ZX80 kit no clock (now no display)

[1024MAK]

Using an oscilloscope, what do you get on IC9 pin 2? That's the pixel dot clock.
What about on pins 4, 5 and 6 of IC20?
Finally, what do you get on IC19 pin 5 and IC21 pin 1?

Pin 26 on IC1 (Z80A) is the /RESET pin. if you short this to 0V/GND, does that have any effect on the background interference?

Mark

[Boydie]

Thanks Mark.

I’ve been trying to get the scope to work all day. For some reason (likely ineptitude), it just won’t give me the clean signals I get on other machines, and I see on this forum.

I’d say it was down to the machine, but I can’t get anything better on the one with the normal background. Planning in spending tomorrow banging my head against the wall…

Shorting the reset pin to ground makes the screen go blank. When it returns the noise is the same.

[1024MAK]

For some reason (likely ineptitude), it just won’t give me the clean signals I get on other machines, and I see on this forum.

Is the 'scope GND clipped to a suitable point - e.g. the metal tab or heatsink of the 7805 (if you are using this type of course) or the case of the modulator?

Are you using a X10 probe with both the probe and the 'scope set to the X10 setting?

What PSU are you powering the board with?

Shorting the reset pin to ground makes the screen go blank. When it returns the noise is the same.

Shorting this stops the processor ONLY. As the processor generates the screen display, the screen should go blank. So the processor can start up okay from being reset.

I think there is an open circuit fault that is causing IC9 to pick up garbage and convert it to pixel data. Or something is missing a 0V/GND connection.

Attached are some 'scope waveforms taken from my ZX80. Please note that I used different timebase settings for some of them. I have compressed them in a ZIP file.

• R2 (IC9 pin 2) - pixel dot clock
• IC20 pin 4 video pixel data
• IC20 pin 5
• R30 RH side (IC20 pin 6) video pixel data
• R32 RH side (IC19 pin 5) SYNC
• IC21 pin 1

Mark

[Boydie]

Thanks. I’ve tried two Sinclair PSUs and a 5V wall wart, soldered direct in. All have the same result.

Thanks for the tips on the scope. Coupled with an evening with the manual, I’ve managed to get some results.
Only IC20 pin 3 and IC9 pin 2 are as you’d expect (about 50% attenuation between the two).
I’ve noticed that the signal on IC9 pin 7 (and IC20 pin 4) have a background low-amplitude square wave even when IC9 isn’t fitted. Which strikes me as odd. Adding IC9 gives an overall bizarre waveform.

I subsequently realised that I can’t tell the difference between 47nF and 47pF so had to change all the caps that should have been nF.
Rectifying that little boo-boo has improved the quality of the background lines, but hasn’t removed them. The overlying text is very readable.

I’ll try the scope again over the next few days.

[1024MAK]

I’ve noticed that the signal on IC9 pin 7 (and IC20 pin 4) have a background low-amplitude square wave even when IC9 isn’t fitted. Which strikes me as odd.

Are you sure there isn't a short (e.g. solder splash) between the tracks for that circuit and another circuit?

Mark

[Boydie]

I agree; there *has* to be one somewhere!

But even with the "advantage" of -10D myopia (makes for one heck of a close-up zoom) I can't see the slightest hint of anything. I'll keep looking because I'm convinced you're right (and not just because of your track record).

I'm temporarily demoting it to "rainy day project". It's getting stupidly time-consuming. And the fact that the power supply is now being awkward (output voltage has started dropping to zero on load), I'm taking the hint.

In the meantime, the best I get is:

Legible, but annoying

[1024MAK]

Do you know the frequency of the unwanted signal?

Mark

[sP1d3r]

I was looking on eBay recently and I saw this;

https://www.ebay.co.uk/itm/256659232251

There's a picture of a built up board and I notice that the regulator heatsink is smaller than the one that you have, perhaps the heatsink is contacting tracks on the PCB?

Worth checking, I'd say.

[Boydie]

That one’s a definite no. I went for total overkill on that insulator - it’s 1mm thick plastic, dimensions slightly larger than the heatsink. Partly out of paranoia, but mainly because I didn’t have anything more subtle.

On the other hand, the insulator under the modulator is less substantial. I’ll double-check that….