Products > Test Equipment
Fun With Low Leakage/Bias Current: Femtompere, Electrometer, Keithley 617
Kosmic:
--- Quote from: r6502 on August 24, 2021, 10:38:24 pm ---the following parts have been defect:
* input double JFET, replaced with 2N5909
* defect LT1012, actual replaced with OPA177
* defect trimpot R348 and R314
* defect -5V regulator LM337LZ
* defect MPS-U60 PNPs in the end stage (Q303, Q305 and Q307), replaced with MJE350
--- End quote ---
I believe those failures are cause by an overload in the volt mode. Don't ask how I know :palm:
In my case I replaced:
LT1012 with a similar LT1012
LM337LZ with a MC79L05
MPS-U60 with STX93003
MPS-U10 with STX83003
Surprisingly, I think the dual JFET survived. Tested with a component tester and it look OK.
Now while I was there, I decided to follow Alex Nikitin's recommendations and made some changes.
On the output:
AD7541AJN replaced by LTC7541AKN
LM308A replaced by OP97
On the input:
Dual JFET replaced by LMC662.
New Keithley 7078-TRX-TBC 3 prong connector 8)
For the LMC662 I decided to use the PDIP package since it's easier to solder.
The new input connector.
Now after adjusting the input offset and input current, everything is working fine and the instrument oscillate +-0.0004pA with the input open.
The 617 measuring 100pA
This thread was really helpful to fix and upgrade my 617. Thank you all!
unseenninja:
I have just recently joined the distinguished club of Keithley 617 electron counter owners!
I found one one eBay, in working order, at an acceptable price and as I've been looking for something to measure capacitor leakage and some insulation resistance for some time, the Buy it Now button was duly clicked. I am now the proud owner of Keithley 617 Serial Number 372756.
It appears to have last been calibrated in 1997.
Turning on and leaving it to warm up for the regulation two hours delivers a satisfying 0.0000pA reading.
And even nicer, turning off the Zero Check function also delivers the same reading a few minutes later.
However, everything is not as happy as it could be in Keithley land...
unseenninja:
As you do, you take it apart! Before I opened the top case, this calibration sticker from 1991 was unbroken.
Some time before that sticker was applied, the meter was repaired, I can only assume, by Keithley themselves. At first glance, things looked OK inside.
The analogue board is a revision K and the date codes on the various chips suggest it was manufactured in early 1987.
As I started looking closer at the analogue board though, I started noticing things. Bad things. Soldering iron burned insulation on electrolytic capacitors, Flux residue in both the power supply area and in the critical shielded circuitry. PCB damage and other evidence of a gorilla having been let loose on this incredibly sensitive piece of equipment.
If you have a sensitive disposition, you may want to avoid looking at the next image (it's even clickable for full size if you have a strong stomach).
I know there is a school of thought which says "cleaning flux residue off after a repair is worse than leaving it there" and I do understand that point of view for smaller and very limited repairs. Here though, many components have either been replaced, resoldered, chewed by metal eating cockroaches, etc and the residue around some of the fixes in the most sensitive area is just waiting for a very humid day.
The thing is, the meter is not unusually noisy, it needs calibration on all ranges as they all have a constant (by scale), slightly low reading against any of my standards. But it settles to a stable zero reading on the most sensitive ranges and it works on all functions and ranges showing no signs at all of any problems.
But now I know the injustices that have been perpetrated upon it. The flux spatter around all the repairs is significant. There are little dark brown, shiny spots all over the components and the PCB. The burned insulation annoys me just as a matter of principle. The fact that this seems to have been done by the manufacturer themselves seems to go along with previous images in this thread where there are components with inexplicable pools of dark brown flux around where they enter the top side of the PCB. (I haven't dared remove the bottom cover yet for fear of seeing more horror!)
My OCD screams - "Take it apart, make it right!" The pragmatist in me says "It works, leave it alone."
What do you say?
Alex Nikitin:
Leave it as is if it works OK. Don't even think about cleaning the flux residue, you might easily ruin the unit. Ugly looks aside, I see no major problems and the Keithley may yet present you an opportunity to open more covers ;) !
Cheers
Alex
unseenninja:
Good advice indeed. I shall restrain myself and leave it well alone!
I was very pleased to discover that Mouser sell H&S G_02330_HT cable by the metre. I trust cables made by me much more than I trust used, overpriced cables from eBay. They will most likely be far better than the Pomona cable I'm using at the moment.
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