ZX81 Composite Video Mod and Restoration

Issue One ZX81 Restoration and Composite Video modification.

 

This is my second entry and it wasn’t quite as straightforward as I hoped it would be.

 

Here is a little background; I won a ZX81 with 16k Ram Pack and a load of ZX81 books at auction, at just under £20 in total it was a very good price. As it was a proper auction and not ebay I had no idea if the ZX81 would actually work or indeed what kind of state it was in.

 

Saying that, as I already have an issue one ZX81 already in my collection I wasn’t really that bothered as the books and ram pack alone would have set me back over £20 on ebay. Confirmation came through that I had won the little computer and I couldn’t wait to get my grubby little paws on it!

 

After picking it up, it looked to be in better shape than the one I already had in my collection!

below is a photo of my haul from the auction house.

Before I plug anything in to the mains, I always check the plug… I am so glad I did as I was greeted with this monstrosity

There is so much wrong with this image

 

 1 -  the cord grip is loose 

 2 -   the cable has a piece of foam to try and secure it in place (if the previous owner had the cord grip the right way up then there would be no need for that dangerous strip of foam)

3 -   if you look closely enough you will see that the neutral has been crushed on the centre boss for the securing screw

4 -   both the live and neutral wires are the same length

5 -   there is bare wire showing on both the live and neutral terminals

6 -   there is a 13A fuse fitted!

7 -   Although you can't see it, and as the plug was very old it didn’t have the insulators on the live and neutral pins.

 

Phew rant over

Here is what it should look like

New plug fitted and voltage checked on the 3.5mm mono jack plug. It read 12VDC with the positive on the tip of the jack plug.

 

Time for some fun now…

 

I plugged it in and it powered up, albeit with a pretty poor picture on my TV. I then tried to load a game using my TZX Duino, this was problematic, it should have worked (as it worked for my other ZX81) but wasn’t keen to load. I had a similar problem with my older ZX81 and it turned out to be a faulty ear socket which I will go into more detail about later on. Finally after a bit of tinkering I managed to load a game, but just look at the picture!

 

 

Horrible! No synch and pretty fuzzy… time to sort this out.

 

After a bit of research I found a circuit on

zx.zigg.net

Below is the direct link to the documentation

http://zx.zigg.net/misc-projects/ZX81_Video_Conditioning.pdf

and below is a pretty in depth explanation of how the circuit works on youtube

https://www.youtube.com/watch?v=1irH3KuGyl0&t=0s

 

Using JoulesperCoulomb’s schematic, I designed my own PCB using Eagle PCB

 

Here is my schematic diagram and below is the board layout with dimensions

(note:  the PCB layout has been modified slightly in the above images)

The PCB is pretty small but was designed to fit into the modulator box of the ZX81, below is a picture of my populated PCB

 

White wire is the Video in

Red wire is +5V

Black wire is 0V

Yellow wire is Video out.

Whilst I was designing and manufacturing my PCB, I ordered some other parts for the ZX81.

 

I bought a heatsink for the ULA along with some replacement rubber feet from

https://www.mutant-caterpillar.co.uk

I was impressed by the price, quality and speed of delivery, he even threw in a worthers original (which my 8 year old intercepted before I got the chance to enjoy)

 

 

 

I briefly mentioned earlier about the problem loading games and my solution last time was to replace the 3.5mm mono ear jack socket (I actually replaced all three sockets in truth) they tend to corrode over time and I found that the contacts were loose from many years of use.

 

I had a dig around in my old connectors box and found a bag full of similar connectors, after a wee bit of modification they would work just fine.

 

 

All I really had to do was to cut a pin off the front right of each jack socket.

 

 

These connectors are probably as old as the originals (showing my age and hoarding skills here) but after a little brush with a fibre pen they came up quite nicely.

 

Next up was to remove the old jack sockets and modulator.

 

As usual, desoldering a plated through hole circuit board proved to be problematic. A wee tip is to add fresh solder to the desoldered joint then desolder again until all the solder has been removed. Take care doing this as the traces are easily damaged as I will show you later.

 

 

There was a fair bit of flux residue left on the circuit board so after a wee scrub with the Mrs’s toothbrush and some 99% ipa it turned out quite nicely.

