Parts Replacement Suggestions for Keithley 155

[Crossphased]

Hello Friends,
I purchased a Keithley 155, and now I’m trying to get it working correctly. When first received, the needle pinned to one side on any range. I started by replacing the electrolytics, and many of the carbon resistors. A lot of the carbon resistors had drifted 30-60% out of range. After doing those repairs, the DC voltages are on point. The multivibrator circuit is working perfectly, and putting out the square wave specified in the manual. It has been adjusted to the proper 220 Hz.  Everything is looking good, but now there is no meter movement with voltage on the input.

The meter itself definitely works, as the unit has battery check function, and the needle pins to either side for the positive and negative voltage check. I started going through the circuit, and the problem seems to be somewhere in the amplifier section (I think). I’ll show some of the waveforms & measurements further down, what I’m looking for some help with is parts replacement suggestions for some of the transistors that don’t show a replacement in the manual, they are listed simply as “Selected at Factory”. Also any tips on debugging the problem are more than welcome.

Here’s the schematic:


Parts List:


Q101 & Q102 on their body have text on them: “HDIG1030”, which apparently can be replaced by 3N163. The JFET Q118 is listed as 2N5457, which I was able to find some of online.

Do you guys have any replacement suggestions for these parts: Q103, Q106, Q107, Q108?

Here's some measurements I've taken below-
Multivibrator waveform at the gates of Q101 & Q102:


Output of Q102 to C105:


R148 into C112:



The other side of C112 seems to be mostly shorted by Q118, unless I'm reading this wrong


Have any ideas?

Also see attached file of K155 board that is has much more clear reference designators than the manual

[Crossphased]

See full schematic attached

[Edwin G. Pettis]

Q103, 106, 107 are 2N5089 transistors, the 'selection' is for a certain minimal gain at a specified current, something like 1000 @ 1mA roughly.
Q108, made by Amperex is a high gain NPN, max. hfe of 400, BVxx is 30V, 200mW, something similar to a MPSA20 selected for hfe between ~300-400 @ ~1mA.
Q118 originally MPF103 by Motorola, 2N5457 is probably an okay substitute.

The HN1030 was likely from Motorola but I cannot find any cross reference or data on it, the 3N163 may be an okay sub but you will need to match units for gate voltage for turn on otherwise you will get an unsymmetrical waveform.

[maginnovision]

The 3n163 is the replacement for the 1030's replacement. Had to do that since the direct replacement is no easier to find than the 1030.

[Echo88]

Q101/102 are listed in the K155-BOM as "24598a", which is also used in the Keithley 164 and can also be found as "TG-51" and "RN1030" in its manual.
Only RN1030-specs i could find are here summarized, line 47: https://www.datasheetarchive.com/pdf/download.php?id=d59dd5e9964d16ee58dec4f9c950f68ea9cfce&type=M&term=RN1030

https://doc.xdevs.com/doc/Keithley/164_164TT/164%28Model164%2C164TT%29.pdf Page 46

[Crossphased]

Q103, 106, 107 are 2N5089 transistors, the 'selection' is for a certain minimal gain at a specified current, something like 1000 @ 1mA roughly.
Q108, made by Amperex is a high gain NPN, max. hfe of 400, BVxx is 30V, 200mW, something similar to a MPSA20 selected for hfe between ~300-400 @ ~1mA.
Q118 originally MPF103 by Motorola, 2N5457 is probably an okay substitute.

The HN1030 was likely from Motorola but I cannot find any cross reference or data on it, the 3N163 may be an okay sub but you will need to match units for gate voltage for turn on otherwise you will get an unsymmetrical waveform.

Awesome thanks so much!! Thats great info. I bought a few 3N163 NOS off ebay, but I just found out Linear Systems still makes them- so I'm going to order a few from Linear Systems and match for gate voltage as you suggested.

Can't remember where I saw it, but I was searching around today and saw that NTE199 might be used in place of Q108. It has the high gain specs like you suggest, so I'm going to order a few of those. Thanks for the other parts suggestions as well! Much appreciated!