"Different" ZX81's

[Lardo Boffin]

Here is my issue 3. The chips are socketed, but not by me.

image.jpg

(2.15 MiB) Downloaded 357 times

image.jpg

(1.78 MiB) Downloaded 357 times

[Moggy]

Looking at some of these issue 3 boards,they seem to have a date code as well next to the ear socket.

[Lardo Boffin]

And here is another issue 3, not socketed. I bought this in the hope of an easy ULA swap for mine to see if it cures my output issues but sadly it's not looking easy!

IMG_5259.JPG

(1.9 MiB) Downloaded 306 times

IMG_5260.JPG

(1.78 MiB) Downloaded 306 times

Lardo

[Lardo Boffin]

And another issue 3, socketed but different to the first socketed issue 3 of mine above!

IMG_5429.JPG

(1.62 MiB) Downloaded 266 times

It looks like the ROM is in an oversize socket?

IMG_5430.JPG

(1.77 MiB) Downloaded 266 times

This one appears to be producing a rather good image without being composite modded. Once I get the keyboard fixed I will test that in a bit more detail.

Lardo

[Moggy]

QUOTE.
"It looks like the ROM is in an oversize socket"?

From the factory any ROM socketed 81's came with a 28 pin socket, your other issue three has been converted by the last owner, I suspect, as issue threes were in the main not socketed possibly as a cost cutting exercise.

The top 4 pin holes on your first issue three are factory soldered over as the 24 pin ROM was directly soldered to the board at the factory.

[Lardo Boffin]

QUOTE.
"It looks like the ROM is in an oversize socket"?

From the factory any ROM socketed 81's came with a 28 pin socket, your other issue three has been converted by the last owner, I suspect, as issue threes were in the main not socketed possibly as a cost cutting exercise.

The top 4 pin holes on your first issue three are factory soldered over as the 24 pin ROM was directly soldered to the board at the factory.

That explains that then thanks!

[Lardo Boffin]

An issue one I happened across...

IMG_5483.JPG

(2.13 MiB) Downloaded 210 times

IMG_5484.JPG

(2.18 MiB) Downloaded 210 times

I like the agricultural feel of the heat sink!

IMG_5485.JPG

(1.71 MiB) Downloaded 210 times

So far I have got an output (dark background not surprisingly) and the keyboard works. Or at least it did until I took the case apart. Fingers crossed it still does when I eventually put it together again!

Not sure whether to composite mod this one or keep it original.

Unusually the case has a serial number 001-003011.

Lardo

[Fwapp]

Erk - bet that gets a bit toasty! Looks like a good candidate for one of those recom regulators, or one of these - http://www.ebay.co.uk/itm/272451377177? ... EBIDX%3AIT

I get the impression that the SN'ed cases were much later in the ZX81's life, perhaps they used returned/serviced pcb's in them? This is Sinclair we are talking about

[Lardo Boffin]

I get the impression that the SN'ed cases were much later in the ZX81's life, perhaps they used returned/serviced pcb's in them? This is Sinclair we are talking about

That would explain why a later case contains an earlier PCB. I guess it could have also been the previous owner - the screws holding the PCB in the case are missing.

[Fwapp]

Also a good chance of that as well