I have had my used Keithley 2015 THD multimeter for just over six years now, and have finally decided to perform some maintenance on it. I purchased it as a fully tested and working unit back in 2016. After it arrived, I did a visual check of internal components and a good cleaning of the exterior.

I had started to have some issues with the unit resetting just after turning it on. Other than that, everything was working fine with the multimeter.
My first suspicion was the electrolytic capacitors in the power supplies.

A visual inspection of the internal boards showed no signs of any observable issues.
The 2015 THD is mainly comprised of the front panel display, two transformers, the DMM (106) board, and the DSP (136) board. Both of the boards were populated with high quality Nichicon VZ and VR series electrolytic capacitors, so I was starting to doubt my suspicions.
The capacitors were 15 years old though, so I removed three capacitors with different ratings and tested them.
While none of the capacitors were what I would call bad, all of them were around 10% below their rated capacity, with slightly elevated ESR readings.

So new Nichicon capacitors were ordered with 8000 and 10000 Hrs @ 105°C ratings.
Some thing to watch for other than the lead spacing is the overall dimensions for the larger capacitors on the DMM (106) board. The 2200 uF @ 35 V pair are placed close together so their maximum diameter is limited. The 6800 uF @ 25V capacitor is mainly constrained by height and clearance from the cover.


After the capacitor replacement, it was time to improve the thermal stability of the LM399 reference Zener diode. I had performed a similar modification to my Hewlett Packard 3456A multimeter with good results. I used some rigid foam insulation with cutouts for the LM399 and surface mount components. I then attached some roVa Flex Plus Aerogel Insulation sheet to the bottom of the foam to isolate it from the circuit board.

I was still having reset issues after power on, even with the new capacitors. The problem ended up being a dirty power switch integrated into the IEC power connector located on the back panel. The switch is very difficult to access, and I found the easiest way to work on it was removing the power input module from the case. The switch is a semi sealed unit so it took a while to get some DeoxIT contact cleaner worked into it around the actuator.
While it is currently working good, I would like to find a switch for future replacement.

After a warm up period I performed a calibration on the DCV/DCI and resistance ranges only, as the unit has not been calibrated since I purchased it and it was well past due. I don’t have the proper equipment to perform the ACV/ACI or distortion calibration, so those were kept as-is.
The 2015 THD multimeter has been a great tool, and I have used it for many of my projects over the past six years. Hopefully after this bit of maintenance it will be useful for many more.
Keithley 2015 THD Multimeter Service Manual PDF