The transistor is soldered to the edge of a discrete chopper amplifier, 
if I am interpreting the circuit board correctly. There are even two of 
them.
The component has four connections so I assume that it could be a fet 
with a separate bulk connection and I am hoping that it could possibly 
replace the defective MEM556C (search on the market) in my Fluke 343.
Unfortunately, the search on Startpage doesn't turn up anything and I 
have no idea where to look on Bitsavers.
By the way, the transistor I am looking for has a stamp that suggests 73 
as the year of birth.
Why I suspect that the transistor is defective: The zero point of the 
calibrator is shifted by 7.997 or 79.98 or 799.9mV in all three voltage 
ranges. The discrepancy also remains across all voltage presets. This 
means that with the 1V setting I have an output voltage of 1.008V in the 
10V range, 1.080V in the 100V range and 1.8V in the 1000V range. 
Adjustment by potentiometer would only give a few 10µV.

I therefore want to replace the MEM556C and would also install the WM135 
if there is no suitable alternative.

Greetings Armin

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