Sinclair ZX80 / ZX81 / Z88 Forums

1024MAK wrote: ↑Sun May 12, 2024 2:22 pm The joystick chip is believed to be the same as the chip in the Sinclair Interface 2. Hence it's not believed to be essential.

Mark

In my excitement of passing the RAM tests, I forgot to put the ROM back in

Bit of progress. :D 20240512_114244_exported_6245.jpg The clue was a stuck key on bit 1 but also bit 4. Both of these bits from part of the joystick interface. So I removed the joystick controller IC and all RAM tests now pass and no stuck keys. Not booting yet but guess that is because I need a joy...

In summary. ZX Spectrum +2 (grey) displays typical RAM failure pattern on power on. Diagnostics ROM indicates IC31 bit 1 faulty. Removed IC31 and tested off board, and passed. RAM is good. Socketed IC31 and replaced with a new 4164 but faster speed. Problem still persists. Other actions taken but st...

If the fault is or has been intermittent, poor IC socket contacts, dry jointed solder joints, and PCB track faults are still possibilities. Mark Indeed, that is what I am afraid of. But the board is clean and in good visual condition and no previous work has been completed. As a precaution I reflow...

I’m wondering if the ULA is receiving all the control signals from the Z80. Mark I have checked MREQ, RD and WR at the ULA and they all look stable. What is worse, I have seen the lower RAM test pass :shock: and on one occasion I have seen it flag IC28 bit 4. :shock: Unfortunately I am wondering if...

Is there, at any point any screen corruption? RAM in positions IC25 to IC32 provides the RAM for the screen display. If IC31 / bit 1 was faulty, or there was an open circuit fault in the D1 data line, you will get screen corruption. So no screen corruption. So is it possible that the faulty RAM IC ...

But there is no screen corruption when the Diga Runs. So the Z80 is reading the ROM ok and writing to the RAM and the ULA is reading it OK.

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 No just swapped for a CMOS Z80 I had lying around, problem persisted then swapped back. Obviously this looks like a RAM fault but I tested the RAM IC and swapped for a ...

Right need to have a think about this one.

Well it's not the multiplexer

ZX8401 fitted and still the problem persists.

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