About to take ownership of a ZX81

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SpaceFlightOrange
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About to take ownership of a ZX81

Post by SpaceFlightOrange » Wed Aug 28, 2019 1:32 pm

Hi.

Apologies if this is the wrong place to ask this question. I'm about to buy a ZX81. The current owner has had it from new and he says its been stored in its box since the 80's and he's not attempted to turn it on since then. It's Poly boxed with a boxed 16k ram pack.

This, i'm happy about, but my question is what kind of tests can I perform on the ZX81 before i attempt to turn it on. I will do a re-cap first also.

I saw the excellent video by JoulsperCoulomb which goes through a series of tests for the Spectrum, and would like to be able to do something similar on the ZX81.

Thanks
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1024MAK
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Re: About to take ownership of a ZX81

Post by 1024MAK » Wed Aug 28, 2019 4:29 pm

There is no need to open the Zeddy, and no need to replace any capacitors before trying it.

Unlike the ZX Spectrum, it only needs a single +5V DC rail, which is provided by a 7805 voltage regulator.

There are in fact, only two electrolytic capacitors in a ZX81. One is used to in the Z80 reset circuit, the other helps smooth the +9V (nominal) supply. Both are non-critical, as in, if they are faulty, powering the machine up will not cause any further damage.

The (nominal 9V) mains power supply has a greater unloaded output voltage (compared to a ZX Spectrum 1.4A PSU), typically around 14V to 16V. So don’t worry if yours has a similar voltage.

The ZX81 uses either a single 1K byte static RAM (SRAM) chip, or two 1K 4 bit wide SRAM chips (combined, this gives 1K byte).

If anything if faulty, you will not get the white screen with a inverse K in the bottom left of the screen. So to test it, start with:
1 REM 234567890QWERTYUIOP
Then continue to test the other keys...

Keyboard membrane failure is a common fault.

I’ll post a link to another post where there is a simple BASIC memory test later.

Don’t try the RAM pack until after trying the Zeddy on its own.

Mark
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Re: About to take ownership of a ZX81

Post by SpaceFlightOrange » Wed Aug 28, 2019 8:06 pm

Thanks Mark. I should get a hotline installed that goes right to your desk! :lol:

I was browsing the schematic before, and I couldn't see anything other than the +5v rail. so its good to know I hadn't missed it, and that I don't need to worry about turning it on.

I'm meeting the guy on Saturday, and hopefully if all ok, I'll take it away.

I notice that RWAP make membranes. Given the success of the RWAP Spectrum+ membrane I'll pick one up should I need it.
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Re: About to take ownership of a ZX81

Post by 1024MAK » Wed Aug 28, 2019 11:16 pm

The simple RAM test, well really printing out the results of the very basic system RAM ‘how much RAM is there” results... See here.

See also here (bottom of the page) ;)

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Re: About to take ownership of a ZX81

Post by 1024MAK » Thu Aug 29, 2019 5:31 am

Here is a version for a 1K ZX81:

Image

Image

Note that after running this, enter CLEAR then press return ;)

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Re: About to take ownership of a ZX81

Post by SpaceFlightOrange » Sat Aug 31, 2019 10:00 pm

Thanks Mark.

I picked it up today. The guy who was selling it was a really nice man in his eighties and he's had it from new, but hardly used it. This is reflected in the condition of the machine and the PSU. both look new. All feet intact, etc.
The poly box is in a bit of a state, the bottom half being fine, but the top in 3 pieces.

Image
Image

We met and had a coffee, and he told me a sad story. He had been in an accident 2 years ago, and suffered serious brain trauma which led to severe bleeding on the brain and damage. He got through it, but his son now handles all his affairs and has told his to clear everything out before he dies.

The ZX81 is one of those things. Unfortunately he copied parts of another advert to make up his, and it didn't have a ram pack. He was very apologetic about this and, given the condition and the fact I felt for the guy, I bought it anyway, so what I thought was a bargain, turned out to be about average price.

So. It works. Had trouble getting a picture, I tried 3 LCD's before I found one that worked, though the picture was very dim.

The keyboard membrane works perfectly.
I tried the memory test and that works perfectly.
I haven't opened it up as all feet are intact, but I shone a torch through the expansion port and found that its an early issue 1. (radial caps). but the big give-away is the piggy back board for the "SQRT" fix (I believe).
Based on this I assume its the early ULA that is missing the "Back Porch" signal. (yes, I've been reading through some threads :lol: )

So. Im stumped. don't know what to do with it. I'd be tempted to make a composite mod and run it on my little B&W CCTV, but from what I can tell its not as straightforward as a spectrum mod. I don't really fancy buying a mod that costs more than I paid for the ZX81.

