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Old 15th Oct 2018, 7:15 am   #21
mole42uk
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Default Re: I've injured my Keithley 175 DMM

The logic around U106 and U109 appears to correspond with the logic states as printed in the manual. The display appears to show that the µP is working but on DC voltage the display reads 'overvoltage', connecting the test inputs together makes no difference.,

I suspect either the A/D or the switching FETs, but those J210 FETs are described as "selected" so I wonder just how I might select useable ones and how many I need to obtain to select a useable number?
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Old 15th Oct 2018, 3:54 pm   #22
Chris55000
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Default Re: I've injured my Keithley 175 DMM

Hi!

The FETs will be selected for equal pinch–off voltage and Idss – to measure Vp, connect drain to a 12–15V positive supply, gate to zero and source to zero via a 1M resistor, then a high–impedance voltmeter across the 1M between G & S will read Vp.

To measure Idss, , connect G and S to zero and D via a 1k resistor to +15V in series with a suitable current range of your DMM – this will be Idss!

Providing at least one of the original J210s in your meter is serviceable, you can use the Vp and Idss as a guide.

If you can find 'em on eBay, etc., cheap enough I'd go for about 20 to begin with and sort thro' these to begin with!

Chris Williams
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Old 17th Oct 2018, 6:23 pm   #23
mole42uk
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Default Re: I've injured my Keithley 175 DMM

Hmmm. Interestingly, I just pulled the four multiplexer FETs and they all test okay on my Chinese tester.

The display wanders around a bit when I put my hand close the the A/D section, so maybe it either doesn't like an o/c input or there's something awry in there.
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