Sinclair ZX80 / ZX81 / Z88 Forums

hello everyone,
this is my first post in the group because I finally managed to find a zx80 in good condition.
unfortunately i can't tune the channel either on my lcd tv in antenna or on an old analog tv.
I also bought an antenna amplifier in the hope that it was a signal voltage problem but no dice.

This is the best I can see by tuning, even 'manually', my lcd tv, on the analog tv the signal doesn't change much. This is what I read via oscilloscope on the RF output. I should do the modification to bring the composite signal but I don't want to open it yet.
Can anyone tell me if the signal coming out of the modulator is correct?
Thanks in advance.

Translated with DeepL.com (free version)

Not being a hardware buff It would take one of the gurus to sort this which I'm sure they will, but at first glance looking at the that screen I would guess that the modulator is the least of your worries and looks like the kind of thing you get when the onboard RAM or one of the chips has given up the ghost. If you have a RAM pack for the ZX81 to hand try connecting that and see if the picture is still the same.

Many thanks for your reply, I hope you are wrong

Hello lemorlenny, welcome to our forums

Despite being an inexpensive computer, the ZX80 is rather a complex design. Just like the ZX81 which came after it, a combination of software and hardware is used to create the video signal.

In order to produce a useful video picture, the processor (Z80), the ROM, the RAM and most of the 74 series logic chips have to be working correctly. As the processor is the part of the system that actually gets the video information from memory in order for the computer to produce a video picture.

As you have a 'scope, I suggest using it, what signals do you get on the following pins of the Z80 CPU?
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
6 Clock (3.25MHz approximately)
16 /INT (should be pulsing)
17 /NMI (should be high)
18 /HALT
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be high)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Mark

The Input of the ZX80 Modulator can be used as video out.so you can rule out RF problems.
The 74lsxxx producing it are insensitive to load.
NEVER try this on a ZX81 because the ULA that produces the output is VERY SENSITIVE.

Many thanks for your replies.

Ill try your suggestions and ill report the results.

Lenny.

I don't know how to tell you this, the guy I bought the computer from had already made the change for composite so I kept trying to make it work from the modulator when it was already working fine in composite, my bad.

Thank you all.

Lenny

Good you got it sorted