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ZX81 Video Problems

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Have picked up a ZX81 that is immaculate inside & out


The RF output is horrific, but is just good enough to prove it seems to be working


Am going to try a simple composite mod (as it has the later ULA) but was wondering if anyone has any other thoughts please?




The horizontal lines (top and bottom) are constantly moving


Many thanks


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Try a much older CRT TV, ideally a B&W one. I have found that consoles & computers ideally should match the age of the TV, not only for aesthetic purposes but because the technology matches. Some of the older systems of mine barely sync, or bleed terribly on a newer CRT, not to mention a LCD/TFT etc, but have a steady and good signal on a 80's TV if you can get ahold of one.

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Well I've tried the simple mod (as I have the newer ULA)


The background is now a perfectly clear white, no interference or anything


However the text is completely washed out and can only be seen if you squint carefully


Don't know what's wrong as I've double checked my component choice and connections etc. (though I know nothing about electronics tbh)


No point me ordering a mod kit as they all seem to have backporch which isn't compatible with my ULA


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After searching the web I found a suggestion that some modern TV's need a stronger composite signal


So I ran the signal thru a composite to component convertor and I get a much better display than I've had so far, stable but a bit too ghosty around the characters (but the converter was previously put away as I didn't like it's output anyway)


When the weather breaks I'm going to get a much older LCD TV out of the garage and see what happens with that just connected thru composite (I put it in the garage as the VGA input on it wasn't great for the TI-99/4a F18A combo)


At least I should get some idea if the problem is with my ZX81 mod or the TV I need to use...

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Perhaps you can use one of those video amplifiers consisting of 1 transistor + 2 resistors, if you have a ~5V source to feed it with? I know that the original video signal also from a ZX Spectrum tends to be on the weak side unless amplified, but most TVs can accept it.


Examples, where I believe the 75 Ohm resistor equals the impedance on your TV:




Edited by carlsson
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10 minutes ago, carlsson said:

Aha, so you did this mod and found out that the 33 ohm was required?


Yes, that's the one I ended up doing. The 33ohm was the fix.


Though my components are outside the modulator box


As I have a 2600 attached to my composite input on my screen, I've now connected the ZX81 to the Green input on the component inputs (still displays white and black ok) as other inputs are taken up by my other retro systems  

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