ZX81 1983 Issue 3 Board, which has two initial problems.

[RedPete]

ZX81 1983 Issue 3 Board, which has two initial problems.

I have fitted a ZX-WESPI ready ZX81 composite mod from You Make Robots.

I have also purchased a new ZX81 Keyboard as the old one was ripped, this keyboard is from ZX Renew.

The new keyboard on inspection looks ok, when i fit it I don't get all the keys working.

1st Problem
The keys 1,0,Q,P,A, Enter, Shift and Space fail to be recognised.
I have checked for continuity from the various solder pads on the mainboard to the keyboard connectors on the ZX81 and they all seem ok.

2nd Problem
The display initally looks great clean and steady but then starts to flicker / flash after a few seconds. It does this with or without the keyboard connected, I am wondering if I have a ULA issue.
ULA is a Ferranti 2C210E 8244

Any thoughts on these issues?

Kind regards,

Peter.

[mrtinb]

1st Problem
The keys 1,0,Q,P,A, Enter, Shift and Space fail to be recognised.
I have checked for continuity from the various solder pads on the mainboard to the keyboard connectors on the ZX81 and they all seem ok.

You can try to disconnect the keyboard, and short one of the solder pads from the 5-group to one of the solder pads from the 8-group. This should "press a key" and you can see it on the screen. You can try all 40 combinations this way. If this works, the ZX81 is ok, and the problem is with the keyboard. If the test does not work, then it might be a broken ULA IC.

2nd Problem
The display initally looks great clean and steady but then starts to flicker / flash after a few seconds. It does this with or without the keyboard connected, I am wondering if I have a ULA issue.

The flickering has more to do with your monitor/tv than the ZX81 itself. You can use all the energy trying to fix the ZX81, but if your monitor/tv only wants a perfect signals, it will never work. You need an older and more forgiving monitor/tv. You might solve the problem with a RGBtoHDMI device if you want to use modern monitor/tv.

[RedPete]

Thanks Martin,

I will try the solder pad solution you mentioned.

I think I expected the solution I have that works for the ZX Spectrum and some other composite devices to work for the ZX81, but as you say it's a very different computer.

I have another ZX81 which I have just acquired and will use that to perform some further tests.

[Lardo Boffin]

Does the composite mod have any adjustment? I fit ZX8-CCBS (and have fitted a lot) and they have two pots that need adjusting to get a good and stable picture:

https://www.ginger-electronic.com/en/re ... -zx81.html

Once adjusted the picture is superb on most TVs. But as Martin said some TVs do not like the signal even with these.

Do you have a VCR? Or a SCART splitter / multi SCART selector or similar? Sometimes routing the output through one of these helps.

[1024MAK]

With respect to simulating the keyboard, you can also use the metal contacts in the PCB connectors.

Unlike the ZX Spectrum where the entire video signal is generated by hardware (the ULA mainly), in the ZX81, most of the work is done by software, as in the Z80 executing code (if the Z80 is unable to run the display system, you get no picture at all). Hence the signal is not fully to the video standard.The

You can see this effect by using FAST mode. With this selected whenever BASIC is not waiting for user input (on the command line or input command) no video picture is produced.

Use SLOW to switch back to the normal compute and display mode.

I do recommend trying other TVs/displays to see if they are better or worse. Modern LCD TVs/monitors sample the input video signal and then work out what to display. Hence some are less tolerant of non-standard video signals than others.

Mark

[Lardo Boffin]

Is it one of these?

https://www.youmakerobots.com/sinclair/ ... it-without

If so there are two pots that look adjustable? Might be worth taking a photo of where they are now (to allow easy roll back) and adjusting them.

[1024MAK]

An I right in thinking that this is the composite video board you have: link

ZX-WESPI READY ZX81 COMPOSITE MOD

There are some adjustments. As I don't have one of these, I am unable to advise on how to best adjust these without more information. A better, close up picture of the board would help.

Mark

Edit: crossed with Lardo's post...

[RedPete]

Thank you all for the suggestions.

It is the composite video board you mentioned

It has a ZX-WESPI connection, hence the extra unconnected cables, which I am planning to use once I can get the keyboard and video working.

I will try those two pots at the top after making a note of the locations as suggested.

I have two different component solutions to HDMI I am using at the moment, I will also try a a component to scart and the OSSC I have, I might also be able to hook the component directly to an AV amp to output to HDMI, so a few options to try.

Again, thank you all for the helpful and quick suggestions

Kind regards,

Peter.

[RedPete]

Just tested the keyboard joints as suggested, KB1 (4) left most point as you look at the MB seems to be faulty, so suspect the ULA as others have suggested unless the ROM is fault (?)

Guessing the the ULA may or may not be affecting the screen as well, I tried changing the pots on the composite mod with the only difference being the screen eventually became unreadable with the flickering still happening, I will perceive with other composite mod sources to see if that improves.

I'll socket the ULA in this ZX81 and order a vLA81 replacement.
I can also socket the ULA on the spare ZX81 and remove that and try that as well in this ZX81, the spare is from an Issue 1 board but presume they are cross compatible.

Plan is to have two (at least ) working ZX81's with different modifications in each.

[1024MAK]

Just tested the keyboard joints as suggested, KB1 (4) left most point as you look at the MB seems to be faulty, so suspect the ULA as others have suggested unless the ROM is fault (?)

Unlikely to be a ROM fault. Mask ROMs normally fail in a fatal way if they fail at all (they are normally the most reliable chip on a Zeddy).
But do check the solder joints on all the connections for this track (circuit) including the ULA pin and the "pull-up" resistor. This is of a schematic (disregard the video circuitry, this happens to be a French version).


http://fetrmartin.free.fr/ZX81/schema_ZX81_Fr.gif

There are three ULA versions. They will work in either board version (issue one or issue three). More details about the versions in the link in my sig below.

Mark