ZX +2: red stripes over green field

[vldmrrr]

I have this ZX spectrum +2 that I installed composite mod. The problem is that it now shows random red stripes over green color (and green stripes over red color). See attached screen recording: it shows a field of spaces printed in green color, with these random red stripes running over it. Note that white color is normal. Also blue color is also normal - printing blue field does not show any stripes. It appears that it is randomly swaps red and green signals.

So what could be the reason for this? I tried different tv sets - same result. Also I have to mention, I have not tested it over antenna, I installed composite mod right after I received it.

 

[cj7hawk]

It is most likely related to RAM performing slightly out of specification. Especially if it's a grey and it boots to the menu. ( I assume the menu comes up?)

 

[vldmrrr]

It is most likely related to RAM performing slightly out of specification. Especially if it's a grey and it boots to the menu. ( I assume the menu comes up?)

It does boot to menu.The screenshot is in 48k basic as I am more familiar with that.
Is there any conclusive test to verify memory hypothesis? I was loading various games from DIV-MMC on it, including 128K, they ran without problems, music, graphics (except those stripes). There were no suspicious freezes in games, so for most puposes memory shouldbe good

 

[cj7hawk]

I've encountered the problem before. There's not much can go wrong in the memory that doesn't crash the CPU, and the timing between the CPU and the ULA is slightly different.

It was a complex board fault I was fixing for someone and despite the video issue, games were playing OK and diagnostic cartridges were working well... In my case I had vertical lines, so I worked out which RAM the bit was and replaced it. In your case, it looks a little harder to solve, but it's most likely either the RAM or the ULA with a slight possibility of the TEA2000 video encoder.
A quick way to check would be find a RGB monitor - If that gives a clear image, then it's the TEA2000. If it shows the problem, it's either the ULA or the RAM.
Next thing to do would be replace the ULA and see if it fixes the problem.
Finally, replace the RAM.

If your RAM is already socketted then replacing the RAM as a first step would be easiest - Just order some new 80ns memory and it should be good. It's pretty cheap from Aliexpress.

If not, then what I mentioned will troubleshoot without removing any chips yet.

If you do follow the instructions, and it still doesn't work then you will be in the unenviable position of having to follow the signal all the way through the computer to see where it's going wrong A lot harder.

Regards
David

 

[vldmrrr]

A quick way to check would be find a RGB monitor - If that gives a clear image, then it's the TEA2000. If it shows the problem, it's either the ULA or the RAM.

Following your advice I soldered together adapter from round RGB port on ZX to 9 pin RGB monitor connector. The picture on monitor was very clean, no color artifacts. So indeed the problem is with TEA2000.

I am not sure I am going to source and replace TEA2000 on motherboard. I've just got a bunch of AD724 and crystals for them, as I was going to put composite encoders in a couple of other project computers I have. It should be easy enough to fit a small AD724 based board on top of faulty TEA2000.

Thank you for good advice!

 

[cj7hawk]

I've never seen that specific encoder before -so thanks for sharing. It looks like a very simple encoder circuit.
The problem I had with older encoders (eg, LM 1377 etc ) is that they get "unreliable" and sometimes give substandard output. So it's good to know where simpler solutions exist.

Mind you, I haven't played much with PAL / NTSC encoders since the 80s.

David