keithley 172 dmm

[cncjerry]

I have a soft spot for bright red meters and can't resist a good deal. I picked up the subject meter for like $20. Powers up fine, ohms readings are right against my standards but the DC measurements have a strange characteristic.  If I place the leads on my 10v DC standard, the count will slowly increase to the approximate voltage.  If I remove the leads and short them together, the reading will again drop slowly. If I select ohms or AC after reading DC, then select DC again, the count will immediately drop to zero. Given that the ohms readings are dead on against my .01% standards, I would think the ADC and associated circuitry is fine. It is almost like a cap is charging and discharging slowly when DC is selected. I guess there is something leaking in there but why would it drop slowly if that was the case?

Anyway, if someone has experience with the older Keithley172s or ideas, please post or contact offline.  I have a number of restores ahead of this one unless it is something simple.  Also, the idiot packed it poorly and the handle broke in shipment which disappoints me to no end.  Almost as much as the other idiot who wrapped tape around the original box for an old HP DMM I bought. I don't know why people don't treat this old equipment with more respect no matter what the cost or how cheap.

Thanks,

Jerry

[Vgkid]

On DC will it zero quickly if it is in autorange mode, if you change ranges, does it also drop to zero/change?
Here is a manual, related schematics start on pg 146.
http://kuzyatech.com/wp-content/uploads/2013/10/Keithley-1737.pdf

[cncjerry]

Auto or manual the zero is the same.  If I come out of AC or ohms into DC, it zeros; once I have a valid DC reading and then short the leads it doesn't drop.  So I guest you are suggesting the auto zero circuit?  another bit of diagnostics, it quickly hits the proper reading then climbs slightly off a valid reading.

I'll check your version of the schematic as mine is difficult to read.

Thanks,

Jerry

[Vgkid]

I would say that if you were not manually zeroing the leads, then the slow drop would make sense.
WARNING - THINKING ALOUD;
you could apply a small signal to the input(thenremove power from input), and with a wire attatched to ground(neg terminal) starting after the DC attenuator working through the active filter through the A/D circuitry.

[cncjerry]

Good idea on the ground wire. Also, I was just thinking that there must be a cap switched in and out for A/C voltage measurement to block DC.  If it doesn't get switched out or shorted in DC mode for some reason then I guess I would see these symptoms, no?  I am going to search this idea as well as the filter.  Thanks.

[cncjerry]

I finally got back to this project and after an hour or so of troubleshooting found relay k1 contacts to be dirty.  This caused very long integration times and reset as well.  I cleaned the points and off it went.

This old meter is incredibly accurate and stable.  Once I calibrated it even up to 10 megohms it seems to be right on the money.  The high-lo bias is still a little off but another calibration should fix it.  I don't know how accurate it is over 100meg a I don't have a standard that high.

Anybody know how much these things cost when new?  I can't find much history on it.  Having four wire ohms, 300meg range and some crazy digital interface back whenever must have been pricey.  You can't fit the battery option in it with all that digital stuff but I would prefer the battery, I would probably use it occasionally if so. Now that I know my way around this thing I bought another last nigh for $15.  I'll be checking that relay first this time.

[KeepItSimpleStupid]

You basically can only have one option at a time  digital (BCD), IEEE or Battery Pack.

BCD was very common at one time.

[retiredcaps]

@cncjerry, If you like Keithley bench multimeters, check out scullcom's youtube channel.  He has at least 4 Keithley DMM videos.

https://www.youtube.com/channel/UCDqryeq1kMDSEQwltWqASrA/videos

[TeslaPunk]

Spot on with the dirty contacts on K1! I picked up a 172A at a surplus store for $5 and that fixed the problem.

Thanks for following up on your fix.

[cncjerry]

Glad it helped.  I never put the relay cover back on mine, don't forget yours.