Sinclair ZX80 / ZX81 / Z88 Forums

Hi everyone. New here but already asking for some advice I’m afraid.

I’ve acquired a ZX81 and I’m getting a poor picture (no contrast) on a modern TV. I’ve been following the well-known Video Conditioning mod instructions and trying to mount externally (leaving the modulator connected, to try and be as nondestructive as possible). I’ve run out +5v, 0v and Video through the side of the case.

Once or twice, on breadboard, I’ve got a beautiful clear bright picture, free of any ghosting etc, getting my hopes up. Every time I’ve transferred it to stripboard it fails, I just get a grey, barely visible picture again.

Now I can’t even get it to work on the breadboard (except, once or twice, I connected +5v to 555 pin 4 (reset) but not to pin 8, and strangely I got a clear picture but with the back porch encroaching on the left-hand side, similar to an effect I think others had seen in another thread where there were timing problems. This is no longer working at all though, so how it happened isn't clear.)

I’ve stuck as close to the spec (R, C, 1N4148, PN2222A) as per Zigg’s site as possible; I’ve tried replacing R1 and R2 with 10k pots and swivelling them around, switching out the 555 (I believe the ones I have currently from CPC are good TI NE555Ps), wiggling a bunch of components, and I’m wondering if I need to understand this more:

• Is there anything internal in the ZX81 that could be wrong with this machine and is known to possibly affect this? I’ve tried two TVs and never got a remotely usable picture from RF.
• Should I disconnect/remove the modulator? Should this work/not work while it is still there?
• Am I just bad at prototyping and should try again, I.e., is this most likely to be a contact/dead component problem, or rather a finicky timing one? I’ve tried a few times now with fresh parts.
• I don’t have an oscilloscope. Is it time to spend a few £100s now to diagnose a 95p mod?

Any advice is much appreciated even if it’s only to say everything I’m doing is a bad idea.

The video input connection to the modulator has to be disconnected, as otherwise it affects the output signal from the ULA.

Which ULA is in your ZX81? Can you post the part number or take a photo and post it on the forum please.

Also post a photo of your set-up including the breadboard or stripboard.

Mark

If you upgrade the ULA to version 210E or one of the modern replacements you should get good video. I use a legacy 210E ULA and run the original RF modulator and I've never had video issues.

Thank you both!

The video input connection to the modulator has to be disconnected, as otherwise it affects the output signal from the ULA.

This turned out to be the issue, thanks @1024MAK! I snipped video in on the modulator and the stripboard version I'd made up some time ago and had no luck with worked perfectly.

And yes I was about to post the picture (it's a 2C184E, issue one board) but followed your advice while I had it open.

(All of which raises the question, given I'm pretty new to this - would it be practical to create a version of this that works without disconnecting the modulator? Perhaps with different values? Or from an electrical point of view, not? Just from the sentimental angle of not cutting up old kit. Sorry if this is a childish way of looking at it)

Anyway thanks again massively, you've unblocked me where I was tearing my hair out for quite a while.

gbejniet wrote: ↑Thu Aug 05, 2021 10:10 pm (All of which raises the question, given I'm pretty new to this - would it be practical to create a version of this that works without disconnecting the modulator? Perhaps with different values? Or from an electrical point of view, not? Just from the sentimental angle of not cutting up old kit. Sorry if this is a childish way of looking at it)

Most likely, yes it would be possible. However, development is partially driven by demand, and these kind of requests are not very common…

Mark

There are some persons who would like to test a ZX81 without modifying it.
For them it would be useful to have the possibility to add video out as a test without modifying it.
I think of Lardo and me
On the other hand most useful would be some device to test the Zeddy that can be attached at the back without opening.
That's far in the future. A nice little winter project possibly.

Paul wrote: ↑Fri Aug 06, 2021 11:12 am There are some persons who would like to test a ZX81 without modifying it.
For them it would be useful to have the possibility to add video out as a test without modifying it.
I think of Lardo and me
On the other hand most useful would be some device to test the Zeddy that can be attached at the back without opening.
That's far in the future. A nice little winter project possibly.

I use a simple transistor mod with tini clamps for fast testing purposes... fortunately my TV doesn't complain much about back porch lacking