ZX81 First line of display distorted

[daveyj67]

Hi Mark

The Traco was added because the 7805 wasn't working, the capacitors haven't been changed.

I'm getting the same readings whether the RAM pack is fitted or not:
12.02v at power connector and Traco input
5.02v at Tracto out, on ULA, ROM, Z80 and the 1K RAM chip.
Also 5.02v power to the composite mod with 3.38v input and 2.6v output from the composite PCB.

Since installing the borrowed ULA I have had another play around with the tuning pots on the composite mod, I have been able to get a stable top line with both 1K and 16K. However with the 16K RAM pack the contrast on the screen is poor. The K prompt appears a washed out grey rather than black.

Thanks

Dave

[didier]

hello everyone

excuse me for my English... I am French (Bordeaux) and my English is not good...

this little messsage to tell you that I have the same problem on my zx81 as davyj67 >

the first line is distorted after 5 minutes of use

I don't understand ...

How to solve this problem?

thank you

best regards

didier

[Moggy]

Hi Didier, is it possible to tell us which ULA you have fitted?

The early 2C158 version was known to cause a bending of the first line once it got hot, I had 3 which did exactly that even with a composite video board fitted and I found a change to either a later ULA ( 2C184 or 2C210) or modern replacement cured the problem.