Keithley 236s Repair

[MaxFrister]

I recently repaired a pair of Keithley 236s.

The first was a challenge.   The initial symptom was that the guard voltage was different than the output voltage and almost every setting was in compliance. 

A fragment of the analog schematic is attached.  I found it challenging because so many things were making the voltages wrong on the main feedback path it was hard to locate the issues.  I eventually used a 34401A 6-½ digit meter to trace problematic voltages back the source(s) [I always wonder how many people notice that that meter defaults to 5-½ digits on startup].

It had a cascade failure on the main feedback loop of the analog board.  In the end I found and replaced:

Q29    NPN transistor
VR2-5  Zeners
U04   AD744KN
U08   LT1055


This machine is a latter serial number and differed in some ways from the only schematic I found for the machine.  For example, a few resistors had been deleted, the K7 relay (only needed for the 237 option) was deleted, U8 and U13 were originally AD744KN but replaced with presumably cheaper LT1055.  On the pcb there was an additional isolation slot and a few components had been moved around.

Unfortunately, the second unit was not as interesting.  Other than a noisy fan and some schmutz on the keypad it worked.

[Hydron]

Yeah I also found some differences to the schematic that's out there when I did a 236-237 conversion, yours looks very similar to mine. Reminds me that I have to write that conversion process up, it's been over a year now since I did it and others may be interested.