Author Topic: Keithley 2000 Repair: slow shift on VDC reading (Read 5168 times)

electricar · « **on:** September 02, 2016, 05:12:33 pm »

Hello,

I'm a happy new user of a 21 year old Keithley 2000

I already replaced the electrolytic caps and am facing now one issue in the VDC reading whne nothing is connected to the leads. If I connect the leads to e.g. 5V the display is not shifting anymore.
The voltage reading is shifting slowly from -0,4V to -1,2V. This process starts over and over again everytime it hits the -1,2V.
I measured around a little bit and in the end I noticed something strange:
Although the positive (4,98V) and negative (2,479V) inputs of the U115-A are stable, the output is doing the same thing as described at the display. I thougt it might be that the shift is coming from the net "UNNAMEDNET146" but after measuring U115-B (+: -9mV, -: 2,48V, out: -15,68V), I think that the problem is either in Q104 or U115-A.
It was very interesting that when probing the drain and source of Q104 (drain: -1mV, source, -1mV, gate: doing this shift thing from -0,4...-1,2V) the shifting on the display stopped and I got a -9mV reading on the display.
The same happens when I probe the source of Q105 (drain: -9mV, source, -1mV, Gate: -15,66V).

I already wanted to order the JFET and the LM339D but I think it's better to ask if someone had an similar issue?
The schematic can be found here:
https://dev.xdevs.com/attachments/download/672/K2000.pdf
The measured voltages are on page 6.

Thank you very much in advance!

Regards

AndyC_772 · « **Reply #1 on:** September 02, 2016, 05:25:03 pm »

If nothing at all is connected to the leads, the extremely high input impedance of the 2000 means the inputs can and will float, and will show a drifting voltage just as you describe. It's completely normal behaviour.

If the voltage reads a stable 0V across, say, a 1MOhm resistor, then your meter works just fine.

Kjelt · « **Reply #2 on:** September 02, 2016, 05:53:25 pm »

Yup normal behaviour, it is alive!

electricar · « **Reply #3 on:** September 02, 2016, 05:58:03 pm »

Wow great! Thank you guys!

How is this prevented in the Agilent E34401A? We got many of them at work and I didn't see such a behavior. That's why I was confused with the Keithley

Another question when I already got it open: the VFD is making a really high pitched noise. Is this normal too?

Vgkid · « **Reply #4 on:** September 02, 2016, 06:14:44 pm »

The 34401a default input impedance is 10Meg ohms. You switch it to 10Gig through a front panel menu.

Nuno_pt · « **Reply #5 on:** September 02, 2016, 06:34:54 pm »

I've one and with the probes connected just laying around, it reads -11.3xxxxVDC, in auto mode.
If I short the probes it will show 00.000xxVDC.

So this will be normal.

I've to try attach the probes to 1MOhm resistor and see if it reads 0VDC settle.

electricar · « **Reply #6 on:** September 02, 2016, 06:42:48 pm »

I tried what Andy suggested and got the same results, although mine is doing this shfting thing between -0,4V...-1,2V after the startup. After some minutes (heated up) it toggles between very low V and mV readings. When connected to 1Meg I get 0,xxxxmV. So I'm convinced now that this is normal behaviour. The only thing I'm concerned about is this high pitched noise of the VFD. Do you have this too Nuno?

Nuno_pt · « **Reply #7 on:** September 02, 2016, 07:13:20 pm »

Nothing that I've notice, no noise.

My readings are stable at -11.3xxxxVDC, on Monday when I get back home I'll do the test on manual 1V, 10V, etc..., and will see if I can find 1MOhm resistor.

electricar · « **Reply #8 on:** September 02, 2016, 07:20:48 pm »

Good for you, bad for me

.
I'm totally sure that the noise is coming from the VFD. It's really bright too, like new.
I had a lot of meters with a VFD in front of me but none had such an annoying noise...

Samogon · « **Reply #9 on:** September 08, 2016, 03:39:33 am »

Quote from: electricar on September 02, 2016, 07:20:48 pm

Good for you, bad for me .
I'm totally sure that the noise is coming from the VFD. It's really bright too, like new.
I had a lot of meters with a VFD in front of me but none had such an annoying noise...

