Non working ZX81

[colday]

Ok gents, here we go.

Acquired a ZX81 which the guy swore was working.

Guess what?

Ok, so checked the PSU, all good.

Checked voltage at the regulator, is spot on.

Checked voltages round the various chips, Ferranti ULA, Z80 etc, all SEEM fine, however I can't find anywhere the voltages at various points for testing...

Anyhow, I'm sure that's easy to sort out, I'm getting no output from the machine but all the IC get nice and warm (not hot, just warm) so I suspect it is likely to be the composite output.

I have a scope arriving any day now, and I used one for about 6 months back when I was doing my A-level Physics, however, that was about 30 years ago now and as such, I've forgotten most of what the esteemed Mr Graves taught me.

So, is it possible for someone to really break it down to simpleton steps for me of what to check for and where?

And not just me, but as a valuable resource for all useless imbeciles like me in the future.

Or, alternatively just point me in the direction of something on YouTube that is foolproof.

[Paul]

He found the appropriate thread.
Kind regards Paul

[mahjongg]

You could try to initiate a csave operation (blind), and monitor if sound is coming out of the audio output, to see if the ZX-81 is otherwise working.

[1024MAK]

Hello colday and welcome to our forum

You may wish to introduce yourself in the Welcome Area

I'm getting no output from the machine but all the IC get nice and warm (not hot, just warm) so I suspect it is likely to be the composite output.

What exactly do you mean by no output?
Black screen?
White screen?
Just static and noise?

Is the video circuitry / output original, hence is a RF/UHF signal that needs to go to an analogue TV tuner.
Or has this machine been modified to output composite video (composite modified)? If yes, do you know which method was used? Or can you post up some photos?

Mark

[colday]

Hey.

Sorry about the delay, and yes, I've found the other thread at last.

In answer to the questions Mark, the TV won't tune to the Zeddy. It tunes happily to my Speccy 48k but not to the ZX81.

The scope shows there is a 50hz output and the TV (LCD analogue thing from years past) almost finds the channel, so it may actually be working and the fact that the TV doesn't have a 'manual' tune could be causing the issue.

I have bought a 80's colour TV from eBay, (rather nice actually, and certainly superb for £1) but I just need to rive to Cambridge to go get it.

Hopefully later this week.

I will wait till I get that before deciding what to do next.

However, I was thinking about the mods that are possible and was looking at your schematic on another thread and had a thought (rare)...

Would it be possible to knock up a mod, that instead of going inside the zeddy, plugs into the Tv out and amplifies the signal?

That way we all get the best of both worlds, working Zeddy's and no mods (for the purist)

Thoughts?

[Lardo Boffin]

http://www.fruitcake.plus.com/Sinclair/ ... erface.htm

Awesome bit of kit.

[1024MAK]

Would it be possible to knock up a mod, that instead of going inside the zeddy, plugs into the Tv out and amplifies the signal?

The output level from the modulator is perfectly acceptable to television aerial inputs, as these inputs are designed to cope with the very weak signals from aerials.

If you are suggesting that a demodulator be designed, I don’t really see much point. Such a device cannot improve the quality of the picture. And it would take a lot of effort to get near the performance of the demodulator inside a television.

For the best video signal, it is best to use something like a ZX8-CCB (other similar devices are available) or at the very least, a simple video amplifier buffer like my circuit. Both can be connected to the PCB without having to disturb the keyboard membrane tails.

If you have a bit more money to spend, follow the link to the Chroma that I see Lardo has just posted (while I was busy typing). This plugs into the expansion port. So there is no need to open the case. I also have one, it’s excellent

Then there is no need for tuning in.

The view of many here, is that the place for original ZX81 computers that are in excellent condition, is in a museum. Leaving plenty of ZX81 computers that can be used for fun.

Mark

[colday]

That is, as you say, a lovely bit of kit that does exactly what I want.

Have to say though, the price does seem a little high.

But I guess you get what you pay for.

Mark, as for your response, I was aiming at something that converts to the same out put that your circuit does, but that can be mounted outside the board. So an external video amplifier so to speak.

[1024MAK]

Mark, as for your response, I was aiming at something that converts to the same out put that your circuit does, but that can be mounted outside the board. So an external video amplifier so to speak.

The ZX8-CCB (and similar video circuitry) can be used outside the ZX81, but it does require some (three in fact) soldered (or test clip) connections to the PCB inside of the ZX81.

As I said, if you want something where you don’t have to open the case, that would be the Chroma.

If you want something that takes the RF/UHF signal and converts it to a composite video signal, that is what I mean by a demodulator.

Mark

[colday]

Yup, makes sense, thanks for explaining Mark.

Ok, an update...

I bought an 80's portable TV (it's actually almost like new, came with manuals and remote and it works perfectly) so that I can manually tune it.

It seems the ZX81 only outputs a black screen. I've read something about that in passing somewhere but wasn't paying particular attention as at the time it wasn't my issue.

I also note that I can see 'jailbars' on my speccy now as well but am aware of that fix/remedy.

So, black screen....