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EEVblog => EEVblog Specific => Topic started by: EEVblog on June 03, 2022, 12:39:42 pm

Title: EEVblog 1476 - Keithley 515A Wheatstone Bridge TEARDOWN & TUTORIAL
Post by: EEVblog on June 03, 2022, 12:39:42 pm
Teardown of the Keithley 515A Megohm Wheatstone bridge, plus a tutorial on how Wheatsone bridges work and their applications.
Manual & Schematic: https://download.tek.com/manual/515A(Model515A).pdf (https://download.tek.com/manual/515A(Model515A).pdf)
1960's Daven precision resistor and attenuator catalog: http://www.technicalaudio.com/pdf/Daven/Daven_attenuator_switch_lab_Catalog.pdf (http://www.technicalaudio.com/pdf/Daven/Daven_attenuator_switch_lab_Catalog.pdf)
Keithley electrometer teardown: https://www.youtube.com/watch?v=LI_5ANmkVqo (https://www.youtube.com/watch?v=LI_5ANmkVqo)

00:00 Keithley 515A Megohm Wheatstone bridge
03:50 - The Magic smoke test
04:00 - Zero check test
04:40 - Standardise calibration step
05:30 - Range calibration
06:21 - Measuring a 200M resistor
09:41 - Confirmation with a Keithley DMM7510 7 1/2 digit multimeter
10:37 - Weatstrong Bridge Tutorial
14:55 - Gaurd traces for high impedance measurement
16:22 - Teardown
28:25 - OUCH!

https://www.youtube.com/watch?v=JDXKrXJloSw (https://www.youtube.com/watch?v=JDXKrXJloSw)
Title: Re: EEVblog 1476 - Keithley 515A Wheatstone Bridge TEARDOWN & TUTORIAL
Post by: PartialDischarge on June 03, 2022, 01:16:18 pm
I find it surprising that that resistor changes that much with only a 90V increase. The voltage coefficient is usually very low in that range, maybe some ppm per volt
Title: Re: EEVblog 1476 - Keithley 515A Wheatstone Bridge TEARDOWN & TUTORIAL
Post by: free_electron on June 03, 2022, 04:06:55 pm
That input fet is made by https://www.linearsystems.com/ (https://www.linearsystems.com/)
i (probably) have the last 3 in existence :)  I repaired two of these years ago and they only had 5 of these fets in stock and i bought them all. 2N5904 or 2N5907. there is a whole range of these fets. depending on what model electrometer you have they use one or the other.
Title: Re: EEVblog 1476 - Keithley 515A Wheatstone Bridge TEARDOWN & TUTORIAL
Post by: EEVblog on June 04, 2022, 12:51:51 am
Quote from: PartialDischarge on June 03, 2022, 01:16:18 pm
I find it surprising that that resistor changes that much with only a 90V increase. The voltage coefficient is usually very low in that range, maybe some ppm per volt

Yeah, I was a bit surprised by that. Maybe some non-linearity in the old 515.
Title: Re: EEVblog 1476 - Keithley 515A Wheatstone Bridge TEARDOWN & TUTORIAL
Post by: Kleinstein on June 04, 2022, 07:44:54 am
For the bridge in storrage for a long time in a possibly slightly damp basement it may take some time to dry up. So nonlineartiy with the 515 is a possible issue. This would effect the reference resistor(s) not the switchable decade resistors.

Another point could be some bias current from the null detector. This could be visible when using an external 0 V bridge voltage.
Title: Re: EEVblog 1476 - Keithley 515A Wheatstone Bridge TEARDOWN & TUTORIAL
Post by: free_electron on June 05, 2022, 09:19:51 pm
-cringe- triaxial bnc ... it's threaded so it should be a TNC  (B = bayonet , T = Threaded )