Author Topic: Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR  (Read 6634 times)

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Offline ve7xen

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Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR
« on: May 28, 2014, 09:50:43 am »
I picked up a Keithley 2000 on fleaBay, advertised as "Measures DC voltage properly, does NOT measure resistance at all." for about $200 shipped to my door. I didn't take any before photos, but thought some repair shots might be of interest. I was about to come here for help fixing it...when I fixed it...so posting a repair log.

On arrival, I expected to find things worse than described, as you'd expect. Surprisingly, that was not exactly the case. The meter was in externally "fair" shape, nothing appeared missing or broken except for the bail and feet which is pretty typical (and I assume not economical to replace). As described, however, none of the resistance ranges worked properly. Volts and amps on the ranges I am able to test appeared to work properly and in spec. I opened it up and had a peek around, hoping for a bad tantalum or obviously burnt components, but none were to be found. Everything looked fine. Boo.

This was the worst of it - some rust on the power trafo:



Self tests passed except for 302.1 and 302.2, which failed initially. The symptom is that intermittently, the meter drifts all over the place, even goes negative, when in any ohms range. It doesn't seem to matter which range it's in, and the range doesn't seem to affect the drift rate. Other times it works perfectly for a while, then starts drifting. Maybe it will come back after switching modes or ranges, maybe not.

So first we'll test that the current source and its switching is working, since it's easy to test and an obvious ohms-only problem source:



Looks good in all ranges, even while spamming garbage readings. I guess that means it's time to start tracing the input sense and see where we're losing it (at this point I should have realized it must be after the Isource is joined with INPUT). The first time through, I traced the signal in the schematic incorrectly and was on the wrong side of the board. Oops. After not much work, I found the signal wasn't getting past the string of 4x24k input resistors. I'd also noticed probing around that flexing the board would bring it into line or cause it to misbehave, so I knew now I was probably looking for a bad joint, cracked part or trace. This was a bit tricky to measure directly, since the 10M input impedance of the other meter I was using would drag the voltage to 0V and reading to 0R almost instantly. Of course, that was until I noticed it! Of course if it is able to drag it to 0 ohms I'm still on the opened side, no need to lean over to the bench meter. A minute or so of poking and aha! Here's the bastard that was acting up; this part of the board is subject to more flex than I'd like:



R108, you naughty devil.

A quick reflow of the string of resistors and everything seems to be perfect now:



And of course, any opening of a precision instrument requires a moneyshot of its reference circuit!



(A few more random pics here if anyone is interested: http://gotroot.ca/media/keithley_2000/ )

Now seems to be fully operational. Thanks to TiN for hosting the 38hot reverse engineered schematics (found here), which made tracing the input a little easier. Thanks also to zucca for found by zucca on his 34401A]fixing almost the identical problem with his 34401A the other day on the forum and lodging in my memory that I should check for broken joints.

I'm not sure if this thing has ever been calibrated. The rear screws said "warranty void if removed," nothing about calibration. Seems reasonably accurate though, agrees with my Advantest R6871E within 200uV and with my DMMCheck within 100uV. Now that R is fixed, it's basically bang on the DMMCheck measured values (I too) and matches my other "standards" on the ranges I am able to test.

PS. Any suggestions on cleaning the front/rear switch? It is quite sticky, the spring is not strong enough to fully return it.
« Last Edit: May 28, 2014, 09:56:31 am by ve7xen »
73 de VE7XEN
 

Offline mimmus78

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Re: Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR
« Reply #1 on: May 28, 2014, 11:42:43 am »
I had exactly the same problem on one I have fixed ... seems to be common problem.
 

Online TiN

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Re: Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR
« Reply #2 on: May 28, 2014, 11:56:22 am »
You just put a tear on my eye.
As I have three of my 2001's collecting dust already a while, waiting repairs.

Anyway, congratz, good score.  ;)

P.S. Do you mind if I steal your photos and host them as well, just for future robots? :)
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Offline ve7xen

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Re: Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR
« Reply #3 on: May 28, 2014, 05:21:36 pm »
P.S. Do you mind if I steal your photos and host them as well, just for future robots? :)
They shouldn't move from where they are, but you're welcome to mirror them if you like.

Quote
You can check last calibration and calibration due date from the menu on the front panel
I believe this is only on the 2001/2002, but I'll check the manual, thanks.
73 de VE7XEN
 

Offline KJDS

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Re: Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR
« Reply #4 on: May 28, 2014, 06:09:53 pm »

Quote
You can check last calibration and calibration due date from the menu on the front panel
I believe this is only on the 2001/2002, but I'll check the manual, thanks.

It's possible on the 2015THD

Offline ve7xen

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Re: Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR
« Reply #5 on: June 11, 2014, 10:25:38 am »

Quote
You can check last calibration and calibration due date from the menu on the front panel
I believe this is only on the 2001/2002, but I'll check the manual, thanks.

It's possible on the 2015THD

Checked the manual and you guys are correct, thanks! Shift+CAL and the dates come right up.

Shows last cal May 28 2010. Fresher than the cal on the Advantest.
73 de VE7XEN
 

Offline dev26

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Re: Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR
« Reply #6 on: August 06, 2015, 09:16:40 pm »
Just for record: +1. R108 too.

My way, if it can help somebody. The DMM bought on eBay; very cheap, probably because it was very dirty on photos. Seller sad it's completely functional, but build-in tests failed.
  • Run build-in tests. 302.1 and 302.2 - fail.
  • Check values of keys, as specified in the repair manual - some values are completely different. So, a wrong way.
  • Select 100K range and try to measure a 15K resistor:
    • Connect another DMM in series - it shows 700nA. It could be correct according to the manual
    • Now re-connect the second DMM, the common to "LO" of the 2000
    • Go throw the circuit (using the reverse engineered schematics), starting from "HI"
    • All points have ~10mV, but after R108 there is a garbage
    • One lead of it is break. So, desolder R108 and just re-solder it bottom side up
 

Online TiN

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Re: Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR
« Reply #7 on: August 07, 2015, 04:23:28 am »
Good job and easy score  :popcorn:
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Offline mimmus78

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Re: Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR
« Reply #8 on: October 22, 2015, 03:51:31 pm »
I had one with R103 and the same problems :-)

Just for record: +1. R108 too.

My way, if it can help somebody. The DMM bought on eBay; very cheap, probably because it was very dirty on photos. Seller sad it's completely functional, but build-in tests failed.
  • Run build-in tests. 302.1 and 302.2 - fail.
  • Check values of keys, as specified in the repair manual - some values are completely different. So, a wrong way.
  • Select 100K range and try to measure a 15K resistor:
    • Connect another DMM in series - it shows 700nA. It could be correct according to the manual
    • Now re-connect the second DMM, the common to "LO" of the 2000
    • Go throw the circuit (using the reverse engineered schematics), starting from "HI"
    • All points have ~10mV, but after R108 there is a garbage
    • One lead of it is break. So, desolder R108 and just re-solder it bottom side up

 

Offline zucca

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Re: Keithley 2000 Errors 302.1/302.2 and wacky ohms - REPAIR
« Reply #9 on: October 23, 2015, 06:29:19 am »
Thanks also to zucca for found by zucca on his 34401A]fixing almost the identical problem with his 34401A the other day on the forum and lodging in my memory that I should check for broken joints.

You are more than welcome! Nice score and congratulations!
Can't know what you don't love. St. Augustine
Can't love what you don't know. Zucca
 


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