ZX81 with vertical hold issues?

[Lardo Boffin]

Hello all

I have a zeddy which seems to have a bit of a vertical hold issue? Or something similar.

When I tune it into my portable b&w TV I get the same picture shown below:

I don’t get this on more modern TVs - they barely tune in at all.

I can always comp mod this but as it is virtually immaculate I don’t really want to do anything to it.

Is ‘tuning’ it via the adjustable pot in the modulator likely to help?

Thanks!

[Moggy]

In the past I had a couple of TV's identical to yours, one with AV input, and occasionally I would get a vertical picture roll which was easily corrected by using the vertical hold control on the TV. I suspect it to be more of a TV thing with this brand of set rather than a zeddy one. As for playing with the modulator screw all that does is move it off frequency from channel 36 and may not solve the problem and that screw if memory serves fits into a fragile "carbon" pot and can easily be damaged.

You say you don't want to alter it because of its good condition, but bare in mind it's doubtful if anyone else will be able to use it on a modern TV so why not just mod it after all unless you intend to keep it?

[1024MAK]

As Moggy says, the adjustment on/in the modulator only adjusts the UHF (or VHF) frequency. It is very unlikely to help.

It’s possible the the ULA video output pin is now (due to a fault) unable to produce the three different output voltages. Is the ULA soldered in?

Mark

[Lardo Boffin]

Thanks all. I have not opened the case yet to see whats what inside.

I didn’t want to mod it out of laziness to be honest.

[RandUsr836]

Hi,

My orginal kit ZX81 back in the day suffered this problem and it was the Cermic Resonator. Swapped it out and the problem went away. But, it was always a question of tweaking the vertical hold when I went back to 625 line B&W TV.

I would recommend the usual fix, a video buffer board with the 555 backporch signal. Yes, you loose the loading bars. But, it saves a lot of hastle trying to service older kit to use it. Easy mod, 3 wires, if you have never tried one.

Yes, you do need at least a screw driver and know which way around to use a soldering iron which is more than I can say for the whom ever built my ZX81 factory build LOLz

With the mod I use it on an old Acorn Green screen or LCD TV with video and no problems.

Thanks all. I have not opened the case yet to see whats what inside.

I didn’t want to mod it out of laziness to be honest.

[Lardo Boffin]

Thanks.

When I mod these I use a ZX8-CCB as they give great results on every TV I have tried. I also fit a ZX8-KDLX keyboard, replace the caps and voltage regulator, fit 16K internal RAM and a Wespi (wifi fast loader) so its a bit involved!

[RandUsr836]

Thanks.

When I mod these I use a ZX8-CCB as they give great results on every TV I have tried. I also fit a ZX8-KDLX keyboard, replace the caps and voltage regulator, fit 16K internal RAM and a Wespi (wifi fast loader) so its a bit involved!

Just been looking at the curcuit for the ZX8-CCB, very nice. I do like the clever use of the 74LS08 which I have used for many voltage level hack like between 5v and 3v logic devices. A versatile lttle chip.

For what it's worth I have been experimenting with buck converters to replace the voltage regulators with SC protection seem to work very well. Yes, younger fingers do try to hot change things without checking the power is off.

Changing the caps for quality Japanese made caps is a smart move for any retro equipment restoration. I have known some Ebay electrolytics start to fail inside of 100hrs, proberbly old stock

Not so sure about the Keyboard. How do you get the authentic experence of the orginal 'dead skin' that you have to press so hard it causing the RAM to wobble and crash the program... without the traditional blobs of Blue-tac LOLz

Seriously, the keyboard looks a nice kit, if you don't have old shakey hands and failing eye sight like mine

[Lardo Boffin]

My zeddies don’t need a RAM pack (unless you want more than 16K) so I’m not too worried about losing the RAM pack wobble to excessive keyboard force nostalgia!

The newly made keyboards tend to be less responsive than the originals so I think the KDLX offers a good compromise between looking fairly original and actually being usable.

If I’m being depressingly honest I suspect most of the people buying my zeddies play 3D monster maze a few times and then don’t switch it on again having scratched the nostalgia itch but I hope I’m wrong. A lot of hours go into each one.

[zx-heinz]

Did you try to adjust the vertical frequency by the potentiometer at the backside of the TV?

[Moggy]

If I’m being depressingly honest I suspect most of the people buying my zeddies play 3D monster maze a few times and then don’t switch it on again having scratched the nostalgia itch but I hope I’m wrong. A lot of hours go into each one.

Unlike just one of my workhorses which runs 6 hours a night 7 days a week!

Nay Lardo I can't see all your efforts getting wasted on the nostalgia itch as you call it. The way you sort em out makes them eminently more usable then when new and if I didn't have enough to see my time out, rest assured you would be the one I would be buying them from.