Non working ZX81

Discussions about Sinclair ZX80 and ZX81 Hardware
Lardo Boffin
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Re: Non working ZX81

Post by Lardo Boffin »

If the zeddy puts out a black screen it could be an early ULA. Try turning the brightness all the way up.
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colday
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Re: Non working ZX81

Post by colday »

Well last night I tried the above suggestion, turned up the brightness and contrast but to no avail.

Just a blank black screen.

Occasionally if I pull the power and plug it back in, I get a sort of slightly diagonal set of stripes.

Pressing the keys seems to do nothing and there are no obvious changes on the screen.

What's next on the agenda chaps and chapesses?

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mrtinb
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Re: Non working ZX81

Post by mrtinb »

If you write SAVE"X" and newline, sound output will come on the EAR connector. Try with headphones.

Type SAVE "X" is pressing these keys:

Code: Select all

S shift-P X shift-P newline
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

Moggy
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Re: Non working ZX81

Post by Moggy »

It's also possible that the internal RAM is shot. Do you have a known good RAM pack to test with?

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1024MAK
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Re: Non working ZX81

Post by 1024MAK »

What test gear do you have?

Multimeter - you say you have one. Is it a cheap one or a higher priced model?
Logic probe?
Logic analyser?
Oscilloscope - you say you have one on the way, has it arrived, and what type/make/model is it?

A fault with any of the four/five chips can prevent a ZX81 from working. But alas, a black screen is often the symptom of a failed ULA (IC1).

Using your multimeter first, then if you have it yet, your ‘scope, measure the voltage on IC1 (ULA) pin 16.
If your ‘scope has a suitably high bandwidth and sampling rate (if digital), also see if you can see if the CPU clock input is active on IC3 (CPU) pin 6.

Mark

colday
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Re: Non working ZX81

Post by colday »

Yes, have a couple of multimeters, the newest is true RMS 6000 count so should be fine.

Goldstar Oscilloscope OS-9020P 20MHZ. Works nicely.

My drawback on the scope is the settings, however I can muddle through with it.

I don't have a logic probe as yet and am not actually 100% what it is, but I'll google it. Same goes for logic analyser.

Had a rubbish night last night so am off to bed imminently as shattered.

Will have a look at pin 16 on the ULA as soon as I can.

Quick question in the mean time, can I use the Speccy ULA in it's place to see if I get an output? Are they the same?

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mrtinb
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Re: Non working ZX81

Post by mrtinb »

The ULA in the ZX81 and the Spectrum are different.
Martin
ZX81, Lambda 8300, Commodore 64, Mac G4 Cube

colday
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Re: Non working ZX81

Post by colday »

Arse

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1024MAK
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Re: Non working ZX81

Post by 1024MAK »

The different ULAs are a large part of what gives the ZX81 and ZX Spectrum their personalities and characteristics.

Mark

colday
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Re: Non working ZX81

Post by colday »

Ok, gonna try Pin 16 and Pin 6.

What sort of values should I be looking for?

From memory last night, I got something like 3.6v on pin 16 and 4.1v on pin 6.

They are approximate as brain has switched off since then.

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