New to repairing ZX's

[Moggy]

As I said earlier I'm not the smartest here but I have seen similar patterns when the onboard RAM chip(s) were shot.
The only way I could test was by using a RAM pack, which cuts out the onboard memory, as for for scoping and probing tests then hopefully a forum guru who knows their way around such things will be able answer more fully.

[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.

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?

Wasn’t suggesting you abandon one (never would!!!) but more that if you swap chips and get one working you can then work out which chips are dodgy and what to replace.

I’ve bought one or two of these and don’t remember having one where the problem wasn’t with a chip.

The main culprits have been Z80 and ULA in that order for me.

[Lardo Boffin]

It will probably be worth cleaning the legs on the Mostek RAM - they always seem to tarnish badly.

[colday]

So the IC4 is the RAM?

I presume they had both sizes just to go for the cheapest at the point of manufacture?

[Paul]

Yes IC4 is RAM and yes, the PCB is prepared to take whatever was cheaper.
The Idea by swapping Chips is finding a combination that works.
Then swap one chip each to see if the alternative chip also works okay.
As RAM is different you can't swap this. But CPU ULA and ROM are interchangeable.
Once you get a picture (even the distorted) you can try multiple times power off and on to see if it will start after several tries.

[Paul]

Ah, and if there is no picture from a PCB it's also a good idea to have a look under the cover of the modulator.

[colday]

Ah, and if there is no picture from a PCB it's also a good idea to have a look under the cover of the modulator.

I don't get a picture per se with one of them, just a modulated plain black screen.

I very much suspect the ULA as I believe that creates the clock for the machine?

[colday]

As I said earlier I'm not the smartest here but I have seen similar patterns when the onboard RAM chip(s) were shot.
The only way I could test was by using a RAM pack, which cuts out the onboard memory, as for for scoping and probing tests then hopefully a forum guru who knows their way around such things will be able answer more fully.

Ah! I must have missed this comment! It actually came with a 16k RAM pack.

Guess what I'm doing next!

[Moggy]

And you're not leaving until it's fixed!!!

(Or until Lardo sells you one of his Chinese zeddy knock offs)


Moggy hides fearing lardo's wrath

[colday]

And you're not leaving until it's fixed!!!

(Or until Lardo sells you one of his Chinese zeddy knock offs)


Moggy hides fearing lardo's wrath

It may be somewhat later in the week that I check with the RAM pack.

I've no idea of the condition of it either so will likely open it up to check first.