ZX81 with vertical hold issues?

[1024MAK]

I moved the ULA to another zeddy and the hold issue didn’t move with it so I guess I should be ok to refurb this one.

Did you try a different ULA in the suspect board?

Mark

[Lardo Boffin]

No but I will do so thanks.

[Lardo Boffin]

I finally tried a different ULA in this one and got exactly the same issues. I guess the issue is in or related to the modulator?

Regardless I fitted a ZX8-CCB and it now gives a perfect picture.

[1024MAK]

Ahh, the good old tried and trusted process of elimination

Obviously, it’s always the last thing

Mark

[Lardo Boffin]

Lacking skill and suitable equipment elimination is my friend!

Having connected up the ZX8-CCB the picture is the same as my Timex 1000. Interesting.

Normally the K is further up the screen and the top row further down the screen.

[1024MAK]

Now, if you had an item of test equipment that could measure frequency, I would be suggesting you test pin 6 on the Z80 to see exactly what frequency it’s running at

I presume you have checked R30 / ULA pin 22?

Mark

[Lardo Boffin]

Looks like I have R30.

[Exile]

I suspect most of the people buying my zeddies play 3D monster maze a few times and then don’t switch it on again

I'm still using mine regularly, but then I've never played 3D monster maze on it

Looks like I have R30.

I guess that goes with "USA1" being written on the silk screen between the modulator and the ULA

[Lardo Boffin]

Excellent on the still being used front!

[redgatemodels]

I'm still using mine regularly, but then I've never played 3D monster maze on it

Wasn't that the point of a ZX81