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I was recently gifted an old 8-bit BBC Microcomputer. This is a 6502 based computer that's built like a tank. The last time it had been switched on, it went pop with a puff of acrid smoke. Turns out this is a very common problem with the noise suppression (X) capacitors in the PSU, so I replaced these two capacitors, along with an electrolytic that can cause problems due to age.
It burst into life with two tones of square-wave joy, and gave be a lovely (and by lovely, I mean horrible) picture on my 4K TV's composite input. The colours seem weird, but useable. I have since discovered that there's a jumper link that can be made to enable colour composite output, and it looks *much* better.
The PSU capacitor fix is well documented on the web, so I won't discuss it more at the moment. I will however remind you to take photos before dismantling stuff like this, they pretty much always come in handy at some point in the future.
I had already cleaned the keys and used a silicone lubricant (acetone free) on the switches, which had improved the feel of the keys, but it wasn't long before I discovered there were properly faulty keys on the keyboard.
Two keys were completely non-functioning, and two were mostly non-functioning. I checked continuity between the traces on the keyboard PCB, which were all fine, so I desolderer the key-switches and checked for continuity across the pins (to eliminate bad-solder joints as the cause). Still no joy, so I had to fully remove the switches to see what I could do.
It turns out the key-switches can be entirely dismantled to service. The pins themselves unscrew from the shell, and the base comes free revealing the conacts.
Be careful not to lose the little spring that's inside when opening.
So, one switch had a crushed contact - I can't imagine how that happened. All four had blackened pins, whether it was some kind of residue or perhaps oxidation, I don't know. Regardless, a small file to scrape the pins back to shiny silver was all I had to do. I checked each before soldering back into the board.
Teardown pictures of the BBC Micro key-switches.
I'm not sure if contact cleaner or isopropyl alchohol would have been enough to clean the contact pins, but at least two looked so blackened that maybe abrasion was the best approach.
I'm pleased to say that the whole thing went back together easily and is working as it should.
I was reminded to use care with old hardware (I lost half a pad when desoldering one of the switches). Not too much heat on the iron (300C for sucking the solder), mop up excess solder with good desoldering braid (wick carefully at 400C).
Always use flux when re-soldering components, particularly where the board has been exposed to the outside world (like the keyboard PCB). The joints will be messy otherwise, and you may end up overworking the joint to the point of damaging pads, traces, or components.
This old computer is so fixable, it was clearly designed by people who like things to last (I suppose it was designed for education, so it had to survive a hostile environment!). The fact that I could repair keyboard switches is such a striking departure from modern hardware.
It came with two floppy disk drives and a couple of boxes of disks. All the disks seem to still work. That is also quite amazing. The machine was clearly used for education, judging from the software listed on the disk labels. There was just one disk suspiciously without any labelling, so I had to take a peek and found it full of games. Yay!
As an aside, the reason why a jumper link is needed to get colour composite video is because when colour is enabled, the RGB-Out signal degrades slightly. A reminder of just how raw this hardware is. In fact, I can hear the circuits squeal among the background hiss of the speaker. I'm literally listening to the sound of a running computer - how cool! You don't get that with an emulator (more accurately, a simulator).