Sinclair ZX80 / ZX81 / Z88 Forums

I have a zx81 project that I have a few questions about.
I bought an old zx81 (paid way too much) and want to bring it back to life but discovered a few quirks and things that I need advice on.
1. Randomly even though I live in New Zealand for some reason this one turns out to be an American one. I have no idea why it was even in this country as NZ is a Pal territory. Not happy that it turned out to be an American one. I was going to get a composite mod from ZX renew so Im now wondering because this one was American, is the motherboard different and will the mod even work the way it should with a pal tv. Outwardly it looks identical to the standard English/New Zealand version aside from the RF output being slightly further back and there a ch2/ch3 selector switch on the bottom.

2. I have an official power supply on the way but I wanted to test it out so i used a standard 9v adapter for a few moments and realised very quickly that the polarity was wrong. it was maybe 30 seconds before i realised this and it generated some heat. Im worried I might have damaged it.

If anyone would help me with the above questions Id really appreciate it. To be honest i kind of feel like throwing it away after discovering it was an American one, but if I can bring it back and use it as if it was a regular pal one then the trouble will be worth it. Im hoping that using a compostive mod from ZX renew will be possible.

Hello and welcome gl3nnium

First, there are two versions of the ZX81 PCB (Printed Circuit Board). And one version of the TS1000 (the USA version of the ZX81 marketed by Timex) PCB. But a USA version of a ZX81 or TS1000 can use either a ZX81 PCB or a TS1000 PCB…

I hope that suitability confuses you

The important point being that which PCB you have does not matter. The ZX81 PCB was designed to be flexible so that it could be used in either 625 line 50Hz TV areas (now known as PAL, but as PAL is a colour system and the ZX81 does not generate colour, it technically is not PAL) or 525 line 60Hz NTSC areas.

So, to put it simply, yes if converting it to composite video, your ZX81 can (with suitable modifications) output 625 line 50Hz video.

Do however note that the modulator (the metal can) used in Europe has its output socket in a different position compared to the modulator used in 525 line 60Hz NTSC areas.

Can you please post a photo of your PCB (top, component side only) so that we can see if resistor R30 is fitted. For 625 line 50Hz video, R30 should not be fitted. If it is fitted, it can simply be cut out.

I would recommend the ZX8-CCB VIDEO MOD from a member of this forum. Lots of members here, including myself use this module to improve the video output from our ZX81, TS1000 or ZX80 computers. It’s both low cost and produces an excellent video picture.

I presume the module from ZX-Renew that you are talking about is this one or this one?
I’ve not tried either of these, but both are based on a design that has mutated from this design. They need to use a genuine NE555 timer chip. I have used a different version from another source, and unfortunately it did not work. The reason being that the ‘NE555’ timer chip used in the items supplied did not operate quickly enough, likely due to it not being a genuine within specification chip.

But now to your most important problem. If the PSU (Power Supply Unit, or power adapter/adaptor) that you used did subject your ZX81 to an incorrect polarity, then almost certainly you have damaged beyond repair the 7805 voltage regulator chip and one (or two depending on which version your board is) electrolytic capacitors. Hopefully the 7805 voltage regulator sacrificed itself quickly enough that nothing else got damaged.

The ZX81 is actually not that fussy on the voltage, but the correct polarity is very important. The tip has to be positive. The recommended voltage range can be anywhere between 7.5V DC to 12V DC for regulated DC PSUs (if not using a third party 16K RAM pack, 8V to 9V is recommended). Or anywhere between 8V DC to 10V DC for unregulated DC PSUs. The current rating must be 500mA (0.5A) or greater (more if using expansions, recommend 1A or 1000mA or more).

Do you have a multimeter or have access to one, so that you can test the PSU that you used? It’s much easier to diagnose problems if we know more about the cause. It may be that the PSU you used did not cause any damage.

If you have blown up the 7805 and damaged the capacitors, it’s not a big problem, as replacements are available and they don’t cost much. However, you may want to consider using a DC/DC converter (or ‘switcher’) switching regulator instead. These cost more, but don’t need the metal heatsink, hence are both far more energy efficient and result in the computer running far cooler.

Note that in a working ZX81 standard (as from the factory) computer, using a Sinclair PSU, the 7805 and it’s metal heatsink will get very hot (too hot to hold your finger on), the ULA chip will get hot in the middle, and the Z80 (or equivalent) processor will get warm (but not hot). The other two or three chips run cooler.

Mark

The difference between UK and US model is:

• R30 is fitted on US model. Just remove the resistor to be UK model.
• US model has 2k of RAM, but UK model only has 1k of RAM.
• On US model, the modulator outputs to channel 2 or 3 VHF. On UK model, the modulator outputs to channel 33 UHF.

So getting a US model is a great catch. You get double the RAM, and the setting of resistor R30 decides whether to use NTSC or PAL (however it is neither NTSC or PAL, as there are no colors).

I have place a jumper in series with the R30 resistor on my board, so I can change between NTSC and PAL. Some games works better in one of the positions.

http://www.mainbyte.com/ts1000/good_schematic_hi.jpg

Thanks very much to both 1024MAK & mrtinb.
Yes i'm assuming that in my stupidity I fried the ZX81 with the wrong polarity PSU. So I'm considering it a repair project now.
I don't know for sure that the polarity was wrong but it got so hot so fast that I knew something was wrong right away. On the plus side it came with a 16k ram pack so its not a total loss. I will post picks of the PCB when I can.
So...in frustration I jut bought another one from the UK and will be pushing ahead with that. I will be attempting to repair the USA one as a future project. Hopefully its just a replacement of the 7805 chip that was mentioned.
I also bought a couple of composite mods from ZX renew. one for an old speccy I have and one for the zx81. Its the first one that 1024MAK put a link to on that line.
https://zxrenew.co.uk/ZX81-Composite-Mod-p294875052

All of that will take a few weeks to get here.
I will continue to post here for the time being about the American one tht I probably fried. Hopefully I didnt damage it but that seems unlikely.
Picks of the PCB will follow shortly.
Looking forward to getting into it. I'm curious as to what I could find a 7805 regulator ship and what price that would be.

