface mount is a bind for sure when you want to instrument anything.

I saw in the video he replaced the buffers, (Were they 74HCT245's) but I can't help but think he should have tried new. I had to use new on my A5000 in the end, they didn't look damaged at all, but nothing worked until I dropped in new.

You've got quite the repair on your hands there.