Author Topic: Keithley 171 vintage nixie tube 4.5 digit DMM  (Read 576 times)

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Offline PfannenHans

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Keithley 171 vintage nixie tube 4.5 digit DMM
« on: July 20, 2021, 12:54:34 pm »
This is my first real post, so I hope I get the formatting right...
I am also not quite sure if Test Equipment or Repairs is the right topic, but since it is not realy a repair I chose Test Equipment.

A few weeks ago I got myself my first bit of vintage test gear, namely a Keithley 171 DMM from the early 70s. Since I could not find a thread about it in the forum I thought I make some photos and share my experiences.
I got it from Ebay for about 80€. It was described as "turns on and the display is working". When it arrived I couldn't get it to measure anything, so I got it open and had a look around:
The culprit was a blown input fuse, which was sandwiched between the analog circuitry board and the range switch board:
After replacing it with a new one i got the DC voltage, ampere and ohm ranges working, but no luck with the AC ranges.
The manual mentions (see https://download.tek.com/manual/171(Model171_Multimeter).pdf) states that both AC ranges use the same AC/DC converter circuit before the signal goes in the common siginal path (see PDF p.17).
An impedance transforming JFET (TO-72 2N4220) is right at the beginning of the circuitry (PDF p.84), which I suspect to be dead.
Unfortunately it is on the range switch board, right under the range switch, which is soldered down and blocks any attempt to measure it in circuit. Q1101 is the Can right in the middle of the picture above the lowest bar:
I tried to get the range selection board out of the unit, but I already fail at the knob on the front panel, which is screwed in with what I think is a weird imperial hex screw for which I do not have a fitting bit (1.5mm slips, 2mm is to big)
But since I will anyway only use the DC and Ohm range it is not so big of a problem.
Last but not least a shot of the Nixie tube display, which uses NL-841s from National Electronics. I can not compare them to other Nixies, but I think they are in a pretty good shape for their age:
Component wise the DMM is full of carbon film resistors which could become a problem in the future. The electrolytic capacitors look good but have no vents. I am not sure if they are solid ones, or they just did not have vents back in the 70s?
I hope you enjoyed my little post.
Edit: Didn't quite get the images right, without having them to large, so i leave them at the end for better readability.
« Last Edit: July 20, 2021, 01:06:07 pm by PfannenHans »
 
The following users thanked this post: lowimpedance, doktor pyta, Gyro, Cubdriver, AVGresponding, duckduck

Offline Kleinstein

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Re: Keithley 171 vintage nixie tube 4.5 digit DMM
« Reply #1 on: July 20, 2021, 02:13:52 pm »
For the imperial hex screws there is a good chance a torx bit can work. It can still be tricky to find a suitable size.

The input fets mainly works as a source follower. So one should be able to see if the signal comes through.
 

Offline TimFox

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Re: Keithley 171 vintage nixie tube 4.5 digit DMM
« Reply #2 on: July 20, 2021, 03:14:42 pm »
The hex is probably 1/16 inch, a typical US size (in conventional units), probably for a 6-32 UNC screw (maybe a 5-40 thread).
 

Offline Gyro

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Re: Keithley 171 vintage nixie tube 4.5 digit DMM
« Reply #3 on: July 20, 2021, 05:59:56 pm »
Nice vintage meter, definitely worth the effort of restoration.

It's well worth getting a set of ordinary right angle imperial hex keys [Edit: Not ball ended!] and keeping them to hand. They're easy to find in Europe and sometimes you only get one chance with a tight grub screw.
« Last Edit: July 20, 2021, 06:17:59 pm by Gyro »
Regards, Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline AVGresponding

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Re: Keithley 171 vintage nixie tube 4.5 digit DMM
« Reply #4 on: July 20, 2021, 08:35:45 pm »
Those brown bodied resistors are carbon composition rather than carbon film, though you are correct they can drift wildly out of spec over time. Film resistors tend not to.

The electros are almost certainly wet, with the venting being out of the base through the lead holes...   :--

At least it won't be an impossible-to-repair multi-layer board if one/more of them do leak electrolyte.
nuqDaq yuch Dapol?
Addiction count: Agilent-AVO-BlackStar-Brymen-Chauvin Arnoux-Fluke-Hameg-HP-Keithley-IsoTech-Mastech-Megger-Metrix-Micronta-Racal-Siglent-Solartron-Tektronix-Thurlby-Time Electronics-TTi-UniT
 

Offline PfannenHans

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Re: Keithley 171 vintage nixie tube 4.5 digit DMM
« Reply #5 on: July 20, 2021, 09:28:32 pm »
Thank you for all the feedback. I will definitely look into buying imperial hex keys, so i can disassemble the whole DMM and give it an overhaul.
Unfortunately i can not measure the signals of the AC-DC signal converter circuit, since it is right under the soldered down range switch assembly, but it should at least be possible to measure the components when I manage to get the board out.
 

Offline Kleinstein

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Re: Keithley 171 vintage nixie tube 4.5 digit DMM
« Reply #6 on: July 20, 2021, 09:55:23 pm »
The link on the axel for the range switch look like that the card may slide up, if the swtich is at the right position. So one may not even have to remove the knob.
 

Offline Cubdriver

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Re: Keithley 171 vintage nixie tube 4.5 digit DMM
« Reply #7 on: July 21, 2021, 06:33:48 am »
Nice!  Welcome to the nut house, and the addiction that is old test gear.  Cool old meter.

-Pat
If it jams, force it.  If it breaks, you needed a new one anyway...
 


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