ZX Spectrum +2 (Grey) issue 3 board problems

[zedexspecky]

Moderator note: split from this topic “zx Spectrum 128k +2 Grey Screen Issue. Help appreciated”.

How are you getting on with this boot issue and garbled screen ?

I've got a very similar fault also with my grey +2 issue 3 board.
Using Smart Card V2 diagnostic ROM the boot fails with both lower and upper memory faults.
The weird thing is that the bad bits are always 0 and 3 in IC6,IC9,IC18. ( this is what the diag rom reports but I think it is reporting the 48K ram IC's) Since I have a 128K the IC's for bits 0 and 3 are different. Not sure why Smart Card diagnostic rom is doing this, maybe not compatible with 128K machiines.

Anyway, I've socketed all 16 RAM IC's with fresh KM4164's 120ns and the same bits and IC numbers are always being reported as faulty. The screen corruption is pretty much similar to yours, I also do not get a white border. The screen stays the same with the default ROM removed and inserted. The diag ROM displays an image fine. I've swapped ram chips around more times than a swinger swapping partners

I'm suspecting this to be a ROM issue as I've checked Z80 and 3.5Mhz clock is there and ULA should be fine as it runs diagnostic ROM without issues. Not sure the correct procedure for testing a ROM chip but could this also be the problem that you are having with your board.

Appreciate any advice, I'm a newbie member but a long term lurker

[zedexspecky]

Just performed some checks with a multimeter on the ROM and Z80 address lines and found a strange resistance reading on A0.
All of the address lines have a resistance of 8.9Mohm with ref to GND apart from A0 which is reading 5.5Mohm.
Not sure if this is normal for A0 to have quite a large difference to most of the other address lines.
There is good continuity between address lines from ROM to CPU though.

[zedexspecky]

Another strange anomaly is that when I power off the spectrum and remove the diag interface, then power the spectrum back up, some of the screen contents from the diags are still on the screen even though the diags are not actually plugged in ? Seems the RAM is still holding the same contents after each power cycle. Maybe the caps are keeping the cells charged, just thought I'd mention it.

Wondering if it's ok to plug a 48K ROM into a 128K ROM socket just for testing as I dont have a spare 128K ROM ?