Spectrum Plus key operation faults

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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Chookie
Drutt
Posts: 8
Joined: Sat Feb 03, 2018 5:28 pm

Spectrum Plus key operation faults

Post by Chookie »

Hi all.

Before I ask my question, I haven't just bought a machine off some bloke on Ebay that doesn't work :D

I've had my Speccy for a good number of years now and have been happily playing all my old software on it until early this week when out of the blue some of the keys stopped functioning. These keys included most symbol shifted characters, the quotation symbol key and the edit key to name a few. The standard keyboard numbers and letters function fine.

Ah I thought, the membrane has finally given up which was not bad for its age. After doing my research I ordered a new membrane from a reputable supplier and fitted it into the machine. Great I thought, time to get gaming ... wrong ... the replacement membrane behaves exactly the same as the original.

So before I start digging out soldering irons and volt meters, could I get some pointers as to what else could be causing this issue, straight off the bat I'm suspecting dry joints with the cable connects on the mainboard.


All advice is most appreciated.
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Ast A. Moore
Rick Dangerous
Posts: 2640
Joined: Mon Nov 13, 2017 3:16 pm

Re: Spectrum Plus key operation faults

Post by Ast A. Moore »

Here’s the keyboard matrix with its logic board connections:

Image

Determine which keys are not working (whether it’s a row or column), and check the respective line for continuity, dry solder joints, or faulty diodes to the KB2 connector or pull-up resistors to KB1.

Another point of note. The membrane has two ribbon cables running from it to the connector on each side. They must be clamped fairly tightly by the plastic brackets so the two metabolized conductors make contact. Make sure they’re not disrupted. You can use the diagram above to test continuity of the membrane itself with a multimeter by pressing the respective keys. (Holding the probes in place will be tricky, though.)
Last edited by Ast A. Moore on Sat Feb 03, 2018 8:04 pm, edited 1 time in total.
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
Chookie
Drutt
Posts: 8
Joined: Sat Feb 03, 2018 5:28 pm

Re: Spectrum Plus key operation faults

Post by Chookie »

Hi and thank you for the guidance.

For some reason I can't see your image, could you email me this ?

Thanks
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Ast A. Moore
Rick Dangerous
Posts: 2640
Joined: Mon Nov 13, 2017 3:16 pm

Re: Spectrum Plus key operation faults

Post by Ast A. Moore »

How about now?
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
Chookie
Drutt
Posts: 8
Joined: Sat Feb 03, 2018 5:28 pm

Re: Spectrum Plus key operation faults

Post by Chookie »

Hello again

No sorry I just don't see it. Can you please email me it instead ?

[email protected]

On another note, carried out checks on those resistors and diodes. Checked KB1 and KB2 connectors back to resistors and diodes with no dry joints. All diodes and resistors ok.

I'm a bit perplexed, maybe there is an issue with the keyboard after all. When I disassembled the keyboard I did notice the hard cardboard type inlay was a bit uneven so I thought this could have been the issue with faulty keys. I remedied this by taking the wife's iron over it :0)
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