Fixing my C128D

[PhilC]

@rubber_jonnie tjats my thought too, the lower half of the address bus initialises just fine every time but not the top half and I’m yet to find out which of the logic chips is not switching correctly.

I’ve ran out of 20 pin sockets too which is a pain.

So I’ll learn what happens on the new C128 and then check the old one. Only thing I can think happened is that we had a spike on the 5V when the caps in the filter blew and caused all this damage.

[rubber_jonnie]

@rubber_jonnie tjats my thought too, the lower half of the address bus initialises just fine every time but not the top half and I’m yet to find out which of the logic chips is not switching correctly.

Yeah, be good to understand what it's supposed to do.

I’ve ran out of 20 pin sockets too which is a pain.

I hate it when that happens...

So I’ll learn what happens on the new C128 and then check the old one. Only thing I can think happened is that we had a spike on the 5V when the caps in the filter blew and caused all this damage.

Sounds feasible to me.

[stephen_usher]

So I’ll learn what happens on the new C128 and then check the old one. Only thing I can think happened is that we had a spike on the 5V when the caps in the filter blew and caused all this damage.

I'd suspect every 74LS chip and memory after that. One of the TTL chips probably acted as a sink for most of the current though, but the others were damaged, possibly only partly causing sporadic errors.

Seeing as TTL chips are easily tested I'd go through every one of them desoldering and either testing or replacing (my preferred option). Once you you have a known good set of TTL you can start with the more special chips.

[PhilC]

@stephen_usher thats what I’m doing one chip at a time, typically most of the chips have come out in bits so couldn’t be tested anyway.

[PhilC]

Still waiting for the new C128 to arrive, so I thought I'd get the scope out again and go through the logic chart for the chips in case I can spot something odd there

[PhilC]

My second C128 has arrived today and it looks reasonably clean and in good condition UNTIL you see the hack job someone did to the RF modulator, so in 40 column mode there is no display. Fortunately the modulator is still in the box, so I'll repair the "missing" track and then get it booting in 40 column mode, then converted over to use as my C128D system whilst I continue to work on the other board.

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You should have seen it before I cleaned it.

[PhilC]

Oh and maybe I’ll fix this too whilst I’m at it.

I’m surprised it booted.

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[stephen_usher]

Eek!

[PhilC]

It might stand a better chance of working now

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Eek was the word @stephen_usher. It’s a right mess where I removed the switch and the switch had three pins broken off as well. Good job that switch isn’t needed in the C128D as the PSU has its own one.

[PhilC]

And it’s up and running now in 40 column mode, running off the C128D PSU, just got to put the keyboard socket on and we are good to go for Cyber Legends.

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I’m keeping this thread open as I’ll continue with it as I fix up the other motherboard.