Sinclair ZX80 / ZX81 / Z88 Forums

Well, tried the series 2 in the rubber key case again. Remember this is now bog standard save for the 80’s aftermarket 48k upgrade and a recapping.

It worked, albeit with a slight yellow tinge and the display had interference whenever a key was pressed. I stripped it down of all socketed chips and cleaned the whole pcb, top and bottom with switch cleaner and an old toothbrush. Chips also had a spray. Pcb was then blown down with compressed air and left to dry.

Reassembled and all was good, slight tweak of vr2 removed the yellow tinge and it was tested by loading a game, yes, even the old sloppy ear socket worked.

Screwed the case together and no keys worked

After much faffing around I switch cleaned the ends of the (new) membrane and it’s fine.

I’m pretty happy with it for now. Might swap the sockets at some point and do a transistor comp mod so I can run it on my work monitor via a comp to hdmi adaptor lead, but yes. Think it’s running at last…….

We’ve been here before with this one though

Issue 2 update:

I fitted a new power socket and mic/ear sockets and this screwed up the colours again

A couple of weeks later I summoned up the will to finally swap the ULA from the series 6. It is a -7 version. Guess what? Fitted to the S2 and perfect colours. Guess the ULA from the S2 is goosed, or at least on it’s way out.

While I had it working I plugged in the Retroleum diag ROM. We have intermittent, variable lower RAM faults. Sometimes no errors, sometimes different RAM chips. Interestingly also reports IC3 & IC4 when there are errors. Does this mean the the logic chips are to blame rather than the RAM??

I will see about a replacement ULA, but should I get some lower RAM and/or IC3 & 4?

Are you absolutely sure that the ULA socket is good?

Compare each of the socket contacts using a magnifying glass and under a good light.

IC3 and IC4, normally, either work, or don't work. It's unusual for them to cause intermittent faults.

Mark

1024MAK wrote: ↑Fri Nov 10, 2023 9:41 pm Are you absolutely sure that the ULA socket is good?

Compare each of the socket contacts using a magnifying glass and under a good light.

IC3 and IC4, normally, either work, or don't work. It's unusual for them to cause intermittent faults.

Mark

I can never be 100% certain but when I put the older ULA back in the S2 the colour issue returned. I didn’t try the older ULA in the S6 as you said they were not forwards compatible.

Lower RAM chips are cheap, so once the new ULA is secured I may as well swap the RAM too

Well, yes. And 4116 DRAM are infamous for failing. So, yes, I normally blame the DRAM. But if either IC3, IC4 or the ULA are faulty, this can, and does look like it's a DRAM fault.

It's also easy to blame a chip. But chips only work correctly if all the electrical connections are good. And random or interment faults are often due to poor connections.

Hence, it's worthwhile checking / testing. By testing, I mean testing on the resistance range (20kΩ or 2MΩ range) with the power off when looking for short circuits. Or testing on the resistance range (200Ω range) with the power off when testing continuity.

On another forum, a member went through the pain of a IC3 and ULA fault. See here.

Mark

Indeed. Maybe a replacement socket for the ULA and check continuity etc at the same time would be a worthwhile next step

I’ve got a 40 pin socket on the way, flat spring type not turned pin. I hop3 this sorts things as I could do without 30 odd quid for a ULA. To compound this issue I would prefer an original one rather than a VLA, want to keep this “archive” Speccy as close to how it was when I was a kid

40 pin socket arrived today, now to find time to get the soldering iron out……

Socket replaced, checked all tracks with it removed using a multimeter from pad to next component, new socket fitted, still got the colour problem. Looks like I need another ULA.

Was worth a try though.

A good friend of mine has several old computers up his loft that were left behind by the previous owner. Hopefully a series 2 Speccy …….

redgatemodels wrote: ↑Thu Nov 16, 2023 9:43 pm
A good friend of mine has several old computers up his loft that were left behind by the previous owner. Hopefully a series 2 Speccy …….

a 4S rubber key speccy, so that's another 6C001E-7 ULA. It will do for now (if it works) but will still prefer a 5C112E sometime.

Oh and a 1A ZX81