Keithley Logarithmic Picoammeter:  Manual/diagram search - Page 1
Forum section: electronics / repair

author: TimFox
author_rank: Super Contributor
timestamp: October 06, 2015, 11:40:40 pm
content: I have a Keithley 26300 Logarithmic Picoammeter that functions but needs repair.There are nine variations in the Keithley 26000 series:[positive, bipolar, or negative input] x [(10^-3 to 10^-11), (10^-4 to 10^-12), or (10^-7 to 10^-13) amp range] = 9 choices.My unit has positive input current, with (10^-7 to 10^-13) amp range.I have a manual for the Keithley 26220, which is bipolar-input and (10^-4 to 10^-12) amp range, and also a working 26220 meter.I believe I need to change a factory-set fixed resistor in the log amp offset adjustment, but I don't have the exact wiring diagram for the positive input versions. The bipolar-input versions all have positive outputs for both polarities, but my positive-input unit has a negative output, which is not possible with the wiring diagram used for the 26220.The usual sources (including Keithley's website and those in the sticky here) don't have the 26300 manual available.Can anyone help me find the wiring diagram for this unit?

author: TimFox
author_rank: Super Contributor
timestamp: October 08, 2015, 11:25:24 pm
content: Working without a proper manual, I had to infer the 26300 circuit, components, and procedure from the 26220 manual (see differences in first post).The manual Keithley provided for the 26220 had some pages that referred to the entire series, some pages that referred to dual-polarity units, and wiring diagrams only for the three dual-polarity units.1. The resistance in the offset circuit is a trimpot with a fixed resistor. The variable resistor is 500k, in series with a factory-chosen resistor that measured 2.86M. (The wiring diagram said 500k, but the parts list said 200k for the trimpot.)2. I disconnected the factory resistor and substituted a G-R 1432Y decade resistor. With the pot set to roughly center, I obtained a good offset (at 10^-11A) with the box set to 2.56M. The required total value is just out of the range available with the 500k pot (+/-250k). I assembled a 2.20M(RN70) and 390k(RN65) in series for 2.59M and installed it in place of the original resistor.3. Thereafter, I was able to obtain good agreement at the calibration values of 10^-11 and 10^-8A. First using my Keithley 261 pA Source, then checked with the front-panel high/low calibration switch.

author: garrettm
author_rank: Frequent Contributor
timestamp: May 10, 2016, 04:53:13 am
content: Hi there Tim.I picked up a 26220 off of ebay the other day for ~$40. I've just got done cleaning off all the old sticker residue and grime from past use as well as checking for functionality. Since everything looks A-okay, I was hoping to calibrate the unit but I've been unsuccessful at finding the user manual for any of the 26000 series logarithmic picoammeters online. Since you mentioned that you had a copy, I was curious if you could make it available for download? I know this is an old thread, but it doesn't hurt to ask.

author: Shock
author_rank: Super Contributor
timestamp: May 10, 2016, 10:25:58 am
content: Quote from: garrettm on May 10, 2016, 04:53:13 amUpload a copy toKO44BBas well.

author: TimFox
author_rank: Super Contributor
timestamp: May 10, 2016, 10:46:42 pm
content: It will take a while to get the document scanned. I sent a PM to garrettm with calibration details from the manual.

author: garrettm
author_rank: Frequent Contributor
timestamp: May 28, 2016, 01:49:26 am
content: Hi Tim, I was curious if you had been able to scan the user manual yet? If not no worries, though I was hoping to give it a read through now that I've finished calibrating it. Thanks for the help from before.Regards,Garrett

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: April 06, 2017, 02:30:07 am
content: Any update on manual and schematic for this gear. I have one that is certainly bad. Upon turning on the needle reflects all the way to maximum blue stays there.Clue how to approach it.Also maybe somebody can post some Picts of the insides so I can even verify that's complete.Thanks