ZX80 *extremely* low video output

[UKRetroGamer]

Hi

I just bought a "working" ZX80 from eBay and whilst it (sort of) works, the video output is so low, I can only just make out the characters.

This is on an old "tube" Sony Trinitron set from about 1990.

I'd hate to think what it'd look like on my 2 year old Bravia!?

Is there a particular component (output transistor or similar?) which could easily be replaced to bring up the video output?

Are they easy to mod for composite output?

It's a really nice example with very little yellowing and no marks. I'd really like to restore it to its former glory.

Regards
Mark

[sirmorris]

Hi Mark,

first things first - if you can possibly try it with another CRT tv then that would eliminate one thing from enquiries.

The least intrusive composite mod is simply soldering to a ground point and the video input to the modulator. If it's the video stage of the machine which is at fault you'll see it in the raw signal. If the composite is clear then the problem lies in the modulator. Unlikely though.

Do you have access to a scope?

C

[UKRetroGamer]

No. I don't have a scope. I wouldn't know what to do with one...

My multimeter has a capacitor test and I also have a tool made by Peak (An Atlas DCA55 IIRC - It's downstairs JATM) which can be used for testing Transistors & Diodes, etc.

I'll strip a cable later and test the composite output. Is it *really* that simple?

[sirmorris]

The video signal which feeds the modulator is more or less composite - I think the voltage levels are different - but it will work. I've got a couple of '81s which work with this hack.

C

[Fruitcake]

It sounds like this could be due to the lack of back porch signal within the composite video output (the same issue that affects early ZX81s). It certainly sounds similar to my ZX80 on certain TVs. If it is due to this issue then it is possible to improve the ZX80 video output by attaching a very simple clip on circuit.

The circuit is explicitly designed to attach to the legs of resistors on the ZX80 circuit board and so allows the possibility of using clips to connect the circuit. This means that there is no need to solder to the ZX80 board or cut its tracks, and so the new circuit can be easily disconnected thereby restoring the ZX80 to its original state without any sign it was ever present. The circuit derives a back porch signal from the sync signal and then mixes it into the composite video signal. The resultant video signal is still not compliant to TV standards but is closer to it such that a wider range of TVs will happily accept it.

The circuit requires only 3 readily available and cheap components (1 logic IC, 1 resistor and 1 diode - all available from Maplins) and can easily be built on a small piece of stripboard. The biggest issue is to locate small enough clips to allow the circuit to be attached and fit under the top case of the ZX80.

You can find details of the circuit and photos showing the output of my ZX80 with and without the circuit attached on this page of my website:
http://www.fruitcake.plus.com/Sinclair/ ... ration.htm

I hope it helps.

[UKRetroGamer]

Thanks fruitcake.

The image I get on all my TVs is like this one (borrowed from your web-site):

As it's a good 20 years or so since I built anything on Veroboard, would you be willing to make a circuit up and let me pay you for it?

BTW, the image which shows how to fit the board apears to be missing from your page.

Thanks ever so much for your suggestion.
Regards
Mark

[Moggy]

Or If ,like Me, You are No good at reading schematics seek out an old style closed circuit tv monitor then You only have to solder one wire to the modulator input and one to the modulator case itself. Plug the lead end into the monitor job done crystal clear picture always worked for Me,
but, as stated elseware, no good on modern tv's just another angle to try though.

[Fruitcake]

As it's a good 20 years or so since I built anything on Veroboard, would you be willing to make a circuit up and let me pay you for it?

BTW, the image which shows how to fit the board apears to be missing from your page.

Hi Mark

Thanks for pointing out about the photo. Yes I am aware of it and will be uploading one soon.

Regarding the circuit, the only difficulty would be in how to attach it to the ZX80 PCB. In theory clips can be used, and this is what I did when developing the circuit. However, I had the circuit on a breadboard and connected via long leads and large clips to the the PCB. The circuit itself should easily fit under the hood of the ZX80 but the difficulty would be in finding small enough clips that would also fit. I don't have a solution for this at present, although I have not actually spent much time searching for suitable connectors. You could solder the circuit to the ZX80 PCB but I can understand if you did not wish to do this. So at the moment I could build you a circuit but the issue of how to connect it would still need to be solved. Can you drop me an email via my website to discuss this further.