ZX81 First line of display distorted

[daveyj67]

Hello all

First time poster … I’m looking for some ideas on a problem I’m having with a recently acquired ZX81. It’s an issue 1 board which has had a composite mod (looks like one of these https://www.ebay.co.uk/itm/155289070172) and the 7805 has been changed to a Traco Power. When powered on from cold everything looks OK, but after 30 seconds to a minute the top line of the display becomes distorted, as shown in the pictures.

I have tried different TVs with the same result on all. The problem occurs whether the 16K RAM pack is attached or not.

So, if anyone have any suggestions on what the problem could be and how it might be fixed I'd be very grateful?

Thanks

Dave

[1024MAK]

I think that may be a problem with the ULA.

Mark

[Moggy]

I agree especially if it is the early 2C158 version, I have had a few over the years that exhibited this problem once warmed up and believe this is quite common to this early chip.

If the top of the ULA has the part number visible, either 2C184 or 2C210, then as Mark says probably a problem within the chip itself.
If the number isn't visible then it will be on the underside of the chip denoting a 2C158.

[1024MAK]

Hello Dave, welcome to our forum

Sorry, I was still mostly asleep when I first posted

Mark

[daveyj67]

Thanks for the replies. It's a 2C184E ULA chip - just my luck
Anyway, there's lots of good information on here - so I'll likely be posting again.

Dave

[1024MAK]

Your options are:

• Obtain another Zeddy (the easiest option).
• If the ULA is in a socket, buy a vLA81 and try that (this is a ULA replacement).
• If the ROM is in a socket, try a different one (as originals are hard to find, in practice, that means burning (programming) an EPROM and doing the modification due to the pin-outs being slightly different).
• If the Z80 is in a socket, try a different one (although it’s not likely to be this).
• Wait and see if another member here can do a run of his latest ULA replacement.
• Live with it…

Mark

[daveyj67]

Thanks for those suggestions Mark. I had a suspicion that it might be the ULA so have had a quick look for a replacement, just to see what's available and the prices. The ULA, ROM and Z80 are all socketed so it should be easy enough to put in a replacement. Are there any complications with replacement ULAs, would any ULA work with my board or does it have to be the same model number as the one already in it?

Dave

[1024MAK]

Any ULA will work. But, only the 2C210 has the back porch circuitry (needed by most colour TVs). The two earlier ULA versions did not have this, hence it’s a bit random on if you get a very dark or normal image on a colour TV. Hence why some “composite video” boards include a back porch generator.

Original ULA chips go for silly prices, because production stopped many, many years ago.

Mark

[daveyj67]

Thanks Mark, I'll keep an eye out for something suitable - might just have to live with it until then.

Dave

[daveyj67]

So, there's been an interesting development since my last post. Whilst I haven't found a replacement ULA yet, I have been able to test my ULA in someone else's ZX81 - and it worked perfectly Sadly his ULA doesn't work at all so I can't try it in mine. I also tested my Z80 and ROM on his motherboard and they were both working.

So if it's not the ULA, can anyone think of any other potential causes and how I might be able to check them. I have a multimeter and access to a basic oscilloscope, but only limited knowledge on how to use them

Thanks in advance.

Dave