Sinclair ZX80 / ZX81 / Z88 Forums

Hi, I recently bought a reburbished ZX81 which has the vLA81 modern ULA replacement, 32K RAM and a composite mod.

Now I'm really happy with this as it mostly works great, and I've had a lot of fun playing all the old games I played as a teenager, and typing in some programs for fun.

With the composite mod It gives a sharp, bright, high contrast picture which is way better than the one I had on my original ZX81on a B&W TV back in the 80's.

But the problem I do have is that the screen can't sync while loading - it either goes completely black (On a relatively modern Sony CRT portable TV), or gives a blank screen with occasional flickers on an old dumb CRT analog monitor (Actually a Commodore 1702 monitor which is almost blasphemy using for the ZX81!) Also fast mode gives a black screen instead of the grey I remember.

I am using a ZXUITape emulator to load tapes, and it works pretty reliably to load the games, it's just the screen is blank while loading.

I feel I'm missing out on the authentic loading experience here (funny how this makes such a difference to the 'feel' of using the machine). Is this an issue with the ULA replacement or do all ZX81's with composite mods suffer from this? I have another ZX81 (an issue 1 with no back porch signal) with a battered case I was thinking of composite modding to see if that works better.

Thanks!

It’s more to do with the TV/monitor.

Only when the ZX81 is running in SLOW mode, or is waiting for user input in FAST mode, does it generate the correct signals for a TV or monitor.

When running a program in FAST mode, no valid signal is generated for the TV / monitor.
When loading or saving from/to tape, a non-standard signal is sent to the TV / monitor.
In both cases, the TV or monitor, especially if it’s relatively modern (even if it is a CRT type), will use specialist chips that will be confused by this, and as they can’t detect a valid video signal, will just blank the screen.

Mark

Thanks for the quick reply.

What confuses me is that an old analogue composite video monitor from 1984 doesn't display a proper picture while loading either. I'd have thought that this would work as it doesn't have any fancy signal processing. I've not seen any mention of this issue anywhere else.

I'm just trying to work out if it's worth refurbishing my other ZX81 to see if it gives a more authentic loading experience. Possibly just adding a back porch generator rather than a composite mod with back porch as the TV has an RF input.

There is also the option of changing the vL81's dip switches as one of those apparently alters the signal strength.

Cheers.

If you want a good picture and loading screens try a Chroma 81 (http://www.fruitcake.plus.com/Sinclair/ ... bility.htm), plug it in the back (no mods required to the zeddy) and attach to a telly via a SCART cable. I’m pretty sure you get a loading screen when using tapes.
You also get the option of colour amongst other cool features.

Wow! That Chroma interface looks amazing. Unfortunately it's sold out otherwise I'd be buying one right now.

Thanks for the info. I'll keep looking out to see when it becomes available.

Ravenger wrote: ↑Wed May 12, 2021 12:01 pm Wow! That Chroma interface looks amazing. Unfortunately it's sold out otherwise I'd be buying one right now.

Thanks for the info. I'll keep looking out to see when it becomes available.

Its well worth the wait!