Sinclair ZX81 is this a ULA issue

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pudwink
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Joined: Sat Jun 26, 2021 2:40 pm

Sinclair ZX81 is this a ULA issue

Post by pudwink »

Hi all
Plugged my ZX81 in and it is pulling from the bottom and cant see the text but something seems to move when a key is depressed is this a sign of a faulty ULA.
Many Thanks
zx81.jpg
redhawk668
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Re: Sinclair ZX81 is this a ULA issue

Post by redhawk668 »

That looks indeed like an ULA issue.
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1024MAK
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Re: Sinclair ZX81 is this a ULA issue

Post by 1024MAK »

Is this via the TV/RF/UHF output? Sometimes some TVs just don’t like the output from the ZX81. If the TV has fine tuning, see if you can adjust it. If you have another TV, try that.

And yes, it could also be a problem with the ULA or with the ceramic resonator (this is used to control the master oscillator in the ULA, it is used for all the timing including the timing for the video generation).

Mark
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citizenfish
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Re: Sinclair ZX81 is this a ULA issue

Post by citizenfish »

1024MAK wrote: Tue Jun 29, 2021 5:02 pm
And yes, it could also be a problem with the ULA or with the ceramic resonator (this is used to control the master oscillator in the ULA, it is used for all the timing including the timing for the video generation).

Mark
Is that the tiny capacity linked to pin 35 on the ULA?
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1024MAK
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Re: Sinclair ZX81 is this a ULA issue

Post by 1024MAK »

citizenfish wrote: Thu Jul 01, 2021 5:24 pm
1024MAK wrote: Tue Jun 29, 2021 5:02 pm And yes, it could also be a problem with the ULA or with the ceramic resonator (this is used to control the master oscillator in the ULA, it is used for all the timing including the timing for the video generation).
Is that the tiny capacity linked to pin 35 on the ULA?
The identification on the silk screen printing on the board is ‘X1’, it looks like a rectangular capacitor, but it has three legs/pins. One two pins are electrically connected on the board, one goes to 0V/GND. the other connects to capacitor C6, and then C6 connects to pin 35 on the ULA.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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