[C128][CP/M] Problems booting CP/M disk images

[orion70]

Hi everyone. Long shot ...

I have two C128s, one perfectly working from the beginning, and one which I had to change a chip to (character ROM 390059-01 in U18) because it gave random characters when turned on in 128 mode, as specified HERE.

Now it was working, apparently. After a quick check and careful restoration and cleaning of the motherboard, chassis and keyboard, I reassembled everything. Unfortunately, I realized that the 128 does not load CP/M image disks from SD2IEC, which the other 128 loads without any problem. In particular, whenever I mount a CP/M D71 disk image and reset, it loads all the way through CP/M up to the prompt, but then it crashes, presenting a repeated prompt. I.e., it prints a string of A> A> A> A> and so on, adding one A> every two or three seconds. I can't type anything in from the keyboard, it's all blocked.

From the troubleshooting file I linked above, it seems that the partial failure of CP/M can be related either to the MOS 8722 MMU chips, or to the Zilog Z80 (obviously): the latter is unfortunately soldered into the mobo, and I can't solder.

Before trying swapping chip(s) with the other 128 or buying spare parts, is there anyone who has had a similar problem, could suggest a solution, or other attempts to identify the problem? The computer for the rest of its functions (40 and 80 column 128 mode, 64 mode, all inputs and outputs) behaves perfectly.

TIA guys.