ZX Speccy erratic behaviour

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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Speccyspooky
Drutt
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Joined: Sat Feb 09, 2019 4:35 pm

ZX Speccy erratic behaviour

Post by Speccyspooky »

Hi all,

Recently bought a ZX Speccy off Ebay. Have got it going using a correct power supply, and seems to more or less work. Have successfully written and run a very basic BASIC program that prints a phrase over & over. Have taken it apart and done the composite video mod using capacitor.

However, it does seem to have this problem, of basically doing erratic things. It will type random keypresses on it's own now & again. Not that often but enough to be a problem. This seems to interfere with loading anything. Don't know what it could be. It seem to basically be a bit 'noisy' , to use an analogue electronics term. Any ideas?

Many thanks.

edit, forgot to add, it is 48k version. probably issue 3.
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Ast A. Moore
Rick Dangerous
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Re: ZX Speccy erratic behaviour

Post by Ast A. Moore »

The symptoms as you describe them are a bit too vague. This could be a noisy power rail issue, for example. I rarely advocate it, but this might be the case for the board needing to be recapped.

Do you have an oscilloscope, by any chance?
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
Speccyspooky
Drutt
Posts: 3
Joined: Sat Feb 09, 2019 4:35 pm

Re: ZX Speccy erratic behaviour

Post by Speccyspooky »

Thanks for the reply.

Sorry I guess I was a bit vague. But I suppose all I can say at the moment is that, the Spectrum is acting like keys are being pressed, when in reality they aren't. Unfortunately I don't have a scope.

I have heard some people advocating the capacitors should be replaced as a matter of course, but this is just one attitude.
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Ast A. Moore
Rick Dangerous
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Re: ZX Speccy erratic behaviour

Post by Ast A. Moore »

Speccyspooky wrote: Sat Feb 09, 2019 6:48 pm the Spectrum is acting like keys are being pressed, when in reality they aren't.
If that’s indeed the only issue, than something is probably wrong with the ULA. Another possibility is something creating intermittent shorts between the two keyboard connectors. Try unplugging the keyboard membrane from the computer and letting it sit like that after powering it on. If the fault clears, then it’s a mechanical issue with the membrane. If not, then it’s something else.
Speccyspooky wrote: Sat Feb 09, 2019 6:48 pmI have heard some people advocating the capacitors should be replaced as a matter of course, but this is just one attitude.
I usually don’t advise doing that in most cases. In and of itself, the operation wouldn’t hurt anything, except for the possibility of accidentally reversing the polarity of the electrolytic capacitors and heat-stressing the vias and the traces. On the other hand, those capacitors are almost forty years old, and can potentially cause issues, including killing some ICs.
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
Speccyspooky
Drutt
Posts: 3
Joined: Sat Feb 09, 2019 4:35 pm

Re: ZX Speccy erratic behaviour

Post by Speccyspooky »

Thanks for advice again, interesting, will do some fiddling about and get back to y'all.
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