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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 1st May 2021, 8:54 pm   #1
Cobaltblue
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Default Sinclair ZX81 AKA door stop

Split from this thread

https://www.vintage-radio.net/forum/...d.php?t=179455


I thought seeing as I have been using it as a door stop I would see if this particular ZX81 showed and sign of life.

Of course it didn't no surprise there!

The 5V reg is working and getting warm (Not Hot).

There is a signal going into the AZTEC modulator (Looks reasonable on the scope) but seemingly nothing coming out. I am not yet geared up to pull the video out ( two year house rebuild nightmare) maybe I will set up in the loft to deal with this sort of stuff.

I will start a new thread on it I must have 3 or 4 of the things hanging around (when I can find them!).

I repaired a few (lots) back in the 80's then it was mostly ULA failures.

Not really a fan of Sinclair computers but to give them their due they supplied parts and not expensive back in the 80's.

As the Aquarius is working until I find the interface its going back into hibernation.

Cheers

Mike T
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Old 2nd May 2021, 9:30 am   #2
Timbucus
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Default Re: Mattel Aquarius

When you start your thread if the 81 is old then it likely has no backporch as only the later ULA had that so any reasonably modern TV will fail to get a signal.
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Old 2nd May 2021, 10:10 am   #3
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Default Re: Mattel Aquarius

Quote:
Originally Posted by Timbucus View Post
When you start your thread if the 81 is old then it likely has no backporch as only the later ULA had that so any reasonably modern TV will fail to get a signal.
Thanks for that tip I had completely forgotten about that.

Disappointingly it means this one is probably also working

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Old 2nd May 2021, 10:39 am   #4
SiriusHardware
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Default Re: Mattel Aquarius

Cbb, maybe you'd better start that '81 thread and move these related posts to that thread, since you happen to be able to do that.

The important number on the ULA is '210'. If it has a 210 ULA you are in luck, as that was the final version which produced a properly formed video signal complete with back porch.

Any earlier version (such as 184) will have the back-porch problem. The usual effect is just to make the overall picture very dark, you should still be able to see a black / dark UHF signal on around channel 36. The effect of the missing back porch is usually just to fool the display into adopting the wrong signal level as black-level.

There are little add on circuits which can fit inside the modulator can which convert the output to composite, but the missing back porch will still cause problems even then. The more sophisticated circuits also add the missing back porch signal, but as you don't seem to be much of a ZX fan I doubt you will want to go to that sort of trouble with this machine.
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Old 2nd May 2021, 11:22 am   #5
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Default Re: Sinclair ZX81 AKA door stop

You said the regulator was working - hopefully you measured it - when I was fitting one of the little CCB units to mine to get a good quality Composite and Back Porch I did not notice that it was working but producing around 8V the ULA lasted about 30 minutes...
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Old 2nd May 2021, 11:26 am   #6
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Default Re: Sinclair ZX81 AKA door stop

No I measured it just a shade under 5V

No ripple to speak of either.

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Old 2nd May 2021, 1:27 pm   #7
Timbucus
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Default Re: Sinclair ZX81 AKA door stop

Phew - hate to ask obvious questions but, as I should have known better having repaired many things I always feel it better to do so these days. You could hook up audio and type S (SAVE) which SHIFT P (") anything SHIFT P again and NEWLINE - it should screech...
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Old 8th May 2021, 12:56 am   #8
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Default Re: Sinclair ZX81 AKA door stop

Keep in mind when doing tests entering commands via the keyboard, these old Sinclair's do suffer with the common fault of membrane failures. So test results might be misleading.
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