Sinclair ZX81 Repair & Composite Mod 

Just like my boyfriend. Bought, repaired (and kept hold of) one Commodore 64, one VIC20, one Commodore Plus4, 6 Sinclair Spectrums, one QL, oh and a Sinclair Spectrum +2. Nice to think that there are people out there that can keep these fascinating and grand old machines going :)

Nothing better than a clean, working, and modded ZX81.

Wow that is a rock solid picture. At school we used to ZX81 with huge old black and white TVs somebody had donated. They wobble and ripple and do all sorts but we were very grateful for them. I suppose you could pair it with an MP3 player now for storage and have a completely digital system...........

Thank you so much for this video. Got a ZX81 with the same revision board where the commonly found composite mod produces a very dark picture. My machine has had some strange hackwork done to it in the old days. The RAM module was hardwired directly to the board with a wide ribbon cable. It also had a hardwired 3rd party power supply with very hazardous wiring (completely exposed mains). I obviously removed that and found a more appropriate power supply with the correct connector.

I still have my old ZX81. Bought a Maplin 1 K.Byte RAM kit back in the early 1980's, then designed my own 8 K.Byte RAM pack using a 74138 3 of 8 decoder and a Harris 8K x 8 static RAM chip. I then spent £17 on a Harris 32K x 8 Static RAM chip and made my own 32 K.Byte RAM pack. I added a Maplin ZX80 / ZX81 motherboard port extender and 2 x Maplin IO cards. Then came the proper Memotech plug in keyboard with real keys plus Memotech RS232 port module. It was all consigned to the cupboard when I bought a Sinclair ZX Spectrum +3, to which I added an external IBM 10 M.Byte HD (the +3 could only address 8.5 M.Bytes of disc storage). This required a boot disc to reset the expanded disc parameter base to explain the HD to the +3 onboard drive controller.

WOW, 50p for a ZX81. I would of been all over that at the car boot, Nice repair and mod by the way, JoulesperCoulomb knows his stuff i could watch his videos all day.

Your parents are gold mate, ZX81. My first computer in '82. Im pretty sure it had 3k of ram and 4 shades of grey.

Oh wow, after you connecting that little ZX81 to a better lcd... my mind was blown, would luv to see it on a crt with my own eyes. I recently picked up a toast rack with the same keyboard problem so I'm going to have to purchase a repro one, anyway great restoration, would be nice touch to re-apply the deep red paint on the ZX81 badge, I've done this before with a old paint pen maker I had from an old job, colour is near bang on the money.

Hi i read all you needed was to put two MK4A4 diodes in series in between the output of the composite video out to brighten up the picture

Oh, that glorious hand-routed board!

The board looks like roller tinning process before adding the soldermask only on the solder side. Not seen this for a long time.

Really nice picture quality you got now. Thanks for the hint with the hair dryer, I have to rermeber that when I need to change my keyboards. I also "modernised" my 2 ZX 81´s recently, heatsinks and Tracopower DC-DC converter instead of 7805. ;-)

Interesting video, thanks. Regarding the 'dry joints', I imagine that the through-hole plating would have provided adequate connection for those, provided of course that the soldering top-side was OK. Regarding your initial single transistor modification, I was going to criticise your circuit as doomed from the start, because a composite video signal is only 1V in amplitude to begin with and you fed it through an emitter-follower which will have reduced its amplitude by about 60% and potentially cocked up the sync pulses as well. This would definitely be the case if the ZX81's output adhered to the standard composite video signal specifications. But then I followed your link to the information provided by Joulespercoulomb, and all became clear. The ZX81's output is actually 4V in amplitude, with a massively oversized sync-pulse, so the emitter-follower actually _helps_ bring the signal back into normal proportions. I like the circuit for adding a back porch - very good. If I ever get a ZX81, this will be the first thing I do to it! Your picture results are excellent. The only thing I would have done differently to you regarding mounting is that I would probably have added a tack of hot-melt glue to stop the board flapping around in the breeze. But I don't think there is any problem with what you did. Regarding the uncut ends on the RAM pack... that's shockingly bad! They can't all be made like that, surely?

Yes, clean motherboard, and reflow all joints with low-temp lead-free solder . If you have good soldering skills, socket main chips. Replace 9 volt regulator with proper new DC-DC converter, as old one became very hot. And last, replace (paint) any missing green solder mask to protect tracings on motherboard.

Lovely work as always buddy. I really love the design of the ZX81. I do own one but I don't really use it, it's more of a nostalgia piece for me. I need to do a composite mod on it though, then maybe that would encourage me to actually use the thing lol

Excellent video, please do more Sinclair video's

Sad that I recognised that circuit straight from the thumbnail :) Never built one myself, but then again that would mean I'd actually have to source ULAs for my ZX81s to get them working before hand :)

I can also recommend that composite circuit and RWAP's membranes. Inside the RAM pack is a bit like the power supply section of the Spectrum, and can be subject to the same sort of problems if one of the rails fails.

Very impressive as usual.

Mine was factory assembled in 1981; the solder job was pretty bad.

Looks like I need to work, on my box full of Z81!

Hi, great job! Can you tell me where solder yellow wire? I can't see it on video. Thanks in advance

Brilliant mod !

Great video, but all the way through your voice reminded me of someone. Right at the end, I realised who it was - John Shuttleworth.

looks like someone busted all kindzz of fluxxx on that board all kindzz :P

What connectors does that Lilliput monitor have on the back? What is the model number?

Thank you for this video?

Please, where is the circuit diagramm of yout video adapter available? Thank you very much!

would that composite mod board work on a spectrum or is there an equivalent as thats much better than the normal mod you do to a spectrum

Okay, my electronics knowledge is very limited and i have a really dumb question. ^^ You used an 2n3904 NPN transistor, i searched through my electronics junk and only found an c3355 NPN transitor. I rewired everything but got no picture. I found a schematic that basically was the same circuit but the discription of the transistor was just "simple NPN transistor". Is the problem that my transistor does not work for that mod or can it be that my ZX81 is dead?

Why would a resistor be connected to ground? Also, does that company actually manufacture new keyboards for the ZX81? I was thinking that the keyboard would be the hardest thing to reproduce.

He's probably worse than you for collecting old machines! :O He loves his old Llamasoft games by Jeff Minter too (Revenge of the Mutant Camels etc). Really good games they are. It's all a piece of history in a way, ready for other generations too either love or hate, a bit like Marmite! Lol :O :)

I am building the JoustPerColoumb video circuit for my ZX81 but my local electronic component store does not have the 82Pf ceramic. Can I use a 68pF instead?

You can purchase a small RF to HDMI converter for 15 pounds on Ebay no soldering required. :-)

Considering these were built to a price just how well built are they?

If I gave your parents £1.50, could they get me an Enterprise, Lynx, and Jupiter Ace from a car boot sale? ;)

hot glue

Joulespercoulomb ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-1irH3KuGyl0.html Schematics and layout:- zx.zigg.net/misc-projects/

I did the same with my ZX81. A bit different approach. I made a video of it here: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-vNbrYIkUwkw.html