Right now its sitting in my display cabinet. I think I might just leave it there.
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Re: About to take ownership of a ZX81

Post by 1024MAK » Sat Aug 31, 2019 10:41 pm

So what you have is a Zeddy with the first edition ROM chip (with the mathematical error) and either the first ULA version, or the second ULA version. Both don’t generate the back porch part of the video signal (hence the dark screen on some colour TVs).

I use Pokemon’s ZX8-CCB modules ;) These provide a composite video output that adds the back porch to the video signal. And the best bit, is that there is no need to disturb the keyboard membrane tails when fitting it (assuming you can solder) :D

It is also possible to upgrade a ZX81 to have a 16K RAM chip on the main board. However, this does mean disturbing the fragile keyboard membrane tails, as it requires the PCB to be removed. Ask if you want details.

Alternatively, pick up a Memotech RAM pack. These tend to be more reliable than the Sinclair RAM packs. The 16K byte type are the most common.

I would suggest a ZXpand, but I don’t think Sir Morris is producing any at the moment, and I have no idea if/when he will restart production.

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Re: About to take ownership of a ZX81

Post by SpaceFlightOrange » Sat Aug 31, 2019 11:16 pm

1024MAK wrote:
Sat Aug 31, 2019 10:41 pm
I use Pokemon’s ZX8-CCB modules ;) These provide a composite video output that adds the back porch to the video signal. And the best bit, is that there is no need to disturb the keyboard membrane tails when fitting it (assuming you can solder) :D
Ah. That's more like it. I was looking at this which cost more than the ZX81 so I was bit put off. This is good, because since the keyboard is working fine, I really don't want to touch it. This brings me onto another question. The 2 capacitors. Do you think they'll be fine left alone? I know you said they don't really affect the operation of the machine.

Soldering is no issue. I've had plenty of practise trying to keep my Beeb alive :lol:

One more question. How do you tend to fit a connector for the ZX8-CCB? do you modify the case?
1024MAK wrote:
Sat Aug 31, 2019 10:41 pm
I would suggest a ZXpand, but I don’t think Sir Morris is producing any at the moment, and I have no idea if/when he will restart production.
I'll have a look out for a Memotech. I was looking at the ZXPand on SellMyRetro, but it only seemed to be cases and addons. This explains it.

Thanks again.
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Re: About to take ownership of a ZX81

Post by 1024MAK » Sun Sep 01, 2019 1:07 am

With regards renewing the two electrolytic capacitors on the ZX81 PCB, unless you experience problems, it’s fine to leave them be.

See also these threads over on Sinclair ZX World:
https://www.sinclairzxworld.com/viewtop ... f=7&t=1407

https://www.sinclairzxworld.com/viewtop ... f=7&t=2251

https://www.sinclairzxworld.com/viewtop ... f=7&t=2967

More important is the capacitor in the ZX PSU. That I do recommend renewing. Use a 105°C type.

On the subject of PSUs, if using without a 16K RAM pack (or if using a higher capacity RAM expansion) the ZX81 only needs enough voltage to keep the 7805 voltage regulator happy. So will work with a regulated PSU that outputs between 7.5V DC and 9V DC. A regulated PSU does not vary its output voltage regardless if loaded or unloaded. The use of such a PSU will reduce the amount of heat that the 7805 dissipates via the heatsink.

Note that some RAM packs (like the Memotech 16K) actually need the power input to the ZX81 to be between 10.5V and 12V.

If you do decide to disturb the PCB, then the other option is to fit a modern DC/DC converter in place of the 7805. I have fitted RECOM types before (see this thread) but other makes should also be okay. These produce very little heat, so don’t need a heatsink. They can be used with a ZX PSU.

With regards to fitting a ZX8-CCB, if I am disturbing the PCB, I take the lid off the modulator, remove some components to make space, fit the ZX8-CCB inside and then use the existing phono socket as a composite output.

If I am not disturbing the PCB, I connect a phono line socket to the ZX8-CCB so that I can route it and the cable for it just outside the case. The other option is to neatly fit a chassis phono socket to the Zeddy case. Not actually done this to a ZX81 yet though (I have to some other machines, see my Atom over on StarDot for one example).

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