Keithley 2015 THD Multimeter maintenance

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.

Keithley 2015 THD mul­ti­me­ter, 10 volt input — warmup

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.

Keithley 2015 THD mul­ti­me­ter with cov­er off

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.

Keithley 2015 THD orig­i­nal capac­i­tors from DSP and DMM boards

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.

Keithley 2015 THD bot­tom DSP (136) board with new capacitors
Keithley 2015 THD top DMM (106) board replace­ment preparation

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.

LM399 ref­er­ence with addi­tion­al foam and aero­gel insulation

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.

Keithley 2015 THD mul­ti­me­ter reassem­bly bot­tom DSP (136) board

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

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