SOLVED - Odd keyboard happenings

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Colday
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SOLVED - Odd keyboard happenings

Post by Colday » Sat Oct 10, 2020 10:35 am

Morning Guru's.

Have a slightly odd fault with a beautifully original and clean Issue 4 that I acquired the other week.

It fires up and works lovely but I noticed a few of the keys did not work. No obvious pattern to those that didn't work so I figured just needed a little contact cleaner and many cycles of on and off.

Removed the keycaps and then applied the cleaner and started to repeatedly press, for instance, the S key.

Nothing, several hundred presses and sore fingers later, no joy.

Ok, maybe the switch had failed so I removed the keyboard, checked the solder joints for the key (all seem fine), reflowed the solder on the keyboard connector pins, again all seemed fine anyway.

However, while faffing about with the pressing the key, with the keyboard vertical, I noticed 'S' appearing on the screen (I was checking to see if the keyswitch solder joints were dry). Ooh I thought, it's started working. Laid the keyboard down again, and it stops.

Lift it up vertical, it works, lay it down it stops.

However, now I pay closer attention when the board is laid flat and I press the key, I can here a (very) slight change in the (very faint) noise that is coming from the speaker. I can't really explain, if you can imagine faint tinnitus which then changes pitch ever so slightly when the key is pressed, which must mean that somehow the key stroke is being registered somewhere.

I removed the mainboard, checked and reflowed the keyboard connector that end. I've done continuity checks with the cable. I have swapped out the VIA for a known good one with no change.

My next port of call will be to open up a different beeb and swap a known good keyboard in with a different cable. But in the meantime can anyone explain the change of tone (so faint) when the key is pressed?

The fact it works at one angle and not another smacks of it being a duff solder joint and I will be examining in fine detail the boards as soon as I have done my housework. Lol.
Last edited by Colday on Sat Oct 17, 2020 8:24 pm, edited 1 time in total.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

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BigEd
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Re: Odd keyboard happenings

Post by BigEd » Sat Oct 10, 2020 11:40 am

Maybe a duff solder joint, but maybe also a cracked PCB track.

The speaker fizz is a readout of CPU activity, which does change when the OS detects a pressed key. (Related, I had an expected and very slight slowdown once because my case was holding down a SHIFT key just enough to register. That's because neither case nor keyboard has any fixings.)

Bobbi
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Re: Odd keyboard happenings

Post by Bobbi » Sat Oct 10, 2020 6:31 pm

If you short out the contacts at the back of the switch with a length of wire, does it work reliably?

Coeus
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Re: Odd keyboard happenings

Post by Coeus » Sat Oct 10, 2020 6:46 pm

When you say not obvious pattern, have you checked the circuit diagram in the Advanced User Guide? The rows and columns of the matrix don't correspond to the rows and columns as the keys are physically laid out. So it is worth checking if all the failing keys are in the same row or column as that makes it more likely to be a PCB track rather than an individual switch.

I had a keyboard fault BITD which took a while to find. A group of keys didn't work and it was only when I tried sucking the solder off some of the joints and re-soldering that the cause became apparent. One of the switches had not been completely flush to the PCB when it was soldered in so use had caused it to flex the copper track until a hairline crack had formed between the pad into which the pin of the switch was soldered and the rest of the track. When I removed the solder the pad fell off. Obviously this crack had meant the key concerned and anything downstream of the crack didn't work. I ended up removing the resist for a bit on either side of where the pad should be and bridge it, via the switch pin, with a piece of wire.

On the change of soumd, when at least one key is down the keyboard changes from auto-scan mode to being scanned by the OS so that may account for it.

Coeus
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Re: Odd keyboard happenings

Post by Coeus » Sat Oct 10, 2020 6:50 pm

BigEd wrote:
Sat Oct 10, 2020 11:40 am
(Related, I had an expected and very slight slowdown once because my case was holding down a SHIFT key just enough to register. That's because neither case nor keyboard has any fixings.)
The shift key does not generate the key down interrupt, neither does anything else on that row so neither does the control key or the start-up options. The way the circuit diagram is drawn could be plainer but if you look very carefully the NAND gate input is on the VCC side of the pull-up, not on the switch side. The OS checks shift and control when some other key is detected.

Colday
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Re: Odd keyboard happenings

Post by Colday » Thu Oct 15, 2020 6:47 pm

So I resoldered all the connections, and have checked everything.

It now appears that only the 'S' key is playing up. I've removed it from the board, cleaned it and resoldered it in place with no improvement (only works vertical).

I do have a couple of spares so will replace it next and report back.

Having my chimney lined for my log stove this weekend so maybe not till next week.
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

Colday
Posts: 396
Joined: Thu Jul 25, 2019 10:18 pm
Location: North Essex, UK
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SOLVED Re: Odd keyboard happenings

Post by Colday » Sat Oct 17, 2020 8:23 pm

Ok, replaced the key switch, exactly the same issue.

Started to try install some patch cables and while doing so noticed some dodgy looking solder way over on the right of the PCB.

Scraped back a little of the green PCB stuff exposing a little of the copper, fluxed, soldered.

Sorted.

Works perfectly. I left the replacement keyswitch in, but have kept the non faulty removed one.

Now have a perfect Issue 4.

Oh, and a really warm house now the chimney is lined and the stove is alight!
BBC B's... I now have 6!
I also have 1 boxed with manuals, unmolested model A.
And also an unmolested model B. (but not boxed sadly)
12x floppy drives (only 1x currently works I think)...
Learning to repair and refurb keyboards next! No more sticky keys!

Bobbi
Posts: 150
Joined: Thu Sep 24, 2020 12:32 am
Contact:

Re: SOLVED - Odd keyboard happenings

Post by Bobbi » Sun Oct 18, 2020 12:09 am

Congratulations!

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