ZX81 rebuild

[Andy Rea]

Cool stuff, us mad soldering iron weilding maniacs have to make do with whatever space we find ourselves in

Regards Andy

[Lardo Boffin]

Another day another issue...

So the zeddy works perfectly when connected to my Chroma - lovely picture.

I have fitted a ZX8 (with the extra bit to keep the picture going) but cannot get it to work! I have fitted about 15 of these now (maybe more) so I have a reasonable level of skill in tuning them. But I cannot get this one to tune.

I can tell it is outputting some kind of signal because my telly says ‘No signal’ if I turn to the extremes of the adjuster pots. The no signal message goes if I turn back to a normal settings.

I can only assume I have damaged it during fitting? The zeddy still works fine via the Chroma so the zeddy is ok.

The back. No obvious gaffes?

The top.

Please ignore the heatsink. It is how the ULA arrived.

I could of course remove it and just use the Chroma but I really want to complete this zeddy!

[mrtinb]

Lardo, are you planning to get in the World's Book of Records with owning most ZX81s?

[Lardo Boffin]

Lardo, are you planning to get in the World's Book of Records with owning most ZX81s?

Would it help my case if I said I don’t actually know how many I have?

Its probably less than 10. To be fair I did refurbish and sell quite a few over the last couple of years.

[Lardo Boffin]

I removed the ZX8 and tried it again with my test rig ZX8 and got a picture so the zeddy is fine. Before removing it I tried it tuned to the same position as the test one but got nothing. This is why I think I broke it.

Will buy a few more and try and be more careful this time.

[Lardo Boffin]

Clearly this zeddy hates me.

I switched it on today with a view to finishing it off (ZX8-CCB basically) and got nothing on screen.

Following the pin tests used last time all was ok except for Z80 pin 4 having a fast pulse not a slow one. Does this suggest what the issue may be?

[1024MAK]

Pin 4 is address line A14. If you are not getting the expected signal here, the Z80 CPU is not executing the normal code in the ROM. Either the ROM is faulty, the RAM is faulty, the Z80 is faulty, the ULA is faulty (yeah, could be any of the chips) or if the problem is intermittent, there is a poor connection (socket/pin) or dry solder joint on one of the important lines (which is most of them). Less likely, but still possible is a hairline crack in a PCB track.

Just remind me, did you use sockets for all the chips? If yes, have you tried all the chips in another machine? And have you tried different chips from a known working machine in this Zeddy?

If yes to the above questions, then I would suspect a dry solder joint as the most likely problem.

Mark

[Lardo Boffin]

Thanks - all the chips are socketed. The ULA, Z80 and ROM are all taken from working zeddies but I have not swapped the RAM out. Will check that next!
The problem is constant and the board was working before I tried to fit the ZX8-CCB. Something I have done loads of times now so probably got over confident and broke something...

[1024MAK]

Well, there is no harm in double checking by trying the chips in a known good board, AND trying the chips from a known good board in this Zeddy

Mark

[Lardo Boffin]

I have tested this with a full complement of known working chips. Well, the 16K RAM chips are all NOS from a reputable supplier and I don’t have another zeddy with a 16K mod to test with so I am assuming at least one of the four I tried would have been working.
If I remove the RAM chip and fire it up with a RAM pack should it work? Assuming it is working of course.
Not really sure how this could have got a dry joint etc. as it was working fine before trying to fit ZX8. The obvious culprits would be pins 16, 34 and 40 on the underside of the ULA as this is where the board was attached but they are visibly ok and have perfect continuity from the pin to the top of the socket. I also tested every other pin on the ULA in this way.
I don’t mind saying that at this stage it is a little soul destroying. To have had it working and then cock up it up somehow. Argh.
I am kind of tempted to de-solder it back to the dark ages and try again but that is a lot of time I don’t have.
I will reflow the solder on the main four sockets and see if that makes any difference.