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Author Topic: Model M membrane expected resistance?  (Read 1666 times)

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Offline dalias

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Model M membrane expected resistance?
« on: Thu, 01 February 2018, 13:39:59 »
I'm trying to diagnose a Model M SSK that's not working. Put together, no keys respond, but I can generate keypresses by removing the membrane ribbons from the controller and manually connecting matrix rows/columns (so controller is ok). Testing the membranes put together (original rivets, no bolt mod) with a multimeter, most keys are showing about 10-20 ohms when pressed, and 12-18 kohm when not pressed. Is the latter OK or way too low?
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Offline Deefenestrate

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Re: Model M membrane expected resistance?
« Reply #1 on: Fri, 02 February 2018, 08:32:33 »
Try shorting pins on the controller first.  If that works then it is the membrane that is the problem.
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Offline dalias

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Re: Model M membrane expected resistance?
« Reply #2 on: Fri, 02 February 2018, 10:57:36 »
That's what I did, and the controller works fine. The membranes seem to be behaving like a giant capacitor (R slowly increases as long as the probes are connected) and I'm trying to assess whether I have to just take it apart and bolt mod or if it's close enough that I might get it working by some sort of cleaning - presumably moisture with some electrolyte got in between the membranes.
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Offline dalias

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Re: Model M membrane expected resistance?
« Reply #3 on: Fri, 02 February 2018, 13:29:29 »
OK, just tested shorting the controller pins with 200 kohm and it was sufficient to read as keypresses. So I'm doubtful about getting this thing working without taking off the rivets and cleaning the membranes...
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Offline dalias

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Re: Model M membrane expected resistance?
« Reply #4 on: Thu, 08 February 2018, 16:37:26 »
Some updates in case anyone else comes across this thread:

As soon as I took apart the membranes, all the traces near the edges started flaking off, and had to be redone. And while I was able to get keypress events shorting the ribbon connectors with a 200kΩ resistor, it turned out that even around 15kΩ between the ribbon connector and key pads was too high to work reliably, possibly due to additional resistance where the conductive surfaces from the two membranes press together. This posed something of a problem because I'd figured I had a bigger resistance margin to work with and hadn't optimized my conductive paint for low resistance.

In the end I managed to get all of the traces below 7kΩ, and most below 3kΩ, and the keyboard is working great again now.
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Offline Deefenestrate

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Re: Model M membrane expected resistance?
« Reply #5 on: Sun, 11 February 2018, 10:15:51 »
Great news!
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