ICL OPD 3 vertical white stripes fault

[Chr$]

Sorry, it's confusing as I'm talking about 2 different OPD's (both being used with one sort-of working CPU board). Here is the progress, or lack of it, from today.

The one with 5 white stripes: On the SRAM chip /CE2 is at batt voltage with no power turned on and it goes low when powered-on. /CE1 goes high when powered on. As far as I understand it, that's normal so is fine. I found a flow chart for the OPD and gleaned that the SRAM has connections/inputs with the ULA (IC21), the 74HCT257 pair of chips (IC23 and 24) and with a 74HCT245 (IC22). I've checked all those and they all checked fine. So I'm a bit lost now. Could the Sinclair ZX8301 be dud and could that cause an SRAM error? I have a couple of spares of those. And also, I suppose one of the RAM chips could be messin' things up if dud.

The one that goes into a boot loop: I will certainly check the stability of the 5v rail tomorrow. I've also ordered another known working ROM pack, as that doesn't seem to do anything when plugged in.

[stephen_usher]

The ZX8301 does seem to be liable to failure. My OPD has had it replaced "in antiquity".

[Pr0f]

The SRAM chip does not need or use any signals from the 2 Sinclair ULA chips ZX8301/ZX8302 - it gets it's select signal from the ICL ULA, but this is gated with FC2 (high in supervisor mode) so that the RAM can only be accessed when Selected by address decoded by the ICL ULA and FC2 indicating Supervisor mode.

[Chr$]

Success!

My original m/b - the one with the boot loop - had some way out of spec electrolytic caps on it and my cheap Chinese tester also read high vloss (for what it's worth), mainly the blue 33uF ones, of which there are 5. Changed those and tried again, still had the boot loop but with a momentary flash of green screen first time round, which was different. I tried another combination of the 8 CPU/daughter board ROM chips that I have and boom! I got a message about re-setting, inviting me to press 'Start' to go to the main menu and there it was, the main menu! xChange is fully functional so the ROM pack also seems ok. Of the 4 ROM chips that don't seem to work, 3 are EPROMS and one is a PROM and these are the 4 that were originally in the machine and did previously work. The PROM actually has a different part number, namely XBAA02 which isn't mentioned on RWAPs page, here: https://www.rwapsoftware.co.uk/oneperde ... _info.html
I'll have to check the EPROMS and for now assume that they have gone bad and/or that they are an earlier version and only work in combination with the XBAA02 PROM IC4. It still doesn't really explain why the same - now working - combination didn't work before unless changing the caps really did resolve something.

Anyway, I'll now put everything back together and will get around to trying the disk interface. The other m/b with the SRAM issue can be shelved for now.

[stephen_usher]

Yay!

The ROM versions are a set so you can't mix and match. Mine are:

XBAA03
XBAB04
XBAC03
XBAD03

The XB series means mask ROMs. The Q???? versions are the EPROMs containing the same data.