Keithley 600B Input-Board

[TLa]

Hi,
This'll be my first posting, so hopefully it's in the right place.

I have a Keithley 600B Electrometer with a shot input-board, Part No. 21850B
The board is plug-in replaceable and consists of just a pair of MOSFET transistors in a differential-amp circuit with protective diodes.
I was on-line with Keithley today and they regret that no parts are available any longer.
The MOSFET's are labelled HD1G1030 and are Insulated Single-Gate MOSFET's.
I can't seem to find any other info on them and I have no idea of what may replace them.
Afaik Insulated Gate MOSFETS are usually Dual-Gate versions (Like RCA 40673 and 3SK206/207).
Could they possible be made to work as replacements ?

Any help highly appreciated, thanks

rgds,

/TLa

 

[KeepItSimpleStupid]

That thing is ancient. I found this https://www.google.com/url?sa=t&rct...Ytafipz39EEu8YNuGErmAUw&bvm=bv.99028883,d.dmo but I don't know if it's any use. I also found a request for help in another language with no response.

The schematic is on Keithley's website, available with registration. Keithley has been pretty good about giving the OEM p/n for obsolete equipment.

The real question is why is it there and what parameters were optimized? And what might the basic parameters be?

It appears to be a differential amp, so is it Vth or the gain the most important parameter or some other one? They use this differential thing in a lot of their stuff. The Keithley 480 comes to mind.

I haven't found any good books about front end FET circuits, either.

 

[TLa]

Yes, it's ancient.

I have 3 of them; one with a shot input-board, one with a broken meter-glass and one mint.
Obviously fixing a 2nd. won't be a problem.
Also cutting a new glass isn't a hardball issue.
I'm just thinking that any or all input-MOSFET's may break-down soon now.
..and then what !?

Small-signal Insulated Gate MOSFET's are still produced, like:
https://www.onsemi.com/pub_link/Collateral/NTA7002N-D.PDF
They actually give away free samples so I may have to do some experimenting !

I'm registrered with Keithley and have downloaded the service-doc's.
Terrible quality, so I got hold of another scan, from which the schematics are attached here.

Still open to suggestions, thank you.

 

[KeepItSimpleStupid]

I've been registered with Keithley for years. Did a lot of low current semiconductor measurements and basically set the measurement system up. The scan from Keithley was bad.

If you can find them in the same package, selection then would not be a problem. They would essentially be matched.

I have a "bunch" of Keithley stuff at home. Some need minor repairs and some have been repaired.

Have a frequency counter that needs buttons replaced (intermittent)- Have them
I have a voltage source that the IEEE board is bad - nearly dead short somewhere. When disconnected it works fine
I have a current source that's trashed. Bought that way.
I have system DMM that has some NVRAM issues/calibration.
I have 425 picoammeter that was repaired with a $0.25 FET and tightened some screws.
I have a 480 picoammeter that had A/D issues. Repaired.
I have have like two 5+ digit DMM's that appear OK.

In one of Keithley's electrometers I discovered a 100 V spike when the range changed. They did not believe e at first, but they were able to confirm without the instrument. They send a temporary external fix that would work in voltage mode only, then engineered a real fix for no charge.

So, suggest looking at some differential FET design documents and to try to find one that is dual and still made. Make a PCB to replace the input board.

 

[KeepItSimpleStupid]

The TMF3201J MIGHT be findable. Quickly i found this: https://uk.alibaba.com/product/60204685057--Gold-Suppiler-TMF3201J.html

I've dealt with alibaba/aliexpress before with transistors and a special IC. Don't believe their MOQ of 1. I you can wait, China post is a cheap shipping option. I know it's a uk site above. it will work out, if you don't do outrageous shipping options.

and

https://uk.alibaba.com/product/1791279774--HOT-OFFER-IC-TMF3201J.html