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Keithley 2400 SourceMeter review and teardown
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exe:
Congratulations, looks like you are just one step away from having a working smu!
kawal:
Found another issue on the 10UA range.  This range I was not able to calibrate due to offset.
I found this out when i could not measure resistance in Auto Mode but could do it in manual mode on 100uA range. Auto defaults to 10uA BTW
The sense resistor for this range is switched on by a series DG444DY switch ( which seems to work) and a Mosfet transistor Q400.  The transistor  seems to be the issue.
The mosfet q400 which is a VN0605T (obsolete )  must be damaged . Its crating a big offset on this range.  A good replacement ( actually has better leakage current by 10X) is 2N7002E-T1-E3
Ordering yet more parts.

Attached some pictures of resistance measurements  on 2400 and 2700 and a microscope (Just got a nice stereo microscope used locally for good price.) picture of the Voltage DAC.

kawal:
Got some 4094 chips in today and replaced the U253  I guess I was wrong about the output being pulled up. The meter is doing this the same way on the new chip.  Not sure if the ram is corrupted in the meter but the data is definitely being pushed into the register.   |O Seems wrong to me but oh well. not sure what can be done
kawal:
Can someone with a 2400 confirm how the relay K206 is supposed to work. Can someone measure their unit at the test point near Q234 (this is the gate of the mosfet).

Measure this voltage in reference to T500 FCOM when output is disabled
Measure this voltage in reference to T500 FCOM when output is enabled

Thank you in advance.
 
kawal:
Removed Q400 from the 10uA range. The transistor has a short (54 ohms or 20mV  voltage drop with Diode test.) between Gate an Source. This would explain the offset issue on the 10uA range. Still waiting for replacement. 
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