Newbie - ZX81 ULA issue?

[W3bb0]

I'm finally getting back into the ZX81 saddle after 35+ years, purchasing an "untested" issue 1 ZX81 from a well known auction site (I knew the risks but fancied a challenge!).

I'm trying to diagnose a couple of issues but am increasingly beginning to think I may have a ULA (2C184E) issue:

When first booting, I will get a black screen about 33% of the time, removing the power and re-inserting will generally return me to a K prompt and everything seems well (Keyboard works, able to load from tape, run programs etc).

However, after about 15 minutes of use, the screen display starts to become soft with odd wavering rainbow colours and has some instability as can be seen in the attached picture (ignore the moire but the colours are what is seen on screen). Note that the ZX81 still functions in this state.

This seems to be heat related as powering off and on again fixes the problem but only for a short period. After leaving switched off for an hour I get back to the 15 minutes of normal operation.

The steps I have taken so far:

• Installed Composite mod with back porch option (ZXRenew version)

• Cleaned legs / sockets of all chips with contact cleaner (all chips were already socketed)

• Replaced C5 (in case there was a reset line issue with the black screen at startup)

• Replaced 7805 voltage regulator like for like - I did not have a Traco to hand

• Replaced power jack (it was *very* loose)

• Tried a different PSU (from an Atari 2600)

• Tried with and without a RAM pack to eliminate issues with internal memory

• Swapped Z80 with a known working CMOS chip

• Tried several different TV's

I'm not convinced this could be a ROM issue as the machine still functions when the odd display kicks in so I'm starting to think I may have a marginal ULA.

Can anyone think of any other troubleshooting I can do (without a scope unfortunately) other than trying to source another ULA?

Many thanks for any tips anyone can provide!

[1024MAK]

I've not seen that effect (well, not consistently) before

You say you have tried different TVs? Is the TV set to PAL in the menu / user settings?

If you have already done that, either there is a very strange fault with the ZXRenew composite board, or the ULA has a strange fault.

The ZX81, as you know, can't produce colour. There is no colour encoder circuitry. The TV doesn't know this, so something in the signal from the ZX81 is tricking the TV into thinking it's getting colour information.

Mark

[W3bb0]

Thanks Mark,

Definitely a PAL TV (I'm UK based) and have seen the same issue on multiple TV's.

It's certainly very odd as at first the display is rock solid (although with fairly heavy jailbars) but goes this way after 10-15 minutes of use, however the rest of the machine continues to function as normal.

I suspect my next recourse will be a freezer spray then a second ZX81 for troubleshooting parts!

[Lardo Boffin]

Not a glib as it sounds but you could fit a cooling fan if it is heat related. Moggy has done this.

Does it do this with the top of the case removed? That will prevent heat build up and keep it cooler longer.

Not familiar with that comp mod but I have had this issue several times where I have not ‘tuned’ a ZX8-CCB board correctly yet (they have two adjustable pots). On some boards it tunes seemingly fine but goes like this after a while.

[1024MAK]

Definitely a PAL TV (I'm UK based) and have seen the same issue on multiple TV's.

Yeah, but most LCD TVs are multi-standard, hence why I said PAL setting, not PAL TV.

Mark

[Lardo Boffin]

If it is this one:

https://zxrenew.co.uk/ZX81-Composite-Mo ... p364473489

Then I would suggest taking a photo of where the pots are now and adjusting them to see if you can improve the picture. Worst case scenario return them to where they were based on the photo.

[W3bb0]

Thanks for all your advice - will try tweaking the pots further (instructions do not state what they actually adjust!) and attempt cooling to narrow down the issue and report back!

[Moggy]

As Lardo says I did fit a cooling fan just for the experiment and bloody well it worked too, but fitting a Traco reg negates all that.
I have had the colouration issue in the past with an issue three board and 2c210 ULA and put it down to the TV I was using and just turned off the colour on it.

I'm almost sure that a composite mod that uses the 555 chip should have it switched out with a 2c210 ULA as this ULA already provides the back porch.

[Moggy]

Apparently this issue was raised several years ago!

https://www.sinclairzxworld.com/viewtopic.php?t=248

[Moggy]

In the thread I linked to in my post above, one member says he was using an issue one with the "kludge" fix which possibly denotes an early ULA (2c158.)

Would this make a difference with a composite mod given the timing differences this ULA may have had? Just a guess on my part.