Solartron 7075 repair - help needed finding replacement JFET

[TmaxElectronics]

I just bought some old solartron 7075 7,5 Digit multimeters and am having some trouble repairing one of them.

I traced the error into the JFET input amplifier and it seems that TR408 is dead. At least one of the JFETs in it has some leakage from gate to drain/source when applying a negative Voltage to it.


The Part number according to the BOM in the service manual is NDF9410 or WD401 (the one I unsoldered is labeled WD401). For one of those I have at least managed to find a datasheet but the other one didn't even give me that much luck.

Now of course I have the Problem of not being able to find a replacement for it.

Has anybody got an idea how I could get a replacement part for it? (Even if it is just a compatible one and not with the same Part number). Linear circuits (especiall JFETs) are just beyond my capabilities. At least at the moment

[kripton2035]

at least some sort of datasheet...
closest replacement part seems to be 2N5561 (if available ?)
http://www.elektronikjk.com/elementy_czynne/technologie-tranzystory/Tranzystory_N-Channel_Monolithic_Dual_JFET_proces_94.pdf

[tggzzz]

I have some 7075 boards ripped out of a broken 7075. They might have the transistor and it might be working. Or not, of course.

If you tell me which board it is on, I'll have a look.

I suggest you check whether the same type of transistor was used elsewhere, just in case

[py-bb]

I'm very sorry to knowingly "derail" your thread OP but I like Solartron gear, I have a AS1411.2 and the smaller 30V @ 1A supply, I'd very much like a manual or service manual if anyone has one.

One of mine has a CI light that flickers when CV is on sometimes (I think the relay removes the output capacitor from the circuit -very nifty)

Sorry again but yeah anyone have a manual!?

[TmaxElectronics]

I have some 7075 boards ripped out of a broken 7075. They might have the transistor and it might be working. Or not, of course.

It is on board 5. Thats the bottom one of the Analog boards in the middle of the instrument

According to the BOM it is used as: TR403, TR408 amd TR409

EDIT: Oh and its in a TO71 6 pin metal can. Maybe that makes finding it easier

[tggzzz]

Like this...

Looks like I already raided it for the precision resistors.

I also have the board above it (with the rectangular hole), the front panel switch board, and the PSU board.

[TmaxElectronics]

Oh yeah that one is it. Looks like the Fets are still in place too

[tggzzz]

PM containing several options sent.