ZX81 Black screen

[Regiox]

I recently acquired a ZX81. The owner didn't know whether it worked or not, so I took it home and plugged it in. But when I plugged it into my LG smart TV, it just came up with a black background with thick flickering white lines traveling diagonly across the screen.
So I tried my old Sony TV. This worked better, but there is still only a dark grey (slightly off black) screen that is a bit fuzzy. Fhere is not cursor or anything else. Does anyone know what could be causing it to do that?

Thanks in advance.

EDIT: It makes a hummimg sound when on the channel the ZX81 is on - 36.

[1024MAK]

The ZX81 can only do black or white. And it does not have any audio output. So always turn the sound down / off on the TV.

I assume you have tried fine tuning? I’m assuming it has not been composite modified.

If the screen is completely black, then it could be a power problem (does the case get warm?)
Otherwise, it’s a faulty ULA that normally causes this. It is also possible that the modulator is faulty.

But also check that the video lead is okay and that you are getting a good connection on the UHF socket.

Mark

[Bubu]

Try to set the bright on the TV at maximum

[ianace]

Could you try tuning into an old CRT TV?

[Regiox]

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[Regiox]

I assume you have tried fine tuning? I’m assuming it has not been composite modified.

I can't find how to fine tune it, there doesn't seem to be an option. But there must be, as it is quite a useful tool. Do you know where the option may be located?

If the screen is completely black, then it could be a power problem (does the case get warm?)

The screen isn't just black, it's fuzzy and more like a very dark grey. And yes, it does get warm after it has been left on for a while

But also check that the video lead is okay and that you are getting a good connection on the UHF socket.

How would I check them? Use the cable for a TV areal? And how would I check the connection on the UHF socket?

[Regiox]

Could you try tuning into an old CRT TV?

I'm not sure what that is, if that's a type of TV, I don't have any other tvs to try

[1024MAK]

I assume you have tried fine tuning? I’m assuming it has not been composite modified.

I can't find how to fine tune it, there doesn't seem to be an option. But there must be, as it is quite a useful tool. Do you know where the option may be located?

Some TVs have a fine tuning control so that transmissions that are not exactly on the specified frequencies can be tuned in. Unfortunately the location varies between different TVs, even within a make. So it’s hard to help. Do you know the make and model number of your TV?

Because the ZX81 uses an analogue modulator to generate the radio frequency carrier signal for the UHF TV output, and it does not use a crystal controlled system, it is extremely rare that it outputs on the official ‘correct’ frequency. Worse still, some LCD TVs may only work correctly if the received signal is near to the frequency that they are expecting. That’s why there is normally a suggestion to try another TV. And why there is normally a recommendation to try an ‘old school’ cathode ray tube (CRT) TV. That’s the big bulky and heavy type that was common before the flat panel LCD and plasma types were available. CRT types are more forgiving. Obviously if you don’t have one and don’t know anyone that does have one, that you can borrow, then you can’t do this test.

If the screen is completely black, then it could be a power problem (does the case get warm?)

The screen isn't just black, it's fuzzy and more like a very dark grey. And yes, it does get warm after it has been left on for a while

Okay, so it’s getting power. Early versions of the ZX81 were not fully compatible with colour TVs. Can you see what happens if you try changing the TV’s brightness and contrast controls. Turn the brightness and contrast up to see if there is a slightly darker K in the bottom left corner. If needed turn up the brightness to maximum.

But also check that the video lead is okay and that you are getting a good connection on the UHF socket.

How would I check them? Use the cable for a TV areal? And how would I check the connection on the UHF socket?

I meant, does the lead work with other 1980s home computers? And that the symptoms don’t get any worse or better if you turn the plug of the lead where it connects to the ZX81. A faulty lead or a poor electrical connection can sometimes give a very poor quality picture.

Please understand, that although the ZX81 is a simple home computer, the way that it generates a display is very complicated. And there are limitations in the design. Some of which makes it difficult to get it to work with some modern flat panel LCD TVs.
Plus there is a possibility that the ZX81 has a fault. And if the chips are not in sockets, it’s not always easy to fault find on it.

Mark

[ianace]

Could you try tuning into an old CRT TV?

I'm not sure what that is, if that's a type of TV, I don't have any other tvs to try

An old style TV From the 1980s/1990s, your parents might have one hidden away somewhere in a cupboard!