FIXED Spectrum +2 grey RAM fault but RAM is good

[Rabs]

No corruption on the screen, only if I remove IC31 do start to see corruption.

[1024MAK]

So the Z80 is likely able to write to this RAM and the ULA can read from it okay.

It could be that something is causing the signal to be marginal at the Z80, or it could be that there is a strange control fault.

Strange RAM faults in models that use the custom multiplexer chip (IC10, PCF1306P / ZX8401 / 40058) may be caused by a partial failure of it. But new spares are not available.

This fault may be rather hard to diagnose.

Mark

[Rabs]

Indeed, hard to pinpoint.

But the stuck key warning may be a clue.

This sometimes clears and I am presented with the main diag menu and the keyboard works.

I have had AY-3-8912 ICs fail before now and their IO ports are known to be susceptible on the Amstrad CPC.

See how some keys are stuck on and this happens without keyboard also.

Or is this just a symptom of a stuck D1 line?

[redgatemodels]

Strange RAM faults in models that use the custom multiplexer chip (IC10, PCF1306P / ZX8401 / 40058) may be caused by a partial failure of it. But new spares are not available.


Mark

https://www.ebay.co.uk/itm/326030599256 ?

[1024MAK]

Yeah, there are replacement modules, but these are not exactly cheap. Especially if it's more of a guess as to what's wrong...

Mark

[Rabs]

I have a spare Sinclair ZX8401, I bought for a later 48k plus and did not need.

Can I use that?

[Rabs]

Strange RAM faults in models that use the custom multiplexer chip (IC10, PCF1306P / ZX8401 / 40058) may be caused by a partial failure of it. But new spares are not available.


Mark

https://www.ebay.co.uk/itm/326030599256 ?

Thanks, have put a watch on.

[Rabs]

Well it's not the multiplexer

ZX8401 fitted and still the problem persists.

Next

[Rabs]

Right need to have a think about this one.

[1024MAK]

Yes, this is a tricky fault. You said earlier that you had changed the Z80. Did you try the removed chip in another Spectrum?

Mark