My ZX81 on YouTube

General Chit Chat about Sinclair Computers and their Clones
Posts: 449
Joined: Fri Jul 08, 2011 8:52 pm

Re: My ZX81 on YouTube

Post by gozzo »

About modern TVs having trouble tuning into a zx81 - they have trouble tuning into A LOT of 'home' computers, game consoles, and the like, even in my case, my Alba 15 inch LCD TV just will not lock onto the UHF signal from a Philips PM5509 pattern generator... It will 'work' if I feed it the composite video out from it but it thinks it's being fed NTSC 3.58MHz, instead of PAL which is what is coming out of the pattern generator - stupid thing - it has no manual setting for the colour system! Have been trying a few experiments to find the exact reason for all these problems - I have a few suspicions - but no conclusion as yet! As for the ZX VID and similar mods, it is unnecessary to remove the innards of the modulator as many do, just connect the composite output of the ZX VID or whatever straight to the centre contact of the modulator's phono socket, and the input to the modulators video input, this allows both composite and UHF RF out through the same socket, only difference is that if you do want to connect to an older TV using its aerial socket, you will need to use a capacitively isolated cable(use a 0.001 uf in series with the centre conductor , although you MAY get away with a normal lead, that depends on the circuitry of the ZX VID's output).The RF doesn't interfere with the composite signal as it is only about 1 millivolt, and the composite will not interfere with the UHF as it is much lower frequency and will be attenuated anyway if using the isolating cable.
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Joined: Tue Jan 01, 2013 12:12 pm

Re: My ZX81 on YouTube

Post by tezza »

RetroTechie wrote:Very nice vid there, tezza!

Especially like the background you give on things like how it was marketed, Sir Clive, other machines around at the time, RAM pack wobble, working with old TV's / taperecorders etc. That's exactly the kind of thing eg. Wikipedia is often a bit short on. I lived through that period myself (and used a ZX81 back then!) so I know what it was like, but it's hard to convey the feeling of those times to today's iPad generation... :mrgreen:
Thanks for those kind words. Yes, I'm sure the ipad/smartphone generation would be completely mystified by these old machines. Yet things like the ZX81 were a step on the road to that consumer computer-based technology so I feel it's good for them to know about this history.

They might feel differently however. :)

Terry (Tez)
Posts: 133
Joined: Mon Jun 20, 2011 1:28 pm

Re: My ZX81 on YouTube

Post by angus »

I've had various difficulties getting my two Zeddies to communicate adequately with my flat TV. My Issue 2 Spectrum is perfect via the UHF input, barring a bit of frequency drift as it warms up which is to be expected.

In the end, my mum donated a cheap "Wharfedale" branded LCD TV/DCD combo unit because the sound had become dodgy and that takes the Zeddy and the Speccy perfectly via UHF.

My modded Zeddy (composite mod) would work with relatively modern CRT televisions, and for a few minutes with my LCD TV. The unmodded Zeddy works for hours on the Wharfedale.

So...maybe a cheap 'tesco value' type LCD telly is what works.
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