New to repairing ZX's

[colday]

Hi all.

I've acquired 2x ZX81's for repair and am a little, not so much confused, but without the requisite experience.

In the following pics the first one appears to have no clock at all when probed - all the voltages on the board are present and correct, so I suspected the ULA although I do get a blank black screen.

When I opened the 2nd one up, as can be seen, what I believe to be the ROM chip is in a different part of the socket. On this one I do have the clock signal and again all the voltages are correct but I still get a blank black screen, albeit with what appears to be some interference.

My main question is to do with the ROM chip and the discrepancy in it's socketed location.

Thoughts please.

And yes, you may have seen me on other 8bit forums, especially you Mark (1024MAK)

[Moggy]

Top picture wrong, bottom picture correct re ROM position.

I cannot help further than that I'm afraid as I don't have the Latin for things technical.

[abaffa]

First, change all capacitors.

[colday]

Change ALL caps?

Surely just the electrolytic ones?

No reason to change the ceramic caps.

[colday]

Top picture wrong, bottom picture correct re ROM position.

I cannot help further than that I'm afraid as I don't have the Latin for things technical.

Ok, so if that ROM is in the wrong place on the socket, is there likely any damage caused to the ROM or other IC's/parts?

[Paul]

This is no spectrum. The electrolytic capacitors are not critical. No need to change to get the Zeddys running again.
The ROM will not be damaged if in the wrong position. But please don't turn them upside down. That will damage them.

[colday]

This is no spectrum. The electrolytic capacitors are not critical. No need to change to get the Zeddys running again.
The ROM will not be damaged if in the wrong position. But please don't turn them upside down. That will damage them.

Cool.

I note the two different boards caps are of differing voltages, same values, but differing voltages.

I'll pop out and move that ROM chip down and let you know what happens next.

Once done I'll split (or start a new thread) so as to troubleshoot each individual machine.

[colday]

Ok, so moved the ROM down to the bottom of the socket and switched it on.

Much different!

Now get the following...

Had to rotate the pic as it seems the site won't accept them vertically.

I'll have to remember to take the pics landscape mode.

[Lardo Boffin]

Your second one has a quite early ULA at 8106. Nice.

As most of the chips are socketed you could try swapping them around and may get a working zeddy.

[colday]

Your second one has a quite early ULA at 8106. Nice.

As most of the chips are socketed you could try swapping them around and may get a working zeddy.

Thanks, but I'd rather repair them both. Gotta keep these 8bit machines running!

I keep buying BBC's as 'spares' machines. But so far I've managed to fix them all. I now have 8 of them.

Anyone else have any tips on the above TV pattern?