A couple of ZX81s and an upgrade question

[TheFifth]

I've recently picked up a couple of ZX81s to add to my ZX Spectrums and Amstrad CPCs. My plan was to upgrade one as my 'daily driver' and keep the other stock in my collection.

My planned upgrades are:

1. Replace the regulator with a modern DC to DC convertor (Traco etc.)
2. Add a composite mod (one that can add the back porch)
3. Replace the Z80 with a modern CMOS one that runs cool
4. Add 16k internally.

This will give me a usable ZX81 with all the niceties I want added internally (no RAM-pack wobble, no excess heat etc.).

Looking at the boards inside the two ZX81s, one is a well used Issue 1 and the other is a very good condition Issue 3. Both seem to work and produce a picture on my modern LCD TV (albeit a poor one via RF). Now the question is, which do I keep stock and which do I perform the upgrades on?

My instinct was to keep the Issue 1 stock, however someone has already been in the Issue 1 and performed some updates. Judging by the rubber feet and the screws, I was the first person into the Issue 3 machine and the board is immaculate inside.

The issue one has had a turned pin socket added for memory and it's been upgraded to 2K (around 1988 if the date code on the 2K Chip is anything to go by). Headers and a jumper have been added at L1-L2. All the chips have had heatsinks fitted and an extension has been added to the 7805 heatsink. Also, the extension socket connections have been re-tinned. All this work has been completed very neatly.



So with these mods already on the board, I don't feel as bad about making the extra changes. Also, as a socket has already been added for the memory, it will make my life far easier when it comes to adding the 16K expansion.

The Issue 3 has 100% soldered chips, so I would need to desolder the memory (2 chips) and the CPU to make the upgrades I want. Something I'd like to avoid if possible.

So what do you guys think? Should I be precious about making further changes to the Issue 1 board, or doesn't it matter? The board is not really in 'collectors' condition anyway (although the exterior case is in really good shape). It's obviously been well looked after and well loved given the external condition and the upgrades it already has. I do plan to do everything in a reversible way.

Are Issue 1 boards that rare anyway? How many were made in comparison to the Issue 3s?

Sorry for so many questions, the ZX81 is new to me, so I'm sadly lacking in my knowledge!

[patters]

I did something very similar to the two I had bought. From my own experience, make sure you test tape loading on both of them before you invest any time modding. One of mine turned out to have a defective tape loading part of its ULA which I didn't test until after I had done all the same mods on your list.

[TheFifth]

Thanks for the heads up on that. I'd have been pretty annoyed if I'd completed everything and found that out!

I'm waiting for a new membrane to arrive at the moment, so once it does I will test loading on them both. One of the membranes seems OK, but I don't want plug it in and out for testing as that will no doubt break it, so I'll wait for the new one to arrive.

[patters]

The other tip is that the replacement keyboard membranes are nowhere near as sensitive as the original, to the point that gaming is almost non-viable on them since you need such firm pressure. So try to preserve the real thing if you can, by trimming the broken ribbon ends.

Another tip for fitting the Traco power regulator is that you should fit it on its side as although it looks like it would fit, it really doesn't. It's too tall and it prevents the two halves of the case from closing properly. You can see this if you inspect the shadow gap. I would recommend instead buying the Murata regulator that Retroleum sells for Issue 2 Spectrums.

Beware buying the composite mod on eBay. There's a guy selling ones which have a bad design. I wrote about it on sinclairzxworld

[TheFifth]

I did wonder about the height of the Traco. I have one of the Retroleum modded Murata regulators in my Spectrum, so that was my first port of call, however Retroleum is out of stock of them at the moment. I've seen images of people mounting the Traco horizontally, so will do the same.

I've purchased the composite mod and membrane from ZX Renew. It's the one with DIP switches and pot adjustments, so hopefully that will be OK. Not seen any issues reported with them (although not looked too hard!).

I'll try to salvage the membranes if I can and keep the new one for testing. I'm sure I'll be plugging it in and out a lot during testing, so hopefully the new one will hold up. I'll try to put things back together with the original (trimmed down perhaps) and hope for the best!

Did you have any opinion on which board I should upgrade? The Issue 1 or Issue 3?

[1024MAK]

Now the question is, which do I keep stock and which do I perform the upgrades on?

That’s your choice, but as the issue three board has all the chips soldered, I would upgrade the issue one board.

Should I be precious about making further changes to the Issue 1 board

No!

Are Issue 1 boards that rare anyway?

They are not rare. In fact, if only talking about ZX81s (rather than the TS1000 or other versions), you are just as likely to get a ZX81 with a issue one board or a issue three board on the second hand/pre owned market in the U.K.

Pictures of my ZX81 fitted with a RECOM 5V Regulator
Modifying a Sinclair ZX81 issue 1 - RAM

Mark

[patters]

I'll try to salvage the membranes if I can and keep the new one for testing. I'm sure I'll be plugging it in and out a lot during testing, so hopefully the new one will hold up. I'll try to put things back together with the original (trimmed down perhaps) and hope for the best!

Exactly what I did. It's useful to have the new one if only for testing.

[TheFifth]

They are not rare. In fact, if only talking about ZX81s (rather than the TS1000 or other versions), you are just as likely to get a ZX81 with a issue one board or a issue three board on the second hand/pre owned market in the U.K.

Awesome, this is what I wanted to hear. Not being that knowledgable about the ZX81, I just wanted to make sure I wasn't about to do something stupid with a rare version or something!

I shall await the arrival of all the bits and let you know how I get on.

Thanks for all the input!

[TheFifth]

Just thought I'd add an update now that everything has been completed. It's all taken a little longer than I'd hoped as a supplier let me down on the memory chip.

I've completed everything in a reversible way, so the 16k memory has lifted pins. It looks a bit dodgy, but works fine. I didn't want to gut the modulator, so I've kept the composite mod on detachable leads and it's now fixed inside the case with a sticky pad. As suggested, I mounted the Traco on its back. Finally I've installed a modern Z80, which runs very cool.

I managed to salvage the membrane on the Issue 1 board, but the Issue 3 membrane was completely gone. No matter how much I trimmed it down, it just split a little further up each time, until it broke right at the very top. So that's been replaced. The membrane I got from ZX Renew seems to be one of the original reproductions that were thicker. When put in place you can see the white edges of the membrane material, so it doesn't look that great in my opinion. I might grab another one from RWAP (https://www.sellmyretro.com/offer/detai ... rd%29-2529) as they say the ones they supply now are of a higher quality and closer to the original. I think the date on the one I had was 2012 or 2014 (can't remember and it's all buttoned up now). Does anyone have any experience of the latest RWAP membranes? Are they thinner and closer to the original?

It's not the neatest job I've every done, but it works really well. In fact I'm amazed by the picture quality! With some (very) careful tweaking of the onboard trimmers, I've managed to get a completely clear, incredibly sharp image. It puts my Spectrum to shame!

Here's the Issue 1 before it went back into its case:



And finally, here's both ZX81s back in their cases. The Issue 1 with all the mods is on the left and the Issue 3, which is completely original apart from the swapped out membrane is on the right.



Thanks again for all the advice, it's been fun playing around with these little computers!