Help spec input JFET replacement? (DC drift problem)

[cvanc]

Hi all-

Please look over my shoulder here, I don't spec replacement JFETs every day.

I've got two of these amplifiers; one works fine.  The other also works but has excessive DC drift and offset.  My initial assumption is trouble in the input diff pair, possibly degraded silicon as these are about fifty years old.

The amp is modular so there's an audio input card I can swap between chassis.  The symptom follows one of these cards, and both chassis are working well (with the 'good' card in place).

Offset when cold is -150mV and at 20 minutes it is +700mV.  The good card, on the other hand, is nearly perfect (~1mV DCO after 20 mins).

I can't find any info on the ITS30874 N Channel JFETs.  Rails from the main chassis are +/-28V regulated and verified good.  I'll post more as I dig deeper but I suspect I'm gonna need a new JFET somewhere in the input stage.

Thanks...

[cvanc]

more pix...

[David Hess]

Oh, that circuit.

The modern parts you are looking for are the LSK489 and LSJ689, but I would double check that there is not some other fault before replacing the JFETs.

Check out this application note which discusses the LSJ689 dual p-channel JFET and that amplifier topology:

http://www.linearsystems.com/lsdata/appnotes/LSJ689_P-Channel%20Dual%20JFETs.pdf

[Kleinstein]

The offset and drift depends a lot on the matching of the jfets, not so much the actual fet type.

There are 2 relatively independent parts: the simple buffer at the inverting input and the main difference amplifier, both can contribute to the offset and drift. The difference stage has 2 trimmers (IM balance and DC0) that should effect the offset and drift - so if lucky one would only need to adjust the settings and not need new parts.

[cvanc]

Thank you both; replacing the upper JFET pair with LSK489 solved the problem.  All good now.