7805 voltage regulators are fairly common and not expensive from any good electronic suppliers. They are made by various semiconductor manufacturers. Buy one or two 22µF 16V (or higher voltage, as long as the physical size is not too large) electrolytic capacitors as well, as they don’t like a reverse voltage. The number depends on what’s fitted to your board. Position C3 (near the slot of the edge-connector) and sometimes position C13 (next to the 7805, but only on issue 3 boards).

7805 are less than £1 in the U.K.
An Axial 22µF 25V electrolytic capacitor is less than 50p in the U.K., although the better brand is nearer £1.50!

Do you have any electronic suppliers in your country? Or you could buy from eBay, but try to use a good trader.

For more on DC/DC converters or switchers, browse this topic (it now being a bit of an old topic, some of links may not work).

As you are in New Zealand, you may want to browse this site. The site owner, Charlie, is a member here. His username is Bloodnok. See his first post here.

Mark

Thanks very much 1024MAK. I opened it up this morning and took really detailed Hi-res pics of the motherboard. Really interesting stuff. I found the 7805 regulator right away. The motherboard seems very clean with no burn or damage to anything but i know that not necessarily an indicator.
Naturally the keyboard membrane connector was already brittle and broken, but I was expecting that. It's going to take time as often things have to arrive from the UK and that can take weeks. Id like to keep adding my progress here as I go. Im totally new to this and aside from basic soldering I don't have electronics experience so Im hoping people can help me as I go.
Id like to post a couple of pics of the motherboard here but this forum only seems to accept URL images. Is there any way I can post the pics i just took of the motherboard directly to this thread? Id really like to get some opinions from you guys.

Under the text edit box, there is an “Attachments” tab, click this. Or drag a photo into the edit box if using a PC and modern browser.

Do keep in mind that the forum software has file size limits, plus it may automatically reduce the resolution of photos and images.

So if there is a specific item you want to show in detail, process the image yourself (cut/crop) beforehand. For schematic and circuit diagrams, or similar images. Instead put it/them in a zip file, then attach the zip file.

Mark

Right thank-you. Wow I didn't look very hard did I. Okay here are two pics of the USA zx81 I will be doing my best bringing back to life. Most likely will be installing a composite mod and will probably have to replace the 7805 regulator (unless im very lucky). Thanks for your advice on components and where to buy. Ill mostly like try and source parts/components locally, or from Aliexpress and probably ZX renew in the UK for the specific stuff like a new membrne keyboard.
Hopefully the images wont be shrunk but ive also added them as a zipfile as you suggested.
(Sidenote - wow you can buy 7805 regulators in packs of 10 from Aliexpress for only $2.50 NZD. That's crazy cheap. I hope anything else I need will be that easy. Im interested in the Dc/Dc swticher option mentioned too. A local supplier aslo sells 7805 regulators for cheap too. those ones are rated as 7805 +5v 1A so im not sure how that is applicable...sorry for a questions...Im a total newbie but im keen to learn.)
Hey I also just noticed from the pic that the R30 resistor had been removed essentially making it the same as a pal UK ZX81 if I understand one of the above posts correctly.

Other sellers that you may find useful are:

• Retroleum
• Sell My Retro

The person that set up the above site is the person responsible for getting new manufactured membranes for various computers including the ZX81. Their membrane listing is here.

On your board:

• Modulator UM1082LA2/3 date code 8233
• IC1, ULA, is a 2C210E date code 8240, the last and best version. This has the back porch circuitry built in, so you don’t need any additional back porch generator.
• IC2, ROM, is µPD2364C type 649, the bug fixed and last version.
• IC3, Z80 CPU, is a NEC µPD780C-1 date code 8220
• IC4A & IC4B, SRAM, are NEC µPD2114LC, so you have 1K bytes of RAM.
• REG - standard 7805, 5V 1A voltage regulator.
• C3 (near the edge connector), 22µF (so just one electrolytic capacitor that needs replacing assuming you did apply the incorrect polarity supply)

The date codes indicate that this board was assembled after week 40 in 1982, which was October.

More information on the different ROM versions (not chip types) is here.
See my signature for other useful links.

Yes, R30 not fitted means it will generate a 625 line 50Hz video output.

Mark

Wow that's amazing thanks so much Mark. Your rundown is such a huge help.
From the information you've provided ive figured out that I need to do the following to bring her back to life.

1. Buy a replacement membrane keyboard. (although I might see if i can temporailty use a craft knife to square off the existing cable and try to re insert it as a quick fix in the mean time. If theres enough of it left that is)
2. Replace the 7805 regulator.
3. Replace the C3 Electrolytic capacitor located just by the Ram Pack slot. (the top blue barreled one)
4. install the compostive video mod (thats on its way from ZX Renew)

Ill post pics here at each stage so this is going to happen over a few weeks as parts arrive. This is a really fun passion project for me. I had one as a kid. It was already outdated by that time buts its all i could afford. Thanks for all your help so far. As I said ill keep updating my progress as I go.

Glenn.