Page 1 of 1

Old 640 vibrating capacitor electrometer

Posted: August 4th, 2013, 2:11 pm
by DavidDilaura
My apologies in advance for this long(ish) post and if this isn't the place for my inquires.

I used Keithley electrometers 45 years ago when I was an undergrad and graduate student (involving the measurement of photomultiplier tube currents) and became fascinated by the metrology of low current measurement. In the late 60's I expressed my interest to the company, making it clear that I wasn't looking for job; just being very curious and interested. To my surprise I was invited to Cleveland to visit the facilities and talk with some engineers. At the end of the visit that day I was treated to a (brief) meeting with Dr. Joseph Keithly, company founder.

The, after nearly 40 years of teaching lighting engineering, I retired from the University of Colorado and returned to my interest in Keithley electrometers as a hobby. I have since collected most of the instruments in this field that Keithley made (eBay, UsedLine and so on), beginning with the model 200 and 210 electrometer voltmeters that Dr. Keithley first offered. So far I have, in all, 23 Keithley electrometers. Many I have refurbished (replacing vacuum tubes, electrolytic capacitors that have gone bad, old connectors, readjusting/recalibrating and so on. Some are now quite rare, some surprisingly common. I have a beautiful version of the 642 -- gate current ~ 3x10-17a, very low noise. An amazing instrument and one of the best things Keithley ever offered.

I very recently finally acquired two contollers and a measurement head that comprised the Keithley 640 vibrating capacitor electrometer. The do not behave. I have worked carefully with the manual, doing the various voltage adjustments and circuit checks listed -- those are in order. I suspect something is wrong with the DC amplifier in the controllers, but am not sure. Finally my question: is there anyone at Keithley who recalls enough about this instrument to provide some guidance? If not, can anyone make a recommendation for someone who has?

It is my hope and plan to establish an website to showcase these interesting hallmarks or metrological progress.

Regards,
D.L.DiLaura

Re: Old 640 vibrating capacitor electrometer

Posted: August 7th, 2013, 8:06 am
by Dale C
There is one persone here at Keithley that might be able to help you.
He is retired and still comes to the office a couple of days a week.
I will ask him to respond to your request on the Model 640.

Re: Old 640 vibrating capacitor electrometer

Posted: August 8th, 2013, 8:46 am
by DavidDilaura
Dale,

Many thanks for attempting to put me in touch with a retired engineer there. I look forward to, and will certainly appreciate, any help.

Regards,
D.L. DiLaura

Re: Old 640 vibrating capacitor electrometer

Posted: August 22nd, 2013, 3:44 pm
by DavidDilaura
Many thanks to Dale (at Keithley) who put me in touch with a retired (and famous!) applications engineer who worked for many years at Keithley; including the time during which the Model 640 was designed, built, and brought to market. It was, by his account, an arduous process. He was most helpful, explaining in detail some of the issues involved in getting the instrument operative. And with the help of a 1st edition User's Manual (strangely, it is much, much more detailed than the later edition that is commonly available on line) I got the oscillator and demodulator working and the instrument functioning. "Good to go," as they say.

Re: Old 640 vibrating capacitor electrometer

Posted: April 1st, 2014, 11:26 am
by petepdx
David,

Can't wait to see your site !

In Keithly's my vintage T&M collection I also have a few. My latest find is a pair of new 301's

The 301 was a Op Amp one could buy for integrating into ones own design.

Dale ...

What can we do to get your retired engineer to do something like the oral histories at

http://www.hpmemory.org/timeline/compan ... s_home.htm ?


-pete Ex Tek

Re: Old 640 vibrating capacitor electrometer

Posted: April 2nd, 2014, 6:19 am
by Dale C
Not sure.
I will check into it.
Dale

Re: Old 640 vibrating capacitor electrometer

Posted: April 13th, 2014, 12:24 pm
by DavidDilaura
Pete,
Have you seen this video? https://www.youtube.com/watch?v=-V7z5CQFg3U
It's a commemoration of the 60 years (60!) of service that Keithley engineer John Yeager
offered Keithley and the measurement industry. John was kind enough to contact me (with Dale's help)
about the 640 vibrating capacitor electrometer, after I made my initial posting. His memory allowed him
to provide a fascinating retelling of the story of its development. He also provided an explanation of why
it was on the market so briefly (relatively speaking).

David

Re: Old 640 vibrating capacitor electrometer

Posted: July 21st, 2014, 7:49 am
by Laurel B
Hello Mr. Dilaura:

We would like to do an article with you about your collection of Keithley instrumentation. Would you be willing to talk with us?

Best regards,
Laurel Brennan
Keithley Instruments

Re: Old 640 vibrating capacitor electrometer

Posted: September 5th, 2014, 3:55 pm
by DavidDilaura
Ms. Brennan,

I am just now (5 September!) seeing your post. I'd be happy to talk with you about an article. You can contact me reliably by e-mail.
Best regards,

David

Re: Old 640 vibrating capacitor electrometer

Posted: November 14th, 2019, 3:27 pm
by lvcian
Does anybody have a stock or leads on the Phillips xl7900 vibrating membrane capacitor that is at the heart of the keithley 640 ?

I've recently acquired a 640 electrometer and find the dc-ac modulation technique fascinating and would like to acquire a few of the Phillips xl7900 for study and experimentation..

Re: Old 640 vibrating capacitor electrometer

Posted: December 21st, 2019, 10:23 am
by DavidDilaura
Hello,

I just saw your post about the old Keithley 640. I have never seen any source for the Phillips xl7900 vibrating capacitor. I had long been interested in it, though never had one until I managed to buy the complete 640 itself. Though I have acquired (virtually) all the literature available for the Phillips xl7900, I have never seen the device itself offered for sale. My impression was that it was quite fiddly. Via this forum I was put into contact with a long-retired engineer who worked at Keithley during and after the development of the 640. He said that there was considerable difficulty achieving *reliable* performance. He recalls that Keithly had to send one of its engineers to Phillips, spending several months there, working with the Phillips people to get a reliable physical setup and circuit perfected. It was his understanding (and my own historical research seems to verify the fact) that the xl7900 was never used in any other commercial equipment.

In fact, the retired Keithley engineer said that Phillips discontinued production of the xl7900 not long after Keithley instroducted the 640. I guess I'm not surprised. The market for such a thing would have minuscule, they were probably very expensive, and the development in high-sensitivity, high input resistance MOSFETs was very rapid. Most folks must have seen that MOSFETs would become a practical input device for electrometers. Indeed, Keithley's next electrometer was the 645, dual matched MOSFET based instrument.

David

Re: Old 640 vibrating capacitor electrometer

Posted: May 4th, 2020, 8:10 am
by lvcian
Hello Mr. Dilaura,

Thank you so much for your replay and the interesting historical background for this instrument !! Would you be able to share that early edition of the instrument's manual that you mentioned ?