Need Modern Parts for Keithley 197 and configuration questions

[technogeeky]

Hello,

I need to identify some modern parts I can use to replace some of the transistors from the Keithley 197. I have compiled a complete parts list from many sources (three different meters, the K197 manual, other Keithley manuals). The order of the items in the list exactly matches the parts list and schematic linked below.

As far as transistors go, I need replacements for:

Schematic Part	Part ID	Alt. Part ID	Sugg. Repl. Part	Notes
Q104/Q107/Q108	J2067	PN/PF5301	J113	JFET Matched with Q105/Q106/Q109, with a different pinch-off voltage.
Q105/Q106/Q109	J2715	J210	J110	JFET Matched with Q104/Q107/Q108, with a different pinch-off voltage.
Q101/Q102	GES5818			Glass-Passivated something. Used as a protection diode?
Q114/Q115	2N5089			Used for ??
Q103/Q127	2N5089			Used for ??

The schematic is available. I am confident that I need a replacement for one GES5818 (it presents as two resistors). I am also pretty sure I need a replacement for Q112 (not listed, but it's also a PF5301). But, in general, I will need to get a bunch of these and figure out how and why they were matched, and try to match as closely as possible so as to not destroy the calibration.

The parts with notes in bold I don't understand exactly how they are used in this configuration (or at least *why* that particular transistor is used, in that configuration). I am pretty sure Q114/Q115 and Q103/Q127 are involved in the guarding system.

Can anyone shed some light on this? What modern parts should I use to replace those JFETs? Are J110 and J113 really suitable replacement parts? (I found this suggestion in some other forum).

If you are really feeling generous, I would also appreciate someone finding a pin-for-pin replacement for OP01 and OP02.

And eventually, I might want to replace several ICs with modern, low-noise (or lower-offset) parts. So if you can think of replacements for parts like the TLC272LCP, the TLC271CP, the buffer amplifier LF356H, and MP842. If you could provide any guidance here, it would be appreciated.
Thanks,
-tg

[Kleinstein]

Q101 / Q102 are used as low leakage diodes. This similar to Q103, Q127 and Q114 and Q115 - they are also used as low leakage diodes.  Q114/Q115 are even less critical as they are used at near zero voltage only.

The JFETs are used as switches. I am not so sure the J110 is such a good choice, as it is low voltage and has rather high gate leakage. The PF5301 is a very low leakage type.

I would more tend towards the 2N4391-3 series, though I am not sure you still get a THT version for a reasonable price. Though they are rather similar to J111 to J113.

The OPs are usually standard pinout. If you really want a upgrade to more modern, types, one could use an OPA140 for U102 (LF356H ?) to get less noise in the low voltage range. However using a DIP instead of TO99 could be a little tricky - it depends on the layout.

 I don't think the LTC27x are used for anything critical.

The OP01/OP02 are kind of improved 741: more precise and faster. So they can tolerate a high input difference - this is unlike most modern BJT based precision OPs. It depends on the usage of that OP. In some circuits (e.g. U109/ maybe U116) the OP07 / OP177 might be a good replacement for the OP02. The OP140 might be an upgrade for both, though a faster OP might also cause trouble (oscillation). Where in the circuit are the OPs to replace ?

[technogeeky]

Q101 / Q102 are used as low leakage diodes. This similar to Q103, Q127 and Q114 and Q115 - they are also used as low leakage diodes.  Q114/Q115 are even less critical as they are used at near zero voltage only.

The JFETs are used as switches. I am not so sure the J110 is such a good choice, as it is low voltage and has rather high gate leakage. The PF5301 is a very low leakage type.

I would more tend towards the 2N4391-3 series, though I am not sure you still get a THT version for a reasonable price. Though they are rather similar to J111 to J113.

The OPs are usually standard pinout. If you really want a upgrade to more modern, types, one could use an OPA140 for U102 (LF356H ?) to get less noise in the low voltage range. However using a DIP instead of TO99 could be a little tricky - it depends on the layout.

 I don't think the LTC27x are used for anything critical.

The OP01/OP02 are kind of improved 741: more precise and faster. So they can tolerate a high input difference - this is unlike most modern BJT based precision OPs. It depends on the usage of that OP. In some circuits (e.g. U109/ maybe U116) the OP07 / OP177 might be a good replacement for the OP02. The OP140 might be an upgrade for both, though a faster OP might also cause trouble (oscillation). Where in the circuit are the OPs to replace ?

I will get back to you on the location of the OPs to replace. Again, that would be improvement of the device; not repair. For repair, I still need to replace:

* 2x matched GES5818
* 1x J2715
* 1x J2607 or PF5301 ("matched" against the J2715)

So you suggest using the 2N4391 through 2n4393 series; I can only find metal can versions of these; only on ebay; relatively expensive. I will probably need to get a medium sized collection of these to be able to match them to each other and/or to the devices already in my multimeter.

I am not familiar with what THT means. I guess "Through Hole" Type, or Technology, or something.

Again, I want to keep separate things that are for repairs and things that are for potential improvements; so I'll reply to the other stuff in another post.

[Vgkid]

For experimentation the to92 version of the 439x are the pn439x

[Kleinstein]

The GES5818 are used as low leakage diodes - at something like 15-20 V. Not real need to have them matched.

For the JFETs, there is no real matching needed. They are used in pairs, with two fets controlled together, so that they should switch one before the other. It is more like finding pairs with a threshold well separated. So like J111 and J113 or the 2N4391 and 2N4393. There is still a chance you get some that don't work. If you buy them together chances are you get a similar batch.

The trouble with JFETs is that it is difficult to get some in TO92 at all. It depends on the source, which are the ones one can still get. So it is more like list what you get and find two types, one with low and one with a high threshold. So the choice depends on the favorite source.

Mouser still have the PF5102 - might be suitable as the FET with a low threshold.

For the high threshold, the 2N4391 / PN4391 or J111 might be an option.
It depends one the layout (how the pads are placed), but chances are the TO18 can be used in place of TO92.

One might have to select / check for acceptable low leakage, when off. This could be the more tricky part than finding suitable threshold.