Restoring a lovely old ZX81 to its former glory
I have a few items in the repair pile and rather than being greedy and fixing them by myself I decided to turn on a couple of cameras and record them. Its been a little while since I fixed anything, and you can expect a few of these simple repair videos to be sprinkled in amongst the long form episodes I’m mostly making these days!
Paul sent me this ZX81 to repair. He gave it a test, plugged into RF and found it was, in his words “DEAD”.
I have my bench power supply set to 9v and 800mA and the TV tuned to the correct channel. Switching it on, it is indeed dead. Showing no signs of life on the screen. But watch what happens when I touch the power jack…
That looks like a black screen, but if we go closer you can see the expected K cursor is there, only the picture is very dark. That’s actually expected as the ZX81 outputs a very weak RF signal and this TV is modern enough that it really doesn’t like that.
Ok, so the first thing to address is that power jack. Let’s start taking some screws out and dig into this little machine.
The ZX81 was my very first computer and it still gives me warm feelings when I use one now. Even with its incredibly limited abilities it opened up a whole new world for me. I hope I never lose that feeling.
The membrane actually looks in reasonable condition for something over 40 years old. I have high hopes it won’t need replacing.
Inspecting the area underneath the power jack I can’t see any cracked solder joints, so it’s likely the socket is just very corroded. Let’s try some contact cleaner first. The power socket feels a bit loose. I wonder if someone tripped over a lead and stretched the insides of this at some point. Vigorous insertions should remove the worst of the crud. So does it work now? I can tell you it still does the same thing. And you can just see the picture flickering on the TV in the background as I wiggle the plug in the socket. There’s little point in trying to repair this socket. It’s a very cheap part and a new one will be much more reliable.
I’m using my manual solder sucker for this job. I could probably use the Hakko, but the nozzle won’t fit over these legs and the benefit would not be as great. Although I’m finding my Engineer sucker is blocking quite often lately. I might need to give it a thorough clean.
The pins are not yet moving, still a little too much solder, so in with the braid. Just to clean up the last bits.
And then moving the legs of the socket with the end of the soldering iron to free them. They’re all loose now and it just needs a little gentle persuasion.
Then it’s just a case of popping in the new socket and soldering it in place. Ah. It wont fit. Too much old solder left in the holes. Better clear that out first and then it will fit.
Still wont fit.
Ok. I better cheat then. File down the legs slightly and then it will fit!
Right, now with that fitted and all connected up again, will it work as expected?
Yes. Course it will.
And tapping on the power lead doesn’t have any effect. Perfect.
Next problem is the display. This machine will be sold by Paul Universal Retro Boss at some point and the new owner will most likely want to use it with a composite display. I have some really nice composite mod boards to replace the RF modulator. It’s not a simple composite mod like you will find in many ZX Spectrums. This one is a bit more involved and requires the modulator to be removed from the motherboard, replacing its guts with the new board.
It’s not too terrible to remove a modulator. The hardest part is getting all of the solder out of the holes in the heavy ground plane. But first I will remove the wires running from the modulator to the board. The nozzle won’t fit over these big legs so I am jamming it into the solder and sucking up as much as I can. Back to the braid to get the last bits. And now heating each leg and walking the modulator out of the board.
And it’s free.
It’s unlikely I’ll ever use the guts of this modulator again, it’s very old, and doesn’t work very well as we’ve seen earlier, and I have loads of them. But even so I prefer to try to remove the insides non destructively if possible. The old board has 3 or 4 large solder blobs around the outside. I first go at these with the Hakko gun and then work them loose with braid and wiggling.
And eventually it pops out. Now I need to make a replacement.
Here is my box of bits that Fuzzy Lee put together for me. There are enough parts to make dozens of composite boards for ZX80’s and 81’s.
Before starting the build I clean the board with IPA.
Diodes first. Starting with the lowest parts just makes things a bit easier.
Resistors next. Lee has marked all of the parts with the board locations so I don’t even need to refer to the github page. I’ll leave a link to the github in the description. I highly recommend these boards. I’ve tried a few different composite mods and not had much success till I found this one. Some ZX81 ULA’s produce a half decent video signal that includes the all important back porch. The good thing about this mod is that you don’t need to fit the 555 timer if your ULA does indeed produce a back porch. I don’t know if this particular ULA will need this or not. But we’ll find out together when I switch it on. If it needs that signal then fitting the 555 timer will allow this board to produce the full composite signal for a good quality image.
The last parts are the transistors and the legs on these are very close together. I don’t want to have to take this apart again so I check with a multimeter to make sure I haven’t bridged them.
And now it’s time to fit this into the can. You can see around the outside there are three large pads. I’ll be using these to solder the board into the metal casing. But first I need to make room for the wires to exit this metal case. There are two holes in the plastic part but one of them is in the wrong place. The one at the other end has some kind of ceramic plug in it. If I can drill this out it will make a natural route for a wire. The drill bit is not happy drilling into ceramic, and eventually the plug just breaks out anyway.
I want to add the three required wires now as soldering them in place after the board is in the case is a pain. I need one wire for 5v, so red for this. One wire for the incoming video signal, which will be green. And one to connect the outgoing signal to the video socket. I’ll use yellow for this.
Some big blobs of solder around the can to fix the board in place.
Ahh, I forgot to drill out the plastic hole for the other wire. Now I’ll need to be careful I don’t damage the components inside.
There we go. All the wires are ready and all I need to do now is attach this back to the motherboard. I even remembered the bottom lid this time!
With the modulator can refitted I now need to attach the two wires. Before removing the modulator earlier I took a photo to show where the two wires need to go. There are alternative points on the board for different regions. But this one here is the video signal for the UK. And this one here is the 5v supply.
And with the wires in place it’s time to test.
To start with I am testing without the 555 timer chip to see if it’s required. And look at that. Pretty much the same black screen as before. Only this time connected to the TV’s composite input. And with the 555 chip?
Well look at that! Perfect!
Back inside its case and the keyboard seems to be working perfectly. It’s a nice feeling membrane compared to some I’ve tried. And typing in a 10 PRINT program is a joy that brings back all those fuzzy warm memories.
The last thing to do, seeing as this is such a nice little machine, is give it a clean and make it pretty again. Windex and a toothbrush on this textured surface. And then a wipe off with some paper towel. Yup, that was pretty grubby. Finally a coating of renaissance wax to protect it and bring out the lustre of the black plastic. Oh and a sticker for its eventual owner to remove in disgust!
It’s not a very useful machine, but I really like it. And I hope its eventual owner will appreciate just how pretty it is to me.
Thanks for watching! I will see you in the next one.
Bye!