Scrolling K on 48k+ start up

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alanpwhelan
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Scrolling K on 48k+ start up

Post by alanpwhelan » Sat Mar 17, 2018 6:58 pm

Hello everyone, I’ve dipped my toe back into the world of the Spectrum and have bought a 48k+ on eBay.

I’ve gone to switch it on for the first time and as soon as the Sinclair copyright info appears at the bottom of the screen the letter K starts racing across the screen. I thought the key might be stuck so I pulled it out and the letter kept on scrolling. It will only stop if you press and hold down another key.

The seller has performed the following mods: recapped it, fitted a new membrane, replaced TR1 and TR2, composite mod and a heat sink fitted to the ULA. It’s an issue 3 motherboard.

Any thoughts on what could be causing it?

Thanks
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PeterJ
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Re: Scrolling K on 48k+ start up

Post by PeterJ » Sat Mar 17, 2018 7:11 pm

Welcome @alanpwhelan

It sounds like the keyboard membrane may need replacing.
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alanpwhelan
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Re: Scrolling K on 48k+ start up

Post by alanpwhelan » Sat Mar 17, 2018 7:14 pm

Thanks for that.

I’ve seen various sellers on eBay selling membranes, is there a particular seller, eBay or elsewhere, that anyone would recommend?
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Ast A. Moore
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Re: Scrolling K on 48k+ start up

Post by Ast A. Moore » Sat Mar 17, 2018 7:31 pm

Some modern membranes are of poor quality. Looks like the membrane was assembled before the conductive paint fully cured. You should be able to fix that by gently massaging that area.

On the other hand, if by “pulling out” you meant you disconnected the membrane from the logic board, this means that one of the lines got stuck. Since you can’t get the letter K to appear by pressing the “K” key, I assume you mean the K mode cursor. Then it’s likely that it’s the SPACE key that is stuck.

The SPACE key are pin 1 on KB1 and pin 8 on KB2. I’d start by carefully following the trace coming from pin 1 on KB1 and making sure there are no shorts (e.g. solder splatter, etc.) or breaks. Also check R69. Then do the same by following the trace from pin 8 on KB2. Check D8. Finally, reseat the ULA.
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alanpwhelan
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Re: Scrolling K on 48k+ start up

Post by alanpwhelan » Sat Mar 17, 2018 8:05 pm

Yes I meant the K mode cursor. Thanks for your replies

:)
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1024MAK
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Re: Scrolling K on 48k+ start up

Post by 1024MAK » Thu Mar 22, 2018 8:21 pm

Yeah, it could just be dirt or a mechanical problem with the keyboard causing the membrane 'bubble' to be pressed for the contacts for the [space] key.

As suggested above, very carefully disconnect one of the membrane tails and then power up and see if the problem is still present.

New membranes are available from Sell My Retro

Mark
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alanpwhelan
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Re: Scrolling K on 48k+ start up

Post by alanpwhelan » Thu Mar 22, 2018 8:56 pm

Thanks Mark.
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