Fixing my C128D

[rubber_jonnie]

Sounds like we have an accord

Indeed.

It probably needs at least three of us to diagnose a machine with 2 different CPUs and 2 different video chips!

I reckon you're right there

[PhilC]

I got a couple more parts ordered, even if they aren't needed and will be kept for spares, I ordered an 8502 and an MMU. I already have Ram, CIAs, clock chips, SID chips and some Eproms to flash the 27128 Roms onto if needed.

[rubber_jonnie]

I got a couple more parts ordered, even if they aren't needed and will be kept for spares, I ordered an 8502 and an MMU. I already have Ram, CIAs, clock chips, SID chips and some Eproms to flash the 27128 Roms onto if needed.

Any luck trying to boot CP/M at all?

[PhilC]

@rubber_jonnie nope unfortunately not

[rubber_jonnie]

@rubber_jonnie nope unfortunately not

Damn...

[PhilC]

My replacement VIC II chips arrived today. I've managed to get as far as plugging one in and it still doesn't boot but I've not managed to get the scope out yet and see if the signals on the address lines have improved.

One thing I can see is that when you press the reset, the screen changes slightly for a second or so and then returns to the pre-reset image. It didn't do that before.

[rubber_jonnie]

My replacement VIC II chips arrived today. I've managed to get as far as plugging one in and it still doesn't boot but I've not managed to get the scope out yet and see if the signals on the address lines have improved.

One thing I can see is that when you press the reset, the screen changes slightly for a second or so and then returns to the pre-reset image. It didn't do that before.

Double Damn...

[PhilC]

My replacement VIC II chips arrived today. I've managed to get as far as plugging one in and it still doesn't boot but I've not managed to get the scope out yet and see if the signals on the address lines have improved.

One thing I can see is that when you press the reset, the screen changes slightly for a second or so and then returns to the pre-reset image. It didn't do that before.

Double Damn...

I've got the Scope out and we are actually a step in the right direction. With one of the new VICs fitted I get activity albeit brief on all address lines but A10 which refuses to pull-up on reset, so looks like at least A10 and what's connected to it is part of the problem.

[PhilC]

So doing a bit of fault finding it looks like one of the buffers is potentially not working, so I'm going to remove that and put it in the chip tester.

Of course it could be something else on the output side of the buffer causing the issue but everything else on that buffer chips seems to work.

Wish me luck!!!

Screenshot 2023-09-12 at 14.15.39.png (172.14 KiB) Viewed 1325 times

p.s. its pin 7 for those that are interested. we have a pulsing waveform on pin 13 but other on 7.

[stephen_usher]

74LS244's are prone to dying. It's not a MOS part is it? MOS TTL chips are the worst possible for reliability. I'd preemptively replace any on the board with new TI versions.