Repairing Keithley 237

[evac]

I recently purchased a broken Keithley 237, that I then repaired, only to break it again during testing...

The original issue was a shorted +/-150V transformer secondary, likely due to some carbon resistors that failed short.
Transformer was replaced, as well as the blown components, powering the digital board alone showed all supply voltages measured correctly and the display and controls were working fine, no obvious damage on the other modules, so I connected everything and put the shields back in place, and powered it up for testing.

I didn't have any triax cables yet, so used a piece of wire with a crimped pin, which was the right size to fit in the HI connector without any force as to not do any mechanical damage.
I also disconnected the LO-Gnd shunt as to leave it floating, and the SMU was powered through an isolation transformer (but connected to earth ground).

Powered-up fine, in V-source mode, the lower voltage ranges measured OK (didn't check if they were in spec, just in the ballpark), I-source mode worked fine for low currents, but for some reason would not source more than 60mA.
I then decided to increase the voltage a bit. I connected it to my CAT III 1000V rated multimeter just in case (glad I did), set the compliance current to 1mA (IIRC), wanted to start at 100V, but I didn't look properly and entered 700V instead of 100V...total idiot mistake.

I heard a faint "bzzz" almost like a corona discharge, a high pitched sound which I guess was coming from the resonant converter, when I realized my mistake and disabled the output...
I don't know exactly what happened, probably my terrible setup had the exposed pin I plugged into HI too close to the triax shield and it arced over.
Or maybe this was an already existing internal issue that would have happened anyway even with a proper cable setup (that's what I would like to imagine so I feel less like an idiot...).

After that, the SMU stopped to properly regulate the output, even at lower voltage ranges.
Turned it off, and noticed that in the resonant converter, the transformer and other components were pretty hot, there were some faint dark stains on the PCB under some resistors which were measuring low at least in-circuit, so probably that will need some repair.

However, as it doesn't work properly even at the lower voltage ranges, I'm not sure if there was damage to the output stage also.
My understanding of the schematic and principle of operation is that the unit should work fine up to 110V with the resonant converter removed.

Can anyone with experience with this unit confirm if this is the case?

This would help to troubleshoot things as I can focus on debugging one module at once.
The service manuals and schematics are available on Xdevs, but it seems the documentation archive is down, so I attach the schematics for the resonant converter and output module.

Thanks in advance for any help.

[MadTux]

K237 without voltage multiplier board is essentially a K236, should work to +/-110V/100mA, like K236.

I would check the output MOSFETs, along with the zener diode transistors that are used as gate drivers, voltage should be distributed equally over the MOSFETs.

On +/- 1100V range, it's most often C1/2/4, C6/7/8 that short out for unknown reasons. Diodes also seem to fail sometimes, what I've heard.

[evac]

K237 without voltage multiplier board is essentially a K236, should work to +/-110V/100mA, like K236.

That was my understanding as well.

I would check the output MOSFETs, along with the zener diode transistors that are used as gate drivers, voltage should be distributed equally over the MOSFETs.

On +/- 1100V range, it's most often C1/2/4, C6/7/8 that short out for unknown reasons. Diodes also seem to fail sometimes, what I've heard.

Thanks for the hints.

The capacitors looked to be fine (or at least not shorted), but Q16 appeared to be shorted. Since that's part of the HV drive, I will leave it for now.
The transistors driving the lower sections didn't show any obvious shorts, so I decided to turn it on without the resonant converter.

Now the output was regulating properly, but still showing the same issues as my first tests, unable to sink/source the full 100mA.
Voltage regulation seems to be working for all ranges, which is good news.
Current regulation also seems to be OK, except it won't go above ~30mA/below ~-16mA.
(some pictures attached as example behavior)

Will need to study the service manual a bit to see where to look next...

[garrettm]

However, as it doesn't work properly even at the lower voltage ranges, I'm not sure if there was damage to the output stage also.

Check the bipolar output transistors (MJE350/340, TO-225). My 237 had a shorted Q19, which prevented the output voltage from working correctly. You may also want to check the HV diodes (CR1, CR2, CR4 and CR6), as my unit had them replaced by the previous owner.

Read through Balage's thread (which also has my repair it), he had to replace the HV diodes and the HV ceramic capacitors before his unit was working again.

https://www.eevblog.com/forum/metrology/calibrating-keithley-237-(236238)/msg3019486/#msg3019486

As a heads up, my SMU was unstable when the output hi wasn't shielded properly. I can't remember if I was sourcing current or voltage while testing the output with my DMM, but on a few of the ranges, the output would become noisy and jump randomly when I was using low triboelectric BNC cable with a Trompeter 3-lug triax male to BNC female adapter. After changing to some Keithley triax cable and a female triax to BNC male adapter that went away. Could have been due to the guard not being shielded or maybe my BNC cables but I didn't investigate further.