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Forum Index : Microcontroller and PC projects : keyboard repair
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| Volhout Guru  Joined: 05/03/2018 Location: NetherlandsPosts: 5089 |
I though I would share this with you, since it may help someone in the future. I use a PS2 keyboard with the CMM1. Last night the "z" key of the keyboard would not respond. Since it is an old keyboard, I decided to open it up and check for dirt. Maybe it needed cleaning. The keyboard used a transparent foil with carbon contacts, the keys had conductive rubber bridges to establish the contact. After cleaning the keys and the foil the problem remained. Investigation showed that the foil showed a 10k ohm resistance parallel to the "z" key. The other keys where real opens. Visually inspected the foil (transparent), even on a light bed, I could not see anything wrong with the foil. Since the foild was cleaned, it must have been something inside the foil. I decided to take the gamble, and pulled out my bench power supply, set it to 50V, and connected to the 2 carbon pads at the "z" key. Its is only 50V at 10k ohm, so 5mA -> 250mwatt. But that removed the 10kohm impedance, it was simply burned away. The keyboard work fine now. PicomiteVGA PETSCII ROBOTS |
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disco4now Guru Joined: 18/12/2014 Location: AustraliaPosts: 1003 |
Good result. That technique was one of two that we were never told about when we were trainees in the 1970s. I learned them from the older guys that work on Step X Step exchanges. 1. using 50v and a 16A rack fuse to blow away a short you could not find. 2. was the 1 ohm headset. Mines long gone but wish I still had it. If looking for a short in a multiple i.e wires going from point to point to form a bank of contacts, then parallel a couple of lamps in series with the short to get some current flowing through the short. With the 1 ohm headset you can hear a click if current is flowing. Move along the multiple testing from one connection to the next.Once past the short you don't hear the click. Latest F4 Latest H7 FotS |
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vegipete Guru Joined: 29/01/2013 Location: CanadaPosts: 1132 |
Reminds me of an old Printronix line printer I owned decades ago. One day it quit working, so I went poking around inside and found a blown fuse. I didn't have a 1 Amp fuse handy, so I guessed with a strand of wire. When I turned it on again, there was a puff of smoke from deeper inside ... an the printer worked perfectly ever after. I kind of miss that heavy old beast. Visit Vegipete's *Mite Library for cool programs. |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2640 |
"2. was the 1 ohm headset. Mines long gone but wish I still had it. If looking for a short in a multiple ie wires going from point to point to form a bank of contacts, then parallel a couple of lamps in series with the short to get some current flowing through the short. With the 1 ohm headset you can hear a click if current is flowing. Move along the multiple testing from one connection to the next. Once past the short you don't hear the click." Long ago used a 2 ohm speaker from a valve radio the same way. Maybe a small 240/6V transformer connected back to front to headphones would also work. |
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Herry Senior Member  Joined: 31/05/2014 Location: AustraliaPosts: 261 |
'didn't have a 1 Amp fuse handy, so I guessed with a strand of wire. When I turned it on again, there was a puff of smoke from deeper inside'... Not to be recommended with house wiring! Senior?! Whatever it says, I'm a complete and utter beginner... |
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RetroJoe Senior Member Joined: 06/08/2020 Location: CanadaPosts: 290 |
That is an amazing trick of the trade! Given that this "trial by voltage" technique would have a high likelihood of vaporizing something vital, I imagine it should be the penultimate step in the field repair manual i.e. the one right before "Discard non-functioning unit". @disco4now, I'm guessing thick copper bus bars and giant relays lends itself very well to this game of "zap the short", if it's a whisker of light gauge wire gumming up the works. @Vegipete, roughly the same idea with the electromechanical guts of your vintage line printer. Edited 2021-02-13 16:48 by RetroJoe Enjoy Every Sandwich / Joe P. |
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9610 |
My favorite trick to play on sparkie's(electricians), is to have an FM or other transmitter with an output power of 10-20 watts, fed into a tuned J-pole or Slim-JIM antenna. These represent a DC short-circuit, but they are not a short-circuit at RF frequencies. Put a 20W 12v car bulb across the feed line - the bulb will light up from the RF energy. Now ask the sparkie to explain how that can be, with a dead-short across the bulb and coax.   Most of them are tricked by it, cos they can SEE the dead-short across the bulb, and are stumped by how the bulb can light with a DC short-circuit across it. Just a little party-trick taught to me when I was learning RF from my boss of the time. He actually played that trick on ME during my apprenticeship, and I also could not work out how the bulb could light with a DC short across it! But that is one trick I have always remembered. Smoke makes things work. When the smoke gets out, it stops! |
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