ZX81 witk 16K memory pack white screen

Discussions about Sinclair ZX80 and ZX81 Hardware
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kiFla
Posts: 4
Joined: Mon Sep 04, 2017 11:33 pm

ZX81 witk 16K memory pack white screen

Post by kiFla »

Hi.
I have Strange problem with one of mine ZX81

Plane ZX81 with 1K works without problem. i can load and execute programs.

When I connect 16K RamPack (Sinclair 16K RAM) ZX81 most of the time show only white screen without K cursor. Maybe 1 of 10 times starts normally, but very unstable (freezing, crashing, won't work even with 1k programs.
White screen is bright for few seconds then loose intensity and start flickering.

I have 2 Sinclair 16K rampacks. Both of them not working on problematic ZX81.

Ram packs are definitely working because they are working fine on my other two ZX81.

So far I tried:

- Changing ZX Power Supply - its a UK1400, works without problem with other two zx81 with 16K Rampacks
- replace 2 electrolyitic capacitors with fresh new ones
- replace 7805 voltage regulator with switching regulator
- reseated all chips in their sockets multiple times
- change all chips except memory from another full working zx81
- cleaned with eraser and alcohol edge connector on board. (Contacts are shiny :) )

ZX81 has Pokemon's ZX8-CCB composite mod fitted.

Any suggestions?
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1024MAK
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Re: ZX81 witk 16K memory pack white screen

Post by 1024MAK »

If the RAM pack works with another machine, and works occasionally with this ZX81, the only thing I can think of, is that there is a poor connection that only affects the circuits that go to the edge-connector contacts (I presume that all the edge-connector contacts are in good condition). The poor connection could either be a dry solder joint, or a damaged PCB track.

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kiFla
Posts: 4
Joined: Mon Sep 04, 2017 11:33 pm

Re: ZX81 witk 16K memory pack white screen

Post by kiFla »

The edge-connector contacts looks visually OK. I will test with multimeter every corresponded track from edge-connector contacts to destination chips and elements.
kiFla
Posts: 4
Joined: Mon Sep 04, 2017 11:33 pm

Re: ZX81 witk 16K memory pack white screen

Post by kiFla »

I check all traces that leads to edge pins. I didn't see problem. Finally I clean edge pins more radically with sand paper. Now it works OK!!!
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PokeMon
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Joined: Sat Sep 17, 2011 6:48 pm

Re: ZX81 witk 16K memory pack white screen

Post by PokeMon »

With old connectors / pcb's it works sometimes if I pull the device a little bit from the board instead of plugging it as deep as possible. ;)
kiFla
Posts: 4
Joined: Mon Sep 04, 2017 11:33 pm

Re: ZX81 witk 16K memory pack white screen

Post by kiFla »

Yes it's true. That's why I was sure that is only connection problem. So I was little harsh and use sand paper on contacts.
Now it's working perfectly without any issue. And wobble effect is minimum :)
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