A Grand Day Out

[rubber_jonnie]

I hope it's a simple fix. I guess that there's a whole load of buzzing out traces in your future.

You may be right!

[rubber_jonnie]

OK, I removed all the socketed ICs and cleaned their legs and sprayed some Electrolube into the sockets. @PhilC You're right, they are single wipe sockets, but I didn't feel like a marathon desoldering session today, so I will make a count of what sockets I need, see what I already have, and order in the balance for a future socket session. For now I have the new PLA and MMU installed.

Things have improved in that after doing this, I could boot to both modes of operation:

IMG20240217160038.jpg

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And notably the screen colours are correct with no multicolour text. C128 diags still fail calling out a bad PLA and RAM though, and despite arriving at basic, it will often fail almost instantaneously and BREAK. You certainly can't write a Hello World prog before it crashes.

What seems apparent is that the C64 side seems to work fine, I can write small basic progs and carts run as expected, so I need to focus on the C128 side.

One thing I did note, is that there is a 74F245 over on the left hand side of the board, and it was getting quite hot (I know they can sink more current) so I pulled that and socketed it. I don't have a spare 74F245, so I tried an LS and an HCT, but it was no different, so having tested the original IC in my TL866 and found it to be ok, I refitted it.

Worth a try at least. Schematics out later for me I think.

[PhilC]

@rubber_jonnie if you’ve got that far then the 74LS06s should all be fine.

Check out the 257s and the 244 and 245s. They switch over depending on whether you are in Z80 mode or using the 8502 and the 257s do the address decoding of the ram if I remember correctly. It might actually be the ram? And my good C128 fails the PLA test sometimes

[rubber_jonnie]

So, the SD card cassette for the MZ-700 arrived today too:

IMG20240217151738.jpg

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Annoyingly the cable to connect it to the Sharp was missing, so I contacted the seller and heard back within 5 minutes!!! His mistake, and the cable is in the post :)

So fingers crossed that'll be here soon and I can test. By the time it arrives I should have an SD card full of goodies for the Sharp to consume :)

[stephen_usher]

Will it boot into Z80 mode or do you need something special to change into that?

[rubber_jonnie]

Will it boot into Z80 mode or do you need something special to change into that?

Yes, it boots into Z80 mode first, thats what the horrible greeny screen is that says CBM basic V7.

If I hold down the left Commodore key it boots into C64 mode and seems pretty stable overall.

From C128 mode I should be able to type GO64 to switch to C64 mode, but it crashes before I can type that.

[PhilC]

Will it boot into Z80 mode or do you need something special to change into that?

It starts the boot process in Z80 mode, then changes over to the 8502 unless it finds a CP/M disk in the drive.

[stephen_usher]

Fair enough. :-)

[rubber_jonnie]

OK, so, C128 status check.

The C64 side seems to run fine. I can use carts, write small basic programs, and since I Electrolubed the sockets and cleaned the IC pins, it is now giving me a picture without random coloured text.

The C128 side does now boot fine to a BASIC prompt, but crashes fairly soon after to a break prompt.

Looking at the schematic, it really does look like U12 and U13, a 74LS373 and a 74LS244 do sit right in the middle of the databus for the Z80

Pretty sure I have spares for both, so I will replace the 244 first based on the comments made elswhere on this thread. If no joy, then I'll replace the 373. If it still doesn't work, then they are at least eliminated from the investigation.

I'm also going to order up the sockets I need to replace all the single wipe rubbish that's installed on here.

[rubber_jonnie]

OK, so C64 side remains fine and I have replaced U12, U13, U55 and U62 (LS373, LS245 & 2x LS244) and the problem still remains with the C128. BTW all removed ICs tested fine in the TL866.

I had this wild thought that the keyboard might be causing an issue, so disconnected it, and also swapped the 8266's, but no change there. I even removed the 8266's but that didn't help either.

But then it occurred to me, I get these messages when the C128 side crashes:

IMG20240218192428.jpg

It's a break message from the OS, so perhaps out there is a clue as to what it means. I'll keep at it, and it seems I have all but 48 pin sockets, so they are now ordered.

It also has occurred to me that the Z80 might be problematic. I've had some go on me before, in fact quite recently on a Spectrum I was working on. So I'll socket that too and try a spare Z80 from my spare Spectrum board.

I see much desoldering in my future to get all those sockets done.

EDIT: Diving down the rabbit hole a bit further and the suggestion is that it is a bad basic ROM. Interestingly Noel of Noels Retro Lab had a similar issue with a C64 this week, so I will look at what I can do regarding burning a new ROM too.