Electronics > Metrology

3458A, fixing Error 202 - Slave test: convergence

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I'm currently restoring a nice unit with the 202 error, and I found an issue on the A1 board: the area involved is the input amplifier.
The X10 and X100 gains are fine, but when in the X1 configuration the amplifier circuit is in open loop. Q106, a 2N4338, is dead.
This J-FET is a little hard to find nowadays, but I should be able to source a few low risk parts.
Looking for alternative parts with comparable performances is proving a bit difficult, and here lays the question: what is Keysight using on the Black Edition of this instrument? From pictures found on xDevs (many thanks for the formidable amount of information) I can see that the current J-FET, in a TO-92 package, is marked LS304, but I can't find anything more than that. Is that a custom part?
I'm re-posting a detail of the picture found on xDevs

My guess would be its a part from "Linear Integrated Systems" (their parts have LS model numbers), it seems not publicly available though as you can not find it on their product page.

I would also suggest a linear systems part, possibly an equavalent to the J304 which is relatively similar to the 2N4338, though lower voltage specs.

Other possible 2N4338 substitutes may be SK208 or J201 in some variant / case. One may want to select for low gate leakage: a fixed leakage is likely compensated as an offset, but the leakage tends to go up with temperature.

For finding JFET replacements the Fairchild AN-6609 is a nice resorce.

Thank you for having introduced me to Linear Systems!  :-+
They are on my way to the office, I might inquire them directly (will ask about what they might have unpublished in their drawers)

Temporarily bypassed the first issue I think I now came to a full stop: Board A3 very likely has a defective U180.
+12ref is good, -12ref instead is -11.3V and as a consequence +5ref is at 5.7V ...  :(
Any suggestion beside replacing the whole A3 board?
Being in the Bay Area, would be great if someone with a Saint in Paradise (Keysight) could share a miracle and help me find a replacement part  8)  :P


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