Low-level signals may benefit from effective shielding. The metal noise shield surrounds the test circuit and should be connected to LO, as shown. The noise shield is connected to LO. The test fixture can be a metal or nonconductive enclosure, and is typically equipped with a lid.
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This was purchased with a broken power switch and "input and output voltages are incorrect". When I received it I saw that power push button itself was missing and output A did not work correctly - it always displayed "overflow" and had -2 volts on the output. The good news was that output B was just fine.
Fixing the power switch turned out to be pretty easy. The front of the switch was broken off and the push button cover was missing.
Unlike many items like this that use a long plastic rod to a rear mounted power switch this unit has a small PCB with a connector and power switch bolted to the chassis near the front. I was able to find a compatible power switch from Digikey and also a new plastic push button cap for it.
The cap I ordered was grey which matched Keithley plastic perfectly. The internal construction of this unit is very nice. It has two SMU modules that are the same. Each SMU has two input connectors - a power connector and a control connector.
It only takes 6 screws to cleanly lift out both SMU units. I swapped them side to side to verify the problem was with the SMU module and not the input power supply. The SMU units are also easy to open - just a bunch of philips screws to remove a double sided PCB with shielding on both sides.
Well it was my lucky day. One of the transistors moved very easily. I ran a small jumper to the proper location on the board and the faulty SMU unit worked just fine again. I believe the received some sort of shock on the front face which broke the power switch, that same shock must have been enough to cause the PCB damage as well.
I checked the second working SMU unit and the same trace was loose on the PCB but had not yet been ripped off the board. So if the pinout is correct what is it - power handling, or perhaps a matched pair of transistors?
The transistors are a 2N and a 2N - would anyone actually match such generic transistors? I suppose it is possible both were repaired at some point with the through-hole transistors but that seems odd to me as it is identical in both SMU units. Here are some pictures:.
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