A stupid but useful tip for TV-sets users

[yerzmyey]

Hi,

I'd like to return to some old discussion but finally I found something interesting by myself.

About 2 years ago we were complaining for all the new TV-sets that they have automatic "search" options only, and they can't find ZX81's carry-wave or they can't stop when the ZX81 screen visible.
On the other hand, their fine-tunning is deadly slow, so nobody can use modern TV-sets with ZX81 (too weak ULA and too crap TV-sets anyway).

My advice is stupid but useful, as I had the same problem for a year or so, and finally I had to buy completely new TV-set specially for ZX81.
Now I use big 21" TV because I found that ZX81's carry-wave is in almost exactly the same place, where we can find ZX Spectrum 48 and ZX Spectrum 128.
Now I just connect Spectrum, find it easily on any modern TV (Speccy has stronger ULA) and then I simply connect ZX81 instead.
And it works fine.
Sometimes it requires small fine-tunnig, that's all.

Enjoy.

[gozzo]

I don't see how the ULA could make the RF output 'stronger' - they both use a UM1233 modulator - , unless the spectrum ULA's video DC (lower DC level= higher carrier amplitude) level is lower than that of the ZX81, if so try adding a 1k resistor or a 4.7k pot. from the video modulator input pin to ground...???? Maybe the ZX81's video waveform/line frequency/frame frequency is too far 'different' from the official 625 line standard..???? Could even be due to the RF carrier not exactly on a 'channel frequency' , but a bit 'in between' eg. not exacly on channel 36..possibly..??? If the TV in question has a direct channel entry, then set it at channel 36 then fine tune the core in the ZX81 .... that MAY work! I have a problem with my LCD TV in that it won't search tune to an old late 1970's/early 1980's video game, but I think the actual carrier is plenty strong enough (works ok on an old black and white TV)...maybe even be caused by the ZX81 or whatever not having a colour subcarrier present in the signal - maybe modern TV's lock onto this..??? If I get time I'll have an experiment with my TV pattern generator - I can switch the subcarrier on or off as required and also vary the RF carrier output amplitude!

[gozzo]

Slight correction to my previous post - the colour subcarrier is 'carrier suppressed' so that it isn't present unless there is colour in the picture, but the colour synchronising 'burst' is always present - maybe the modern TV's search and lock on to this instead of the line sync as older search systems did.????? As I said, when I get time, I'll do a few experiments - if this IS the cause of the problem, maybe it's possible to rig something up to insert a 'fake' burst into a video signal to make the TV to lock onto it.

[RetroTechie]

Simpler is to sidestep all of the above by doing the composite video mod: http://www.zx81.de/english/video_e.htm. Easy, cheap. I've done this on all (5) of my ZX81's and have yet to encounter a TV that doesn't produce a proper image. This doesn't fix the ULA dependent "missing back porch" problem (dark, dim image with old version ULA's), but you'd also have that problem with the RF modulator in between.

No channel tuning needed anymore, all modulator-related issues become irrelevant, and you get more crisp image as a bonus.

The only situation where this doesn't work, is with a TV that can't deal with a composite video signal. Which in practice, is a TV that has RF input only. Which in practice, means a really, really old CRT tube. When that's all you've got to work with, getting a newer TV is equally sensible solution as dealing with modulator/tuning issues. Anything SCART-equipped is usable, I have a 20" CRT tube here that I got for free. With just a little money to spend you have much wider choice of usable TV's.

[angus]

That composite mod works beautifully with some TV's. However my main TV at home (AOC LCD model) doesn't like it much. I get a perfect picture for 10 minutes or so but as the ULA heats up the picture breaks up.

I get nothing at all when going via the composite input of my VCR or DVD recorder....and I get a pure white screen when I use my projector.

Recently I bought a little cheap USB dongle that has a composite input and it treats the modded Zeddy as the projector does.

Strange....I've tried it with other TV's and it's perfect....just not with any of my equipment at home!

[gozzo]

Angus - have you got an oscilloscope you can check the composite video waveform with - check it when the display is ok,then when it plays up, to see if anything is varying - possibly the DC level.?? Try adding a capacitor, say 10uf 25v in series with the video, with a diode(OA91 or BAT85) from the TV side to ground, anode to ground and see if that cures it...

[angus]

I have access to oscilloscopes at work (indeed I was using 5 today!) but the Zeddy works perfectly on both TV's at work. I'm not really in a position to lug a 'scope home though I suppose I could run the Zeddy at work for 30 minutes and observe the changes on the scope.

First I think I'll add a 10uF capacitor.

[gozzo]

I did some experiments with a philips pm5509 pattern generator feeding its UHF into my LCD tv, and whatever I set the thing at ,my tv just did not like it...did eventually get a picture but it kept losing it ever time I made an adjustment on the generator - damn modern tv's - stupid thing said it was being fed ntsc 3.58 -can't set it manually. it auto detects - it most certainly WAS being fed PAL 4.43!! No troubles with a 21 inch CRT philips tv!! No problems with direct composite video!! I have a possible plan 'B' .......

[yerzmyey]

I suppose I could run the Zeddy at work for 30 minutes and observe the changes on the scope.

Yupp, it could detune by 'warming up' or whatever it is. We noticed it in PAL CRT small analogue TV. Doesn't show on every TV 'though.
Hm.

[gozzo]

I've seen warm-up detuning, but only very minor on an old black and white portable - they didn't usually have AFC as all(?) colour sets did - whether it was the TV or the ZX drifting I don't know - probably a bit of both! As for my plan 'B' - actually 2 plan 'B's, was thinking of something like an untuned 'video detector' and amplifier fed from the computers UHF tv out, giving composite out - or alternatively, using the innards of a now redundant 5 inch black and white TV as the tuner/if strip and tapping off the video from that - must be loads of them,still in working order, being scrapped as we've now been digi-tv-ised here in the UK!