Sinclair ZX80 / ZX81 / Z88 Forums

A14 and A15 are combined (using suitable logic chips) with /MREQ to drive the /CS or /CE pin of the ROM chip. That is A14, A15 and /MREQ should all be logic low. And if all three are, the /CS or /CE pin of the ROM chip should only then be driven logic low. Mark ok so a little confused... First A an...

redgatemodels wrote: ↑Thu Dec 14, 2023 11:40 am No expert, but I'd be leaning towards a problem with the diag card. Does the Retroleum ROM run in the diag card?

So, not excluding any issue with the card, but it works on all other units, a mix of 48k and 128k, I have

Quick update on this. Interesting one tbh. I have removed the faulty Z80 and the M1 line is recovered. Result: DivMMC now works. Diagnostics ROM still has problems, symptoms are pretty much the same. Short story: Diagnostics card work correctly without the ROM chip installed. If I test with any know...

Thanks for the information guys, I'll buy one.

If you want to minimise the risk to the board, and don't have a desoldering station, you could cut each pin, then once the chip body is out of the way, it's easier to remove each pin individually. Mark Yes that's an alternative but I would like to retain the chip working. After all it's almost full...

Yeah, looks like you need a new Z80A. Have you desoldered a 40 pin DIL chip before? Mark Once, and it was messy... :) I mean, didn't destroyed any tracks and unit is working, but a rather ugly job. Was a couple of years ago and i do have more experience now but nevertheless. Oh well Have a couple m...

If you pull the /M1 line high by using a suitable resistor (470Ω to 1kΩ) connected between the /M1 line and +5V, what happens then? Mark Hi Mark, So, just to confirm, I have connected a 630Oms resistor from pin 27 to reg +5V. If I have read the oscilloscope correctly (sorry still learning how to us...

Just to add that MREQ pin (19) is showing fine so yeah I think the issue is the M1 line on z80....

Hi, Tested edge connectors, all have continuity. But I think I have found the culprit, as this M1 line does not seems ok... IMG_1846.jpg Taken from Z80 pin 27 and from the respective edge connector as well. Compared with an issue 2, for example, and it's completely different so I assume this z80 M1 ...

Hi Mark,

I clean it with IPA and then used a fiber glass pen, and then IPA again.

The diagnostic card as the divmmc both works fine on other zx I have.

Pics attached.

Thank you.