 

Now let’s have a look at that modulator

 

 

There are a couple of ways to fit the composite circuit

One way is to simply remove the video in, ground and video out wires from the modulator and house the board outside the modulator.

And the other which I opted to do was to remove all the circuitry inside the modulator (storing it away safely should I decide to restore the ZX81 back to its original state) and fit the PCB into the modulator housing.

Care should be taken when removing the circuit board, it is soldered on to the case of the modulator housing and it took quite a while and a higher temperature setting on my soldering iron to finally remove all the solder so that the PCB fell out the bottom (after a wee bit of persuasion)

Before fitting the pcb make sure that you fit the insulator on the bottom of the modulator as without it you will run into all sorts of problems. Below is a photo of the PCB fitted inside the modulator.

 

 

The ground is soldered on to the chassis, the +5V and Video in are fed through the two white plastic bushes and the Video out is soldered on to the RF socket onto which a standard phono plug fits extremely well.

 

Time to start soldering on to the ZX81 PCB

First up the 3.5mm mono jack sockets

 

 

These need to be sitting as flat on the circuit board as possible so a wee tip is to solder two opposite legs then check that the socket is seated correctly. If not then simply heat the solder on the raised side and push the component flat against the circuit board.

 

 

 

The composite modded modulator was next

 

 

Unfortunately this is where my troubles started!

The trace leading to the +5V via lifted when I removed the original wire so I had to carefully repair the damage done. Using tweesers I was able to hold the lifted trace down whilst I soldered the +5V wire on to both sides of the via. Not the best repair I have ever done but it got the job done, I tested the repair with my multimeter just to be on the safe side.

Here is a wiring diagram showing the solder points on a blank ZX81

 

Here are the solder points on my ZX81

 

 

Next up was to fit the heatsink, I used some thermal paste as shown and firmly fitted the heatsink to the ULA which tends to overheat (especially when the 16k Ram Pack is fitted)

 

 

Now all I had to do was to fit everything back together again.

This is when another little problem arose and I am at a complete loss as to how this happened or what actually happened to the connector in question.

When I went to fit the keyboard membrane to the connector on the ZX81, the 8 pin connector held the keyboard membrane snugly and was a good fit. However… the 5 pin connector just wouldn’t hold the membrane. Time to investigate…

 

 

It appeared to be missing half of the metal clips which meant that the membrane was never going to connect. I pulled a pin out and sure enough, half the connector was missing! This one has beaten me, I have absolutely no idea what went wrong as nothing fell out of the connector when I removed the membrane.

 

I found replacement connectors on www.sellmyretro.com for £5.40 including shipping. Like Mutant Caterpillar I was very impressed by the quality and speed of delivery, and for a reasonable price as well .

Below is a picture of the broken and new connector

 

 

I just had to fit it now

 

 

Before I fitted the keyboard membrane, I decided to clean up the edge connector with some 99% IPA, I was quite pleased with the result. (Mrs Geek probably not so pleased with the state of her toothbrush though ;))

 

 

After fitting the membrane, I screwed the case together for testing… unfortunately my problems with the keyboard weren’t completely solved as some of the keys weren’t working. Time to check the ZX81 schematic diagram for some clues...

 

Here is the schematic for the keyboard matrix; highlighted red were the non functioning keys so it was realtivley easy to trace the fault back to the 5 pin connector.

 

 

Sure enough, there was a hairline fracture on one of the silver traces on the membrane. As this was close to the end that fits into the socket I simply used a pair of scissors to cut off the damaged part (when doing this try and get as straight a cut as possible). Luckily for me the membrane was still long enough to fit.

Time to put everything back together… hopefully for the last time!

 

 

I crossed my fingers, held my breath and turned the computer on…

 

 

After testing all the keys, I decided that I would try and load up a game from my own version of the TZX Duino to test the intermittent fault when loading the games

 

 

And…

 

Success! Loaded first time and with a stable image on the TV screen!

 

Like the Binatone, I have had so much fun restoring this iconic computer from the early 80’s Granted this restoration wasn’t straightforward, but that is where my enjoyment and love of restoration comes from… you never know what you are going to get!

 

If you have any questions or queries or if you would like the gerber files of my PCB design then please feel free to get in touch

 

Alleged_geek@aol.com

 

Many thanks for taking the time to read my ramblings.