ZX81 Some keys not working

Discussions about Sinclair ZX80 and ZX81 Hardware
dessony
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Re: ZX81 Some keys not working

Post by dessony »

These gentlemen are right because I exactly did thing. Can you get your multimeter out and set it for a diode check or a continuity tester? Hold a probe on the 3rd "finger" of the 5 pins connector and hold another probe on an ULA's pin # 29. Inform us of the result here. Check the ULA's socket hole #29 and its solder joint for any irregulars conditions like corrosion.

Find a resistors' area numbers R43-R47, which can be found near the 8 pins connector. These resistors have their colors in its order: Brown / Black / Orange and Gold. It is 10K ohm. Put the probe on a bottom end of the "middle" resistor and put another probe on middle contact of the 5 pins connector. Record your result here.

Set your meter near to 10Kohm as possible or higher. Test the middle resistor, R45 for its value, open, short or burnt. Tell us the result here.

Apply some power to the computer motherboard. Now set your meter to 5 DCv or little higher. Put the black probe on a ground area like a heatsink, a RF modulator or a LM7805 area of your motherboard. Put the red probe on the R45's top side first. Record the result here. Then move the red probe to the 5 pins connector's middle contact. Inform us of the result.

DesSony
Last edited by dessony on Wed Dec 19, 2018 6:59 am, edited 1 time in total.
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1024MAK
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Re: ZX81 Some keys not working

Post by 1024MAK »

Also have a look at the schematic in this post ;)

Mark
ZX81 Variations
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repetto74
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Joined: Tue Dec 20, 2016 8:46 pm

Re: ZX81 Some keys not working

Post by repetto74 »

These gentlemen are right because I exactly did thing. Can you get your multimeter out and set it for a diode check or a continuity tester? Hold a probe on the 3rd "finger" of the 5 pins connector and hold another probe on an ULA's pin # 29. Inform us of the result here. Check the ULA's socket hole #29 and its solder joint for any irregulars conditions like corrosion.

Find a resistors' area numbers R43-R47, which can be found near the 8 pins connector. These resistors have their colors in its order: Brown / Black / Orange and Gold. It is 10K ohm. Put the probe on a bottom end of the "middle" resistor and put another probe on middle contact of the 5 pins connector. Record your result here.

Set your meter near to 10Kohm as possible or higher. Test the middle resistor, R45 for its value, open, short or burnt. Tell us the result here.

Apply some power to the computer motherboard. Now set your meter to 5 DCv or little higher. Put the black probe on a ground area like a heatsink, a RF modulator or a LM7805 area of your motherboard. Put the red probe on the R45's top side first. Record the result here. Then move the red probe to the 5 pins connector's middle contact. Inform us of the result.

DesSony

I will do that as soon as possible and revert thanks a lot! :-)
RWAP
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Re: ZX81 Some keys not working

Post by RWAP »

Thanks for the pictures XavSnap - that is helpful as we can all see the keys link to the middle pin in the 5 way connector.

It will be interesting to see what the issue is....
repetto74
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Re: ZX81 Some keys not working

Post by repetto74 »

Then move the red probe to the 5 pins connector's middle contact. Inform us of the result.
By "middle" you mean the third pin of the strip connector? (the one mapped to the set of keys not working in my case).
Will check and post the result tonight :-)
dessony
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Re: ZX81 Some keys not working

Post by dessony »

Repetto74,

Middle contact = 3rd pin of 5 pins connector. Yes. :-)

DesSony (Bill)
repetto74
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Re: ZX81 Some keys not working

Post by repetto74 »

Hi Dessony,

I removed the brand new strip from the strip socket and look at that!! This is insane, it should not break so easily just by pushing in and out the strip into the socket. I think this is the reason why the keys are not working :evil: :evil: :evil:
This thing is coming into piece just after installing it one time. Very disappointed!

Checked the continuity from pin 3 and ULA pin 29 and it is OK. How to fix that now? Should I trim the damaged part?
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RWAP
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Re: ZX81 Some keys not working

Post by RWAP »

You have a ZXpand Keyboard Membrane (blue tails). We had an issue with the bonding of the silver ink on some of these (the factory are still not sure why) so the easiest is to send you a replacement.

However, it could also indicate that some of the pins inside the molex connectors are corroded or bent, thereby scratching the silver ink. You can test for this by using a thin piece of card - insert it into the molex and see if it comes out scratched on that middle pin.

PM me with your details and I will get a replacement off to you.
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XavSnap
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Re: ZX81 Some keys not working

Post by XavSnap »

"Loctite 3863 Circuit Plus" (19$)
or
cut 3-5 mm ... But, it's a bad good idea! :mrgreen:
Xavier ...on the Facebook groupe : "Zx81 France"(fr)
repetto74
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Re: ZX81 Some keys not working

Post by repetto74 »

I have PM'ed you RWAP many thanks in advance for your kind support :)
I will investigate a little bit more on this strip connector and ensure it is safe to plug a strip into it.
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