Fixing my C128D

[PhilC]

Hi All, @rubber_jonnie has suggested that I might want to do a bit of a blog about fixing up my C128D.

The current situation is its sitting there with a black screen and doing nothing else.

The C128D is not an easy machine to diagnose when things go wrong because of the multiple CPUs inside it, so I've started off as I ususally do when fault finding and get the scope out and look at things like address and data lines to see what looks like a good signal and what looks bad.

So far I've found a few oddities, with some of the address lines being logic high or low when the rest of the lines are pulsing as you'd expect with a working CPU.

The picture below is of a typically good address bus signal

IMG_2810.jpeg (593.5 KiB) Viewed 385 times

And this one is of one that seems stuck high.

IMG_2811.jpeg (497.65 KiB) Viewed 385 times

So with this bit of information I then go and look at what else is on the address lines that are not behaving. So far there is a 74LS138 which is a 3 line 2 8 line address decoder. What this is outputting seems to match the inputs its getting, so I jump to the next step back to a 74LS00 and this as a NAND gate. This chip looks to be not behaving. I get one input pulsing and the other staying low, so I'd expect a pulsing output, instead I get 4.16V on the output. I'm going to desolder both of these chips and put them in the chip tester just to double check.

Will let you know how I get on in a mo.

[rubber_jonnie]

Interesting.

Given that IC has 4.16v, what does the rest of the 5v rail look like? Does it have +12/-12/-5 volt rails in there too?

[PhilC]

Interesting.

Given that IC has 4.16v, what does the rest of the 5v rail look like? Does it have +12/-12/-5 volt rails in there too?

that's a good suggestion RJ but that's sitting at a good 5.12V.

And because fault finding is never that easy, both there 74LS00 & 74LS138 both test fine in the TL866. On to the next part of the puzzle then as something is causing that odd signal.

[chronicthehedgehog]

Are they MOS branded chips? They fail often according to Adrian (Digital basement)

[mrbombermillzy]

@PhilC Did you manage to find a C128 SAMS Computerfacts service guide in the end?

[rubber_jonnie]

Interesting.

Given that IC has 4.16v, what does the rest of the 5v rail look like? Does it have +12/-12/-5 volt rails in there too?

that's a good suggestion RJ but that's sitting at a good 5.12V.

And because fault finding is never that easy, both there 74LS00 & 74LS138 both test fine in the TL866. On to the next part of the puzzle then as something is causing that odd signal.

On to the next thing then. Is there any 12v in there at all?

[PhilC]

Lol thanks guys there is +12v, +5V and about 11VAC.

There seems to be something odd happening on the address lines. I’ve checked the ram and rom data lines and they all seem to be good.

Annoyingly my machine is not wired like a typical C128D but an older C128 with 6581 SID which sent me down the rabbit hole a little earlier looking for a missing 1mhz line only to find this C128D didn’t have a 1mhz line to certain chips like the others.

It’s interesting that on reset the 8502 is halted and the Z80 is online, so I’m starting to work from there outwards now I’ve learnt that.

@mrbombermillzy not managed to find one for this yet.

[mrbombermillzy]

@mrbombermillzy not managed to find one for this yet.

Here it is: https://commodore.bombjack.org/commodor ... ore128.htm

Right at the bottom of the page. (I cant upload it here, as its 20mb+)

The 128 s the same as the 128D (not DCR though) bar 1571 drive internals and PSU.

Im not sure if the 128D manual is for the D or DCR. Perhaps try both?

[rubber_jonnie]

Can you remind us of what led to the black screen situation, was there some sort of 'event', or has it always been that way?

Also wondering if there is any way way you can run some sort of dead test cart? Or is there a minimal config it'll run with so you can eliminate some of the ICs by removing them?

I guess the trick is getting it to boot in it's 'normal' C128 mode as though you'd just turned it on from cold and work through that part of the schematic.

I guess in reality you have 3 machines to worry about - C128, C64 and CP/M.

Tricky...

[PhilC]

Can you remind us of what led to the black screen situation, was there some sort of 'event', or has it always been that way?

Also wondering if there is any way way you can run some sort of dead test cart? Or is there a minimal config it'll run with so you can eliminate some of the ICs by removing them?

I guess the trick is getting it to boot in it's 'normal' C128 mode as though you'd just turned it on from cold and work through that part of the schematic.

I guess in reality you have 3 machines to worry about - C128, C64 and CP/M.

Tricky...

I'm not sure there is a minimal installation of chips to get it to boot.

The black screen came around after a reset,I'm going to have a read of the Sam's manual as they are usually so much better than the manufacturers one. and then go through testing components in relation to their data sheets if I can find nothing else. Going to pull the ROMS and see if any of those have failed now but the activity on them looks okay but you never know.