Commodore 128 only running in C64 mode. Ideas?

[sergioag]

I have a Commodore 128 that I stored for a couple of years. It was completely working before storing it. However, after taking it from storage it wasn't working. Initial symptoms were that it was completely dead. C128 mode gave only a "garbage" screen, albeit with the correct green border and grey background. Don't remember if pressing the Commodore key made a difference. However, using the C64 "dead test" pointed me to a failed U38. I replaced and C64 mode started working fine. All memory tests run fine. The C64 diagnostics 58620++ now give an almost perfect bill of health:


I've tried exchanging both 6526, but it reports the same. Regardless of what diagnostics says, C64 BASIC reports 38911 bytes free, so that looks fine.

However, C128 mode gives a blank screen, green border and grey background. Sometimes, either on power on or when pressing reset, I get this:



Or just random characters, but always with the correct colors. So it's very inconsistent in C128 mode. However, pressing the Commodore key correctly goes to C64 mode and everything works fine there.

I've tried running C128 Diagnostics 785260 and 789010JB, but the behavior is the same as with no cart.

Assuming I had bad high RAM, I exchanged lines GCAS0 and GCAS1 at R29 and R30. C64 diagnostics gave no memory errors, so I guess the high RAM is fine. However, this made no difference for C128 either. I also dumped the ROMs (all tested good compared to the corresponding P/N) and made sure they were in the correct place as per the service manual.

At this point I'm really out of ideas. I've obviously checked for any physical problems and no chip is significantly warmer than usual (i.e. nothing is burning). All voltages from the PS are fine, both in voltage and ripple (tested with the original PS and a modern recreation). Any suggestions would be greatly appreciated.

 

[Ruud]

Ask an Administrator to move this thread to the Commodore section. Then you reach more people for this specific Commodore item.
I myself have no idea, I would just start swapping some ICs with a good board at this stage.

 

[sergioag]

Update: replaced all ROMs with freshly burned 27C128. No change. Fails in exactly the same way. I'm leaning towards a failing PLA, but it works so good in C64 that I'm skeptical of it.

 

[NoPizzaTonite]

Did you replace Basic Lo ROM (U33) - the chip or bad socket will often cause the break on bootup
If that screen comes up only now and then - I would check continuity on the sockets.
I have quite a few C128s and at least more than a couple had bad ROM sockets.

 

[sergioag]

Did you replace Basic Lo ROM (U33) - the chip or bad socket will often cause the break on bootup
If that screen comes up only now and then - I would check continuity on the sockets.
I have quite a few C128s and at least more than a couple had bad ROM sockets.

Thanks for your response. I've not checked continuity in the sockets per se, but checked continuity from the kernal socket to the others (including the C64 ROM). All pins checked ok. The only pin that is not shared was successfully traced to the corresponding pin in the PLA. At least from the top and the bottom, they look in pristine condition. And when inserting the chips, they make good resistance. However, I'll take a closer look at the part where the pins meet the PCB, just to make sure there are no cracks that could be causing this. I don't have any spare sockets, but will buy some depending on the results.

 

[NoPizzaTonite]

Thanks for your response. I've not checked continuity in the sockets per se, but checked continuity from the kernal socket to the others (including the C64 ROM). All pins checked ok. The only pin that is not shared was successfully traced to the corresponding pin in the PLA. At least from the top and the bottom, they look in pristine condition. And when inserting the chips, they make good resistance. However, I'll take a closer look at the part where the pins meet the PCB, just to make sure there are no cracks that could be causing this. I don't have any spare sockets, but will buy some depending on the results.

If C64 mode works and you get that monitor output on 128 mode, key in on U33