Electronics > Repair

Transistor Jfet HP equivalent

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HUGOMAN:
Hi to All,

it's long time i look for equivalent of Jfet 1855-0743, even though i found a list by google but it lacks this reference, please indicate with what we could substitue this transistor by near one or what.

Thank you in advance for the support

Best Regards

dacman:
What is it used in and what is its reference designation?

rs20:
TLDR: OP wants TO-92 depletion mode JFETs suitable for replacing the current shunt selection JFETs in a 3458A.

Googling for 1855-0743 finds https://xdevs.com/doc/HP_Agilent_Keysight/3458A/doc/3458A%20CLIP.pdf

Which lists description as "TRANSISTOR J-FET N-CHAN D-MODE TO-92" and matching manufacturer part numbers as NSID5092/F50188A . But then the trail goes cold for me, at least as far as finding matching datasheets is concerned.

However, looking at the schematic, these transistors are almost connects to nets with guard rings, which implies that extremely low leakage current would be a requirement. Given that this is a multimeter, high-frequency performance seems unimportant. So here's a shortlist:

http://www.digikey.com.au/products/en/discrete-semiconductor-products/transistors-jfets/288

However, the parametric search here doesn't help with finding JFETs with exceptionally low-leakage at cutoff. Does anyone else know of parts specially designed for this purpose? Also, I can't be bothered tracing the nets through to see what acceptable threshold voltages would be.

Assembly 03458-66501
Q201-Q206: p48
Q107-Q108: p49
Q12-Q13, Q15, Q18, Q24: p50

Assembly 03458-66502:
2 others, didn't look into further.

dacman:
If you have some that are working, you could measure their conductance and cutoff voltage and then look for a cross.  About all JFETs are D-MODE (depletion mode).

HUGOMAN:
Thanks rs20 congrats for this details, you reach the target because i am stacked in this position like i have leaky ones, so almost this info i have but i am on a problem with this dmm right now i can't solve the problems that i encoutred on |O.

Thank you dacman, welcome this idea i thought about but i lack this method, i am make inverse i see other hp dmm using the stages which are similar, but the best if to draw a schematic even by hand to show how can find out the rds and it's conduction of the J-fet.

Best Regards

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