working 6s crash with diag rom

[amateus]

Hi guys,

I bought a 6a board on a local flea market. I've done all the preliminary tests, all fine, all voltages present, appears to be working fine, all memory ok (print peek reports 65535) but... as the title says, it crashes with my diag card (bough this one https://www.sinclairzxworld.com/viewtop ... =21&t=5255).

Every time i reset (with the card reset button) screen is garbled,border is a random color, usually black). Also tried divMMC and it doesn't recognize the card and boots straight to copyright message.

Tried to clean the contacts but same result.

Any clue to what's happening here?

Thanks

[1024MAK]

How did you clean the edge-connector?

Can you please describe. Also please post a good quality photo of the edge-connector.

And a photo of the edge-connector socket on the diagnostic card.

Mark

[amateus]

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.

[1024MAK]

The edge-connectors look okay. However, I can see some signs of corrosion on the pins of IC2 and the resistor in the photo.

So it’s possible that one of the important PCB tracks or through-holes (vias) has a break.

Do you have any other interfaces/expansions? If yes, can you see if each of these works with this Spectrum.

You can also test each edge-connector ‘pin’ to its destination on the board. The vast majority go to the relevant pin on the processor. Some go to the ULA. The rest are either video signals (not important or needed) or power supply rails.

If it’s not a PCB problem, it could be a missing/M1 signal from the processor. But normally the symptoms are the Spectrum just ignores the expansion.

Mark

[amateus]

Hi,

Tested edge connectors, all have continuity.

But I think I have found the culprit, as this M1 line does not seems ok...

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 is bad.

What do you think?

Antonio

[amateus]

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

[1024MAK]

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

[amateus]

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 use it and scale it correctly) it goes up to around 4.6v.

Thanks

[1024MAK]

Yeah, looks like you need a new Z80A.

Have you desoldered a 40 pin DIL chip before?

Mark

[amateus]

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 more units to diagnose, should get me to xmas, so perhaps Santa will give me a desoldering station

Thanks Mark for the help