Keyboard Issues .. Some keys not working (new membrane)

Discussions about Sinclair ZX80 and ZX81 Hardware
thedevworks
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Keyboard Issues .. Some keys not working (new membrane)

Post by thedevworks » Tue Mar 19, 2019 11:06 pm

Hey all .. again..

I recently added a new ULA and a new keyboard membrane.. but some keys are not working.

Tried reheating the ULA joins and still same .. but perhaps I can identify the issue to a pin.

8 actually causes the whole thing to crash.. but the following keys don't work at all ;

1 2 3 4 5 (8 crazy) e i d k x m

How would I best figure out why

Thanks again!

James

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Paul
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Re: Keyboard Issues .. Some keys not working (new membrane)

Post by Paul » Tue Mar 19, 2019 11:44 pm

Hi, would you please be so kind and tell us to which computer you added a new ula and membrane? ZX81, SPECTRUM etc and in case of spectrum which type and Mainboard issue.
Kind regards Paul
In theory, there is no difference between theory and practice. But, in practice, there is.

thedevworks
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Re: Keyboard Issues .. Some keys not working (new membrane)

Post by thedevworks » Wed Mar 20, 2019 8:18 am

Oops sorry, zx81 Issue One !

RWAP
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Re: Keyboard Issues .. Some keys not working (new membrane)

Post by RWAP » Wed Mar 20, 2019 8:40 am

Where the keys are not fully working, this can be the ULA, the membrane, or the molex sockets (as a start).

It could be a faulty membrane, but the first thing to check is the molex sockets. Did you leave any bits of the old membrane inside? If not, try using a wire to connect each pin in the 5 way connector to each pin in the 8 way connector as that will produce a key stroke.

See if that shows up any problems - in which case you might need to resolder or replace the molex connectors.

thedevworks
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Re: Keyboard Issues .. Some keys not working (new membrane)

Post by thedevworks » Wed Mar 20, 2019 1:59 pm

Aha thanks for that idea. I tried that and pretty much get the same issue (Apart from 1-5 were actually working.

And most of the 'duff' keypresses cause a crash. So I guess next step is to try and log which combinations are duff and map that to a schematic to figure out the cause. \


My concern with the ULA soldering was that I only soldered under it, I understand that the pads should go through the holes so this should be fine, but I did wonder if I damaged any during the de-soldering / re-soldering despite being careful

thedevworks
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Re: Keyboard Issues .. Some keys not working (new membrane)

Post by thedevworks » Wed Mar 20, 2019 2:07 pm

the latest list is;

3 E D X 8 I K M

looking at the schematic (Is there anything better for the issue one?)

http://www.zx81.de/bilder/ts1500_600.gif

All the faulty keys run to KBD2 on the ULA. So, with that In mind I traced it on the board and made sure the soldered joint was all good

So, the situation now is..

Key presses on these keys are totally dead using the membrane.

Connecting the key presses using a wire and the keyboard pins (middle pin on the 5 pin molex to any pin on the 8 pin) actually prints the character, but every time either the screen flickers or the system crashes alltogether. This even happens with the membrane keyboard removed, so I'm not sure this could be a molex issue. I've also checked and checked again for any shorts or broken joins.

I have also tested this with a second membrane (My original which I guess was never faulty) and the same issue occurs. Also run a small wire between KBD2 pin on the ULA and the molex pin, and still dead.

Aside from a faulty ULA I can't think what else this could be.. Any ideas?


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1024MAK
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Re: Keyboard Issues .. Some keys not working (new membrane)

Post by 1024MAK » Wed Mar 20, 2019 4:29 pm

How have you tested for short circuits?

You really need to test between KBD2 and any other track/trace / circuit that may run close to the KBD2 track/trace / pin. Not just to the +5V and 0V/GND lines.

Mark

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1024MAK
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Re: Keyboard Issues .. Some keys not working (new membrane)

Post by 1024MAK » Wed Mar 20, 2019 4:33 pm

See this post for pictures of an unpopulated issue 1 board ;-)

Mark

thedevworks
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Re: Keyboard Issues .. Some keys not working (new membrane)

Post by thedevworks » Wed Mar 20, 2019 6:03 pm

Thanks for the replies again all!

At this point I'd not even thought of getting my multimeter out despite using it to do initial tests ! :) I'm pretty new to this and learning a lot as I go, these small hints send me down a path that I can investigate and learn more. (I've been working my way through a pile of vintage electronics and re-purposing them rather than throwing them away! I have also discovered that focusing on tiny components breaks my eyes so I just ordered a microscope!

Anyway back to business!

I'm working my way around the board testing things are 'as they should be' , looking for shorts.

The only oddity I have found so far is the resistor pack just above the keyboard set. The centre post (The one I have an issue with) is reading around 6k resistance whilst the rest read 10k (Correct according to schematic) .. Could this be my issue.. or more of an indicator of an issue

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