Fixing my C128D

[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...

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.

I take it it has a reset switch then? Failure of that perhaps, or components in the reset circuit? What's the reset pin doing on the 8502?

[PhilC]

@rubber_jonnie the reset switch works fine and resets the 8502 & Z80A as well as the VDC chips.

All the clocks appear to be okay as well.

This is what I'm looking at at the moment. I don't have a spare 8502 unless there is one in any of the C64Cs, will check but do have a VIC on order.

[mrbombermillzy]

The black screen came around after a reset,

Im far too rusty to do anything but hinder here and Jon is a pretty good guy to help, but I just thought I'd ask:

Did the machine work before the above mentioned reset?

What video output are you using to display?

Lastly, if the answer to the above 2 questions were 'yes' and 'VICIIe' then what sort of keyboard are you using?

The reason being, the 40/80 column keyboard switch key may have been brushed over and set (or is otherwise acting erratically with a third party C128 keyboard solution), which would render the VICs video output blank/black screen - display output now being transferred to the VDC - while the system would seem to be otherwise working ok electronically.

Im pretty sure you are aware of this, so just double checking. :)

[rubber_jonnie]

The black screen came around after a reset,

Im far too rusty to do anything but hinder here and Jon is a pretty good guy to help, but I just thought I'd ask:

Did the machine work before the above mentioned reset?

What video output are you using to display?

Lastly, if the answer to the above 2 questions were 'yes' and 'VICIIe' then what sort of keyboard are you using?

The reason being, the 40/80 column keyboard switch key may have been brushed over and set (or is otherwise acting erratically with a third party C128 keyboard solution), which would render the VICs video output blank/black screen - display output now being transferred to the VDC - while the system would seem to be otherwise working ok electronically.

Im pretty sure you are aware of this, so just double checking. :)

Definitely questions worth asking for sure.

[PhilC]

@mrbombermillzy Yes the computer has been working up until the reset occurrence.

I'm using a PS/2 keyboard and adaptor, so can't click on/off the 40/80 column mode like an original keyboard but had tried a monochrome cable in the RGBI port and that screen is blank with no sync etc.

I believe something is holding the address lines low but it's not like its A0-A7 or A8 - A15 its ones smack bang in the middle, so likely a part failure. I'm hoping it's not an 8502 as they are mega expensive £40.

BTW, I've removed the SID chips, so its not them causing any issues and the C128 motherboard is sitting on the desk by itself, with 1571 etc also removed.

[rubber_jonnie]

@mrbombermillzy Yes the computer has been working up until the reset occurrence.

I'm using a PS/2 keyboard and adaptor, so can't click on/off the 40/80 column mode like an original keyboard but had tried a monochrome cable in the RGBI port and that screen is blank with no sync etc.

I believe something is holding the address lines low but it's not like its A0-A7 or A8 - A15 its ones smack bang in the middle, so likely a part failure. I'm hoping it's not an 8502 as they are mega expensive £40.

BTW, I've removed the SID chips, so its not them causing any issues and the C128 motherboard is sitting on the desk by itself, with 1571 etc also removed.

So, power is good, clocks are good but some address lines are being held low. Do those lines stay low regardless, or do you see activity on reboot? Is the reset line staying low? Or does it cycle correctly with power?

[PhilC]

@rubber_jonnie Yes they stay low, unlike the others that go high whilst the reset button is pressed.

[rubber_jonnie]

@rubber_jonnie Yes they stay low, unlike the others that go high whilst the reset button is pressed.

I was wondering, is it possible to remove the Z80? See if that's holding things low?

Also, do you see any signal at all on the video out?

[PhilC]

@rubber_jonnie Yes they stay low, unlike the others that go high whilst the reset button is pressed.

I was wondering, is it possible to remove the Z80? See if that's holding things low?

Also, do you see any signal at all on the video out?

I've got to desolder both the 8502 and Z80 which I intend to do anyway to pop them in sockets for future servicing ease, so will try then. I do have a few Z80s to hand.

The Screen is what I'd call a fully sync'd black screen. You can see the vertical lines you'd usually see in between characters.

[rubber_jonnie]

I was wondering, is it possible to remove the Z80? See if that's holding things low?

Also, do you see any signal at all on the video out?

I've got to desolder both the 8502 and Z80 which I intend to do anyway to pop them in sockets for future servicing ease, so will try then. I do have a few Z80s to hand.

The Screen is what I'd call a fully sync'd black screen. You can see the vertical lines you'd usually see in between characters.

Interesting, so screen is active, just completely black.

Out of curiosity, do you have any C64 cartridges and if so can you plug them in and get anything out? Sounds like you could do with a dead test cart TBH.