Keithley 614 Electrometer - Repair [Success]

[HighVoltage]

This was a great deal, as long as I get it repaired and restored, my first Keithley Electrometer.

Keithley 614 Electrometer
Made in the USA
Rev. F

Problem:
- Does only turn on intermittently
- Shows random numbers on the display
- Has a part of the front bezel broken out

This meter is also battery powered, but the batteries are dead

I will first have a look at all the switches, which feel a little sticky.

The front panel has a old style Triax Input connector with two pins like a typical BNC connector and not the new 3 pic triax connector.
Hmm, I wonder if I should exchange the input connector to a more modern type.
But first, lets get it back to work.

Anyone here have experience with this 614 Electrometer?

[HighVoltage]

OK, I was so much bothered by the damage on the front bezel plastic part, I fixed it first

Step 1: Clamp two aluminum pieces with double sided tape against the bezel.
Step 2: Fill the slot very slowly with some epoxy resin.
Step 3: Use fine sandpaper to grind it to perfectly matching shape
 
The color is not completely matching but the shape is perfect.
May be I will have a calibration sticker right there and then nobody will notice.

I am happy with the plastic repair but just not with the color.

[HighVoltage]

Here are some specifications of this 4.5 digit Keithley 614 Electrometer
   
- Detecting currents as low as 10^-14 A
- Resistances up to 2 x 10^11 ohms.
- It provides more than 5x10^13 ohms input impedance on the voltage ranges
- Can detect charge over a range of 10^-14 to 2 x 10^-08C.
- Voltage ranges are 0.2 V to 20 V in 3 ranges with accuracies to 0.08%.
- Current ranges from 20 pA to 2000 µA in 9 ranges with accuracies from 1.5 to 0.3%.
- Resistance ranges are from 2 kohms to 200 G-ohms in 9 ranges with accuracies from 0.5 to 2%.

[TiN]

Beauty. HighVoltage is running ahead, getting first electrometer  . I should do something about that asap. 

Interesting trick with panel repair. Perhaps paint it with matching color later to have it look genuine?

For example, repair 41422A SMU module...

[vindoline]

HighVoltage, nice job on the plastic repair! What kind of epoxy did you end up using??
Also, you look lucky with the triaxial Jack. It looks easy to replace if you want. I have a Keithly 616 and it looks to be a bear to get to the Jack to change it. The mod I did (and it's a bit of a hack) is to file off one of the lugs on my input jack. I can now use either 2 or 3 lug cables.

[Kleinstein]

The 614 manual include schematics, which should make a repair relatively easy. The ADC is a standard ICL7135 - so easy to change if needed. IF the input FETs are broken it might get difficult to find some as good as the original, but at least reasonable performance should be easy to get.

Intermittent turn on points toward the power supply part. Could be trouble with caps or the oscillator. This would be the first part to check anyway.

[Smith]

I know there is a trick for the 617 input. There are some good and cheap opamps (LMC662) wich offer a beter and more (temp) stable input. Just take a look at https://www.eevblog.com/forum/testgear/fun-with-low-leakagebias-current-femtompere-electrometer-keithley-617/

Btw before replacing the triax with a three lug triax you might want to check for some cables with two lug triax. I know the 3 lug are very expensive, the 2 lug might be a bit less expensive.

[HighVoltage]

Some interesting suggestions, Thanks
I might paint the bezel later.

Here are some more pictures.
Display PCB
And the 50 T ohm input arrangement on Teflon standoffs

[HighVoltage]

HighVoltage, nice job on the plastic repair! What kind of epoxy did you end up using??

This is the resin type epoxy I used.
I added some black and little yellow color, to get a matching brown, but that failed.
It is pretty strong attached to the original plastic.

[Johnny10]

Didn't FeedbackLoop just go over one of these on You tube?

I think problems with switches.
Will have to look at that again.

[HighVoltage]

Today I cleaned all switches but it did not help
Next is the power supply testing
+/- 5V is ok
But it seems the 32V rail is sitting at 25 Volts hmm

[feedback.loop]

Didn't FeedbackLoop just go over one of these on You tube?
I think problems with switches.
Will have to look at that again.

That is right. The switches were the culprit in my case. I also fitted a 3-lug triax connector.
Here is the video:
https://youtu.be/txH6EpqN5V0

[HighVoltage]

Thanks Feedback.Loop for the link to your great video.
After cleaning all the switches on my 614 it turns on perfectly but still shows strange and random numbers
Time to get out the schematics and start at the power supply

[HighVoltage]

By total surprise, I found an old original Keithley 2-lug connector and cable in a local store here in town.
I will just install some banana plugs on to the cable and see how good it performs
It will be a good start and I don't have to exchange the connector in the instrument.

[HighVoltage]

The repair was a success !
The power supply had a bad regulator, not sure why, no obvious reasons are visible and it works perfectly now.

Calibration was easy as well.
But my lowest current source is the Keithley 2450 SMU and it does "only" do 1nA
And the Keithley 614 has a pA range
Hmm, no way to test it.

I think I will hold off on the painting. The repair of the bezel does look ok as it is.

Now I need to build a metal enclosure for high resistance measurements.
I am amazed by this little and old Keithley 614 and its capabilities with 50 Tera Ohm input resistance.

Verification:
1 nA
1 uA
19.0 V

[Smith]

One method i found out to be helpfull, in need of some Tohm resistors: use a standard LED as a current source, and take down the light until your range reaches about 10% full scale, then range down one scale. Not absolutely accurate, but good enough for testing. BTW, it does differ a lot depending on your light and the type and color LED. You could  even use a second LED to shine on the first one to make it a bit more stable and controlable, but rember the setup is quite temperature dependant (due to the leds themself).