Sinclair ZX80 / ZX81 / Z88 Forums

Hi,
Thought I'd share this repair in case anyone else gets the same problem. It took me a while to work out what was wrong and I couldn't find anything on here or the net about it.
The screen image on the the Toastrack I was restoring tonight had an annoying video fault. The screen kept intermittently jumping up and then back down again by about 5 lines every few seconds randomly.
It turned out to be the 17.734475 crystal oscillator that had gone unstable. I replaced it with one from another toastrack and the screen is stable now.

So it turns out this was not a fix after all for my toastrack 6U board. Seemed to be working fine after I changed the crystal oscillator, but then the intermittent fault came back!
The whole screen intermittently jumps up one line then back down again randomly.
If I turn the green pot next to the crystal clockwise the jumping stops and everything is OK when using RGB cable, but then the colour goes and I just get a black and white video signal with the RF cable ( not able to get the colour back with retuning at the modulator or TV. I've tried turning the pot to the point where the jumping just stops on RGB, but this is not close enough to get the colour back on RF. If I turn the pot anticlockwise to just get the colour back on RF all is good, but the screen is still randomly jumping on RGB.
I've fully re-capped all the elecrolytics, re-flowwed any dodgy looking joints and checked all the diodes and transistors.
I've tried freeze spray on the semiconductors and ICs. I've tried pressing all the components with a chopstick and can't worsen or lessen the problem with any of this
Has anyone else had this problem?
Has anyone got any ideas where to start looking to fix this?
Is there any information on the net about tuning the video with the variable inductors, L3, L4, L5 and the green pot next to the crystal? I can't find any. I'm not sure what they are all supposed to do.
Any help would be really appreciated.

Do you have a 'scope? What's it's analogue bandwidth?

Or a frequency counter that can cope with 20MHz signals?

Mark

I've got a digital and analogue scope, both 100MHz.

The "adjuster" near the 17.73447MHz crystal (X1) is a small value trimmer capacitor (C124). It can change the frequency that the oscillator (that the crystal is part of) by a small amount.

This should be adjusted with a plastic or ceramic tool (not a metal screwdriver) to get the oscillator frequency as close to the specified value as possible.

Connect your 'scope to pin 6 of IC37 (74S04).

The Toko coils/inductors:

L3 is for the colour encoder chip IC36 (TEA 2000) and is do with the PAL colour on the composite video and RF/UHF output.

L4 is for the audio sub carrier frequency on the RF/UHF output.

As all horizontal and vertical timing is determined by counters in the ULA (which gets it master timing clock from IC37), L3 and L4 are not relevant to your problem.

L5 is not shown on the schematic that I have (the toastrack is the least well documented of all the models). But I don't think this would be relevant to your problem either. Note see my later post.

I suggest you see what signal you get on pin 4 of the RGB/Monitor DIN socket. Pin 4 is a composite/combined sync signal (made up of the vertical and horizontal sync. signals). You will have to find the correct trigger setting, as this is a complex signal.

A couple of other questions:
Have you tried this computer on another display/TV/monitor?
Are you using a video converter?

Mark

One more question. What PSU are you using?

Mark

Okay, L5 is in the composite video output going to pin 1 of the Monitor DIN socket. So this shouldn’t affect the signal timing.

I'm about to edit my earlier posts to add some extra details.

Mark

I'm using an old LCD TV that has an RF input and a Scart. I'm using a Retro Computer Shack RGB to scart lead for the Toastrack with a separate lead to the mic output for audio.
I have another old portable CRT TV that I can try, but this just has an RF and composite input
I'll try it on this to see if the RF input is jumping on that.

I'm using a Grey 2.1 amp Spectrum PSU.

I tried swapping the ULA for a ULA 128, but it he problem remains.

If I put my fingers on the 15k resistor and capacitor above the green variable capacitor (I thought was a pot) above the 3 diodes the jumping stops. All the diodes and resistor measure OK.

Your finger is acting as a extra small value capacitor...

These oscillator circuits can be a bit sensitive. IC37 (74S04) pins 1, 2, 3, 4 and 5, R101, R102 (both 180Ω), C120 (100pF), the crystal (X1) and the adjustable trimmer capacitor (C124) will be especially sensitive to fingers or metal objects.

If you can see the problem with your 'scope, it could be a problem with IC37. But as these are not so easy to get these days, I don't recommend changing it unless there is a good reason to.

Mark

Thanks Mark,
That's extremely helpful. This is what I get at pin 6 of IC 37. It's a steady frequency and can be varied slightly with the trimmer cap as you say
Does it look ok?
When I set the frequency at pin 6 to the stated crystal frequency (17.7345 MHz), I still get the jumpin but much less, but this stops if I go above 17.38. However the RF colour goes at or above this point.
It's a bit intermittent. I kept thinking it's solved, but it's still there.
This is the signal I get at pin 4 of the RGB..
The positive pulses keep going low many times a second.( Seen as a rapid line flickering and moving at the 0v baseline.
I captured one below: The frequency in the low states does not appear to change with the increase or cessation of the screen jumping when I increase or decrease the crystal frequency, but I can't tell if they are longer in time.