ZX80 kit no clock (now no display)

[Paul]

That seems quite close

[Boydie]

Okay, so the machine now has a 6.5MHz clock.

I was using a uhf->composite converter, with backporch. Powered it up and no display.
The CPU, ROM and RAM all seem to be working and have activity on address and data lines.

So I swapped the composite converter back to a UHF modulator.
Best I get is a poor quality black screen, with a vague outline of a black square, which moves when keys are pressed.
Presumably this explains why the composite converter didn’t work - a poor quality black screen with a ghost cursor probably doesn’t generate enough signal.

Any hints on what could be the issue? Could it be the capacitor value that’s wrong (although I chose the value that gave the closest to 6.5 MHz)?

[1024MAK]

This isn’t a frequency issue.

This is a DC level issue with the video circuitry. Or a digital signal is missing somewhere in the video circuitry.

Tell me or show me what you have connected to the video circuitry.

Mark

[Boydie]

That hint may have solved it.

The BOM on the site I bought the board from lists R31 as 1K and R33 as 5.6K.
From other threads, I’ve now discovered that you’ve previously said R31 should not be fitted (it was intended for other markets).
Opinion seems to be divided on R33. Some PCB photos online have it fitted, others don’t. The revised schematic has it greyed out and doesn’t give a value.

I’m assuming R31 acts as a pull-down to attenuate the signal and R33 pulls it up to make it stronger (in noddy terms)? So fitting both will (a) defeat the object and (b) result in overall attenuation because R31 is lower?

I’ll remove R31.
What’s the general consensus on the need for R33?

[Boydie]

Partial success...

Removing R31 has resulted in the following behaviour:

On power-up, the screen is blank until a key is pressed - took me a while to realise this one!
Then I get a white screen with a black square at the bottom (no 'K'). This (for want of a better word) cursor will then advance across the screen with every keypress. Occasionally, a second black square will appear (I'm guessing some kind of syntax error) before the original cursor carries on moving.
The entire screen remains white - no text appears.

The presence/absence of R33 makes zero difference.

Edit:

Reading around, I've found other threads that suggest the problem is something to do with IC9. I'm investigating. Trying a different 74LS165 hasn't made any difference (but they were sourced from the same supplier at the same time). Tests as working on my TL866 (not definitive, but better than nothing)

[1024MAK]

R31 and R33 should not be fitted for a U.K. board.
R30 should be 1kΩ
R32 should be 330Ω

Resistors in the video circuitry

Only the input of the modulator should be connected to the junction of R30 and R32. Or, the modulator should be disconnected and the signal sent to a suitable composite video circuit that includes active electronics to buffer the signal.

On power up you should get a screen that is completely white (well, very light grey) with an inverted K in the bottom left. You don't need to press any keys to get to this stage.

White screen with an inverse K

The ZX80 produces a video picture whenever it is waiting for user input. It does not produce a picture when it's NOT waiting for user input (unless you have a "flicker free" game running).

Whenever you do press a key / type, the screen will flicker. Some TVs, monitors, converters don't like this.

Mark

[Boydie]

After a bit of digging:

All data pins on IC9 are active
Pin 7 is somewhat intermittent
Pin 2 shows a 6.5 MHz clock, but the voltage is only 0.01V, whereas it's leaving IC20 Pin 3 at a somewhat more healthy 1.2-1.3V. This seems a bit meagre. This is true for both my lad's kit and one that I'm building. R2 is 470ohm as per the schematic, and continuity either side of C2 (definitely 47pF) , to IC9 and IC20 is fine.

But the signal voltage seems to be inappropriately low, compared with measurements on other threads...

[Boydie]

R31 and R33 should not be fitted for a U.K. board.
R30 should be 1kΩ
R32 should be 330Ω

42F72182-3CFC-4A4F-B6B8-17E506ACEE3A.jpeg
Only the input of the modulator should be connected to the junction of R30 and R32. Or, the modulator should be disconnected and the signal sent to a suitable composite video circuit that includes active electronics to buffer the signal.

On power up you should get a screen that is completely white (well, very light grey) with an inverted K in the bottom left. You don't need to press any keys to get to this stage.

The ZX80 produces a video picture whenever it is waiting for user input. It does not produce a picture when it's NOT waiting for user input (unless you have a "flicker free" game running).

Whenever you do press a key / type, the screen will flicker. Some TVs, monitors, converters don't like this.

Mark

Thanks, Mark. That's what I thought should happen.
R30 and R32 are as you say. R31 and R33 are now removed.
I get the white screen and a black square (definitely not a K). The black square moves but leaves a trail of white behind it.

Are the measurements I'm getting at IC9 pin 2 within spec? They seem very low.

[Boydie]

Progress:

It *was* IC9, in both machines. I swapped the new 'RS' 74LS165s for really old 'TI' ones from a Nascom 1 Graphics Board, and both machines now have a working display (one better than the other). They're still not perfect...

Machine 1 still doesn't do anything on power-up until a key is pressed. At which point, the display fires up but the key press hasn't been acknowledged on the screen.

Machine 2 fires up straight away, but the white background is noisy as heck.

(2) will hopefully be a straightforward fix (famous last words)

(1) seems to be a very nebulous fault. Anyone got any suggestions where to start?

Edit: (1) seems to be a grounding issue somewhere, as touching any part of the ground plane also makes it start. It's getting increasingly flakey on the clock circuit. Again, could be ground issues, but there's only 0.4ohms between the ground plane and ground connector on the power socket

[Boydie]

Famous last words indeed. I've re-flowed everything in the video circuitry, checked all values. Best I get is

The other machine, with exact same parts, has a pristine white background. I can'd find anything different between the two machines, but there clearly is. Any tips where to look next?