I had one VFD on HP 34401A making high noise because it had shorted lines. I would check voltage for VFD and possibly current consumption, it can be that something shorted or drawing to much current.

electricar · « **Reply #10 on:** September 08, 2016, 06:29:19 am »

I measured the voltage a few days ago. It was around 36-37V. I think that should be ok. Unfortunately I didn't measure the current. I probed a little around and couldn't detect any shorts. But I'll give it another try. Shouldn't it be visible somehow on the VFD if some lines are shorted?

Samogon · « **Reply #11 on:** September 08, 2016, 04:45:10 pm »

Quote from: electricar on September 08, 2016, 06:29:19 am

I measured the voltage a few days ago. It was around 36-37V. I think that should be ok. Unfortunately I didn't measure the current. I probed a little around and couldn't detect any shorts. But I'll give it another try. Shouldn't it be visible somehow on the VFD if some lines are shorted?

It was looking catastrophic

anode wire was lit like buzzing incandescent bulb, will setup my scope camera to get good picture. But this was result of something or DMM was dropped or some sort of over voltage. I have no history of it, was bought on ebay for part or repair.

Samogon · « **Reply #12 on:** September 09, 2016, 08:06:54 pm »

As promised closeup of burnt VFD from Agilent 34401A

electricar · « **Reply #13 on:** September 12, 2016, 11:29:37 am »

Wow. Is this repairable or did you buy a new display?

Samogon · « **Reply #14 on:** September 12, 2016, 12:36:37 pm »

Bought from China for $40

Nuno_pt · « **Reply #15 on:** September 12, 2016, 12:58:56 pm »

I never notice that was displays for the K2000 selling on epay, beside buying a parts unit.

All the display from China that I see are for the HP 34401a.

Can you put the link.

Samogon · « **Reply #16 on:** September 12, 2016, 01:08:21 pm »

It is for agilent 34401a. No Keithley only from tektronix site and they sell whole front panel display assembly quoted me $430 for keithley 2001, so i bought used keithley 7001 switch system for $170 just to use as a donor of VFD.

Nuno_pt · « **Reply #17 on:** September 12, 2016, 01:15:57 pm »

Yes, that is usual to buy the cheap 7001 to take the VFD for the 2001 and 2002.

The only one that I see that should fit the 2000 is the display of the 2015, since they are basically the same.

I still looking for 2001 cheap to fix.

Samogon · « **Reply #18 on:** September 12, 2016, 01:31:45 pm »

He he no cheap 2001/2002 even in bad shape. I dont know where TiN gets his $100 units

Kjelt · « **Reply #19 on:** September 12, 2016, 01:45:01 pm »

Perhaps if you run a multimeter repair business ?
I know a guy who repairs tvs and if the unit is defective and it costs >$400 to repair and you charge the customer $100-150 research costs they often never show up again to get the tv back (and pay the research costs). You end up with a pile of units with good components to repair other stuff. Ofcourse if the tv is not worth $100 on components anymore you charge the research costs up front.

Samogon · « **Reply #20 on:** September 12, 2016, 01:50:36 pm »

TiN works for EVGA, so repair is his hobby.

Kjelt · « **Reply #21 on:** September 12, 2016, 01:58:13 pm »

Yeah that is a second source of multimeters, defect ones from the company you work for

TiN · « **Reply #22 on:** September 13, 2016, 12:32:48 pm »

I got zero defect or otherwise gear from work, actually quiet few times situation was opposite, as I have some gear which was needed at work, which wasn't there

Those "cheap" 2001's were collected off ebay over more than 6 years, so it's just takes time and patience. I still see some go for 300-400$ mark, which is sorta reasonable for 2001. There are no other alternatives for such price mark in 7.5-digit realm anyway. As of 2002's, deals below 1k even badly broken would be rare "take-it-now-quick" things.

p.s. I'll be selling 2001's I currently have (4pcs) in the end of this year, preparing for other sorts of gear and upcoming projects. Need some room!

Samogon · « **Reply #23 on:** September 13, 2016, 03:52:11 pm »

No offense TiN

I am "new" in this, it is my comeback to electronics after 20 years break. I monitor ebay only few months.
Got few good deals from people selling items they even dont know how to turn on.


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Author Topic: Keithley 2000 Repair: slow shift on VDC reading (Read 5168 times)

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