geekhack
geekhack Community => Keyboards => Topic started by: chimera15 on Mon, 14 February 2011, 01:44:00
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So a while ago I bought a minitouch that was non functional from a fellow geekhacker. He only sent me the board and plate and switches, but I thought I'd try to repair it nonetheless if at all possible.
It's completely non functional. I've tried every possible way to get it to function and tried to physically examine the board to find evidence of damage somewhere to no avail.
I started probing the board with a multimeter and trying to compare my results to a functioning siig. I found that there was voltage between switch terminals and most places on the board where there shouldn't be any. Voltage between switch terminals is 1.5v where on a functioning minitouch it's barely noticeable if any.
There's only one diode on the board. My thoughts is that it failed, other than that there's 2 large caps and 2 small caps and a lot of resistors, and of course the chip.
Anyone know anything about electronics that can help me diagnose this better. Anyone think the chip is destroyed, or a way to test it?
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(http://farm5.static.flickr.com/4079/5444131631_73faaa426a_b.jpg)
(http://farm6.static.flickr.com/5136/5444733300_3db6ed0d20_b.jpg)
Anyone know how to tell the rating on the diode?
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I desoldered the diode and tested it. It looks like it's alright. .5 volts one way, overloard the other. So that leaves the resisters and caps to test. Is there a way to test caps?
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I checked all the resistors. They all seem to be resisting. The green one is suspicious. It's only 1 ohm. It's gold, black, brown, red. Anyone know what that's supposed to be?
In fact I think it may be really suspicious, because if I just put the leads of the multimeter together it's .4 ohms.
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Alright, looks like it should be 21 ohms according to the resistor calculator here. http://www.hobby-hour.com/electronics/resistor_color_code.php (http://www.hobby-hour.com/electronics/resistor_color_code.php)
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Strange, couldn't find any 21 ohm resistors on ebay. I found some 27 ohm ones though. My feelings are that the second band could be either brown or purple/violet...
Yup, looked at it under a magnifying glass, definitely purple, so 27 ohms. Now I wonder how many watts. Going to go with .5w and hope that's alright.
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Based on the test of the resistor I had high hopes, and desoldered the chip from one of my working boards and soldered in the chip from the bad board in hopes that I could prove that that chip was good as well. Ended up that it wouldn't recognize, so unfortunately that's the end. It's a bad chip. Anyone have any spare minitouch chips? lol
I suppose I could reprogram a teensy to work with the pcb...lots of trouble though...
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So I've got a lot of boards with similar sized chips. Anyone know how important the resistor and capacitor layout is? Anyone know if it would be possible to reprogram these chips somehow? What do the resistors and capacitors do on an older board like this? I'm used to seeing modern boards with only chips on them, why would they need so many resistors?
I'm kind of curious if I soldered in a chip from another keyboard what would happen. lol