Zx81

[John Earland]

I’ve tried the modified ZX81 on a LCD TV on bot SCART and AV1 both produces the picture attached. The lines give a strobe effect. On the crt via SCART I don’t have a problem. I guess it’s not a great lcd? The Spectrum works fine on it lcd though so maybe the mod just doesn’t work well on this tv.

[SiriusHardware]

You can always try inserting the diode and the diode + resistor as per the slightly more refined versions of the ZX81 composite mod. If it makes things worse, just go back to how it is now.

[Slothie]

I would guess the CRT has a lower bandwidth and filters out the noise. So perhaps the LCD is too good rather than bad! Sinclair wouldnt have bothered with noise you couldnt see.... Or maybe you have a bad capacitor somewhere thats not decoupling the noise.

[John Earland]

Thank you for that-I think I might need to replace the caps. I have noticed a couple of things now:

1. The ZX81 performs well at first but after a while the picture distorts (pictures attached)
2. After the distortion the computer switches itself off whenever you press a key. So for instance when I unplug and plug the psu back in I get the ‘K’ display. When I press a key, Sat P, the picture goes blank and I either have to reboot or sometimes it resets itself.
3. I’ve tried 3 different TVs-crt and 2 lcds -same issue
4. I’ve tried two separate psu’s and with difference
5. The picture quality on the lcds is not as sharp as I would expect- the ZX Spectrum by contrast was very sharp. Same modification as on the ZX81.

This all occurs after the computer has been on for about 20 mins or so. I would appreciate any thoughts you might have.

[SiriusHardware]

Annoyingly I can't remember where I saw the version of the composite mod using a diode but I do remember the OP had said he needed to add that specifically to get a signal which was acceptable for his LCD TV. So it may be worth adding just a diode in the video-out path, just a resistor in the video-out path, and a resistor and diode in series in the video-out path to see if your LCD TV finds any of those combinations more soothing.

If you do find a combination which works better with the LCD TV, then go back to the CRT TV and make sure it still works there. What you really want is whichever arrangement works best with them both.

Edit: Just caught your latest. Try this: From cold, power up the '81 without a display connected. Leave it on for longer than it normally takes for the distortion / odd behaviour to happen, maybe half an hour.

Then, connect whichever display usually works best.

Is the picture already distorted / key problem already present? Or does it take ~ 20 minutes from the moment you connect the display for that problem to happen?

[John Earland]

Ah, right. Thank you for that. I will give it a go when I get home.

[John Earland]

I’ve actually ordered a ZX8-CCB video output module which has to be soldered to the main board but it comes from Germany. Not sure because of COVID-19 whether I will get it for some time. I ordered it to solve the issue of the ‘older’ ZX81 I have which has the 1st ULA but I think I will use it on my second one. However, I am intrigued about the additional modification. I too have seen this somewhere - I shall endeavour to find it and share it with you.

[Slothie]

The mod with the diode was on the page i linked in post #69

[SiriusHardware]

Thanks, that article shows variants both with the diode and diode+resistor - I failed to click through on that one.

Originally Posted by John Earland

...I’ve ordered a ZX8-CCB video output module....

It will be interesting to see how well that works. I did wonder whether it would be suitable for a 2C210 ULA because it generates the back-porch portion of the signal and the 210 ULA already does that, but on reading through the info about it I see that it does work with all versions of the ULA.

[John Earland]

Originally Posted by Slothie

The mod with the diode was on the page i linked in post #69

Ah yes-thank you for that.

[John Earland]

Would you believe the module arrived this afternoon. Now which mod do I do? I think I might like to try the one Slothie posted-any advice would be welcomed. Mind you I don’t have that particular diode so I’d have to order one.

[Slothie]

Pretty much any signal diode will work if it has a forward voltage of 0.6v - check with a multimeter on diode check mode.

[John Earland]

I’m afraid I don’t have any diodes only resistors! I shall order some.

[Slothie]

If your going to experiment much more get a mixed component pack e.g. https://www.ebay.co.uk/itm/1390pcs-E..._trkparms=ispr search on your favorite auction site for "electronic components" . A breadboard, some wite and a £5 multimeter and youve got months of fun!

[SiriusHardware]

Originally Posted by John Earland

I’m afraid I don’t have any diodes only resistors! I shall order some.

In the meantime, try the version of the mod which places a resistor (only) between the emitter and video-out.

[John Earland]

I think I have already done that one I think? #57?

[John Earland]

Originally Posted by SiriusHardware

Just caught your latest. Try this: From cold, power up the '81 without a display connected. Leave it on for longer than it normally takes for the distortion / odd behaviour to happen, maybe half an hour.

Then, connect whichever display usually works best.

Is the picture already distorted / key problem already present? Or does it take ~ 20 minutes from the moment you connect the display for that problem to happen?

Okay, I tried your suggestions:
1. The zx81 was switched on for about 30 mins before connecting to the crt tv - I get a better picture at the moment on this one. The picture was very distorted and couldn't make out anything.

2. I let the unit cool down and then I plugged it in and the picture was great (again on the crt) and strobe effect on the LCD

However, after an hour of continuous use the picture became distorted but there is a pattern before it gets to this stage.

The unit began not to load games and when it did it it sometimes reset itself and sometimes it froze. When I rebooted it it then went through the cycle of not loading then resetting and sometimes freezing. After this behaviour a few times, the picture became very distorted.

[SiriusHardware]

And this also happens when you don't have a RAM pack connected?

Sounds like a thermal-related problem. The best way to hunt for this would be using freezer spray - let the unit run until the faults appear and then spray each major component - regulator, ULA, Z80, memory IC, one at a time to see if that makes the fault disappear. Any electroyltic capacitors in the unit are also a possibility (spray those too).

Freezer spray made for the purpose of diagnosing thermal electronic faults is usually quite expensive but if you take a look at this thread, someone has just discovered more or less the same product being sold for an entirely different purpose, and much more cheaply.

https://vintage-radio.net/forum/showthread.php?t=164937

If you can bring yourself to buy some of that, that should work quite well for the purpose.

[Slothie]

Originally Posted by SiriusHardware

Freezer spray made for the purpose of diagnosing thermal electronic faults is usually quite expensive but if you take a look at this thread, someone has just discovered more or less the same product being sold for an entirely different purpose, and much more cheaply.

https://vintage-radio.net/forum/showthread.php?t=164937

If you can bring yourself to buy some of that, that should work quite well for the purpose.

Just be careful not to buy freeze cracker spray for removing bolts - it contains release oil that will remain on your circuit and possibly lift off solder.mask or screen printing! Its aboult half the price of "proper" freeze spray.

[John Earland]

Originally Posted by SiriusHardware

And this also happens when you don't have a RAM pack connected?

Sounds like a thermal-related problem. The best way to hunt for this would be using freezer spray - let the unit run until the faults appear and then spray each major component - regulator, ULA, Z80, memory IC, one at a time to see if that makes the fault disappear. Any electroyltic capacitors in the unit are also a possibility (spray those too).

Freezer spray made for the purpose of diagnosing thermal electronic faults is usually quite expensive but if you take a look at this thread, someone has just discovered more or less the same product being sold for an entirely different purpose, and much more cheaply.

https://vintage-radio.net/forum/showthread.php?t=164937

If you can bring yourself to buy some of that, that should work quite well for the purpose.

Thank you for this. I shall take a look.