Zx81

[Slothie]

I would recommend replacing the capacitors smoothing the power supply because they are likely to be worn out and it will be hard to diagnose other problems. Also if they finally give up the could cause a fire!
I also noticed that the page i linked to in my previous post shows a diode in the video output which is supposed to improve working with modern tv's - might be worth a try.

[John Earland]

Yes-I noticed the diode. I’m willing to try anything!

[SiriusHardware]

Originally Posted by John Earland

Hi, I found this which seems promising? https://www.sellmyretro.com/offer/details/3712

I had a look at that, apparently not too bad on price and it appears to do what needs to be done, it provides the missing 'back porch' portion of the video signal.

I would prefer a tidier, but more expensive approach like this

https://retrorevivalshop.co.uk/index...roducts_id=560

Here you just remove the original ULA and replace it with that modern reverse-engineered version - that particular one is quite expensive though, they seem to have been available for about half that price in the recent past.

If you consider the current price to be reasonable, check with the seller because that item is sourced from New Zealand - if the UK seller has local stock, fine, but if it has to come from New Zealand there could be considerable delays due to current events.

[John Earland]

This looks great. I would have to get my friend to do the replacement for me if it’s soldered in. Would this be okay on my original ZX81 with the distorted video output too?

[SiriusHardware]

In theory yes, but hold that thought until you have tried the composite mod on ZX81(1). If it still has buckled / wobbly video-out even then, then the ULA is one possible cause for the fault.

If doing the composite mod produces good, steady video out from ZX81(1) then it may be that the modulator was actually at fault. No sense changing the ULA without due cause.

[John Earland]

I agree. I shall try the mod first. Did you see the post from Soothie which included a link which shows this modification but with an additional diode? What do you think?

[SiriusHardware]

I've seen variants where there is both an additional diode and an additional resistor in series with the diode. Try the simplest version first, then add the other components if it proves necessary.

[John Earland]

Thank you, will do.

[John Earland]

Originally Posted by SiriusHardware

Turning the transistor pinout into a practical layout, this is how to physically wire everything up. (See attached). I've laid it out so that the incoming +5V and the incoming video-in arrive from roughly the right place and direction.

When you cut the incoming video-in and +5V wires do it about 5-7mm inside the modulator housing, the transistor's base and collector leads can then be soldered directly to the ends of the incoming wires.

The resistor can be soldered between the transistor's emitter and the inside of the modulator case which is a handy 0V point - if you do that, do it low enough down the inside so the modulator lid can still be fitted on afterwards.

Does it matter which way round the resistor goes?

[Dave Moll]

No, a resistor is non-polar (i.e. it works the same either way round).

[John Earland]

Thank you for confirming. I've just found 'Electronics for Dummies' online which said the same thing - coincidence. I may invest in a copy! I need it.
Thanks again!
J

[John Earland]

I thought I had posted this resolution but I can’t find it.

I have completed the mod of the ZX81 and the image is strong and stable! Though the screen is slightly darker than it was before it really is a good crisp image. I will try it on my lcd but on the crt, I am happy. Image 1 is showing the ZX81 without a memory pack, the second is one with the memory pack (tv screen has very slight waves which I’m not bothered about) and the third image is the modification.

Thank you everyone for helping me with this project! Really appreciate it.

[SiriusHardware]

Well done, not bad at all for someone who keeps saying he doesn't know how to solder. Quite difficult to work in that confined space as well.

It would appear, then, that the real problem may have been a faulty modulator (possibly intermittent, as you did manage to get it to look better occasionally by putting pressure on it). That presumably doesn't matter if you plan to keep it modified this way.

[John Earland]

Yes-I’m really pleased! Thank you for all your advice and guidance!

[Slothie]

Very neat, especially considering your experience as Sirius said. I prefer it when mods like this are "hidden", it would have been easy to make this messy which would have been a shame with a machine that was actually quite elegant for its time.
I loved the old ZX80/81 though i never had one of my own i borrowed a friends for more time than he was happy with!!

[John Earland]

The first computer that I owned was the Spectrum! I never had a ZX81 either. I was going to have one for Christmas (I was 13) then the Spectrum came out instead! The first computer I ever used was the Commodore PET, so I am fond of them all!

[emeritus]

Mine was a Z81 too. The modulator failed and as I already had an Amstrad PCW8256 by then I converted it to a dummy laptop for my daughter's school play by making a flip-up hinged lid for it from black card. I printed a screen dump of the windows desktop from my work PC, covered it with shiny self adhesive plastic film, and stuck it on the inside of the lid. From a distance it was quite realistic! Probably sill buried in the dephs of the loft. I well remember spending ages typing in games programmes in BASIC from magazine listings.

[Dave Moll]

Originally Posted by emeritus

I well remember spending ages typing in games programmes in BASIC from magazine listings.

... then spending further ages downloading them to cassette tape.

In my case, though, it was a BBC micro. It was only more recently that I acquired examples of the ZX80, ZX81 and Spectrum.

[SiriusHardware]

I have two ZX81s, one is my original machine, built from a kit and still in working order.

The other had been in an auction lot bought by a fellow forum member here and I acquired it from him, it is housed in a 'Dktronics' full sized enclosure / keyboard which is in almost as-new condition - clearly very little used by its original owner because when they were used regularly, the glue which secured the stick-on key legends would soften and the key legends would start skating all over the key tops. ( I know this because my original Spectrum spent most of its life in the Spectrum version of the same Dktronics enclosure / keyboard).

Both were updated to 2C210 ULAs when those parts were still readily available and not too expensive, and both are internally fitted with 16K RAM because, despite what I have said elsewhere about originality being key, the original 16K RAM pack is a dreadful abomination which neither looks good nor works well, due to the inherent unreliability of the connections (AKA 'Ram Pack Wobble').

It is a relatively straightforward mod to fit a 62256 SRAM in place of the original 1K RAM and you then get a machine which retains Rick Dickinson's classic design looks combined with a respectable 16K RAM, and nothing dangling off the back. Because the RAM is internally fitted, the connections are secure and solid and the machine can withstand a thump or three without the memory contents getting scrambled. The mod can also be done in such a manner that it can be completely reversed if required.

(You may point out that the 62256 is a 32K RAM, and you'd be right, but the most basic form of the mod only maps 16K of the 32K - there is arguably no point in mapping more of it because there is no mainstream software from that period which needs more than 16K.)

[John Earland]

That sounds like a great idea. I have, as I mention, a 16k RAM pack but I like the idea of fitting the RAM inside.