EEVblog 1476 - Keithley 515A Wheatstone Bridge TEARDOWN & TUTORIAL

Teardown of the Keithley 515A Megohm Wheatstone bridge, plus a tutorial on how Wheatsone bridges work and their applications.

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!

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#ElectronicsCreators #Teardown #Keithley

[SeanBZA]

50 year old capacitors, probably a little leaky, but it still works well. A bit of work and you can use it again properly. Date on the board is 1967 January, but looks like the meter movement was made 1968, so probably this one dates from then, though I can guess a lot of the precision and high value resistors date from the early version period. There they used pentodes and abused the screen as the second grid for the first stage, but they used similar voltages, and the second transformer was a switched power supply. AC output socket to power the optional high voltage supply to give 500V to use the top ranges, none of this 2 power cord rubbish.

My bet inside the can is a dual Jfet in the classical opamp circuit, specially selected matched pair with ultra low leakage, and all select on test resistors around this input.

[mikebarrett]

That PSU was obviously made for the Aussie market. Inverted components, so that operation in the Antipodes will prevent the expected loss of electrons due to gravitational effects.

[jenniferwhitewolf]

I love how top gear of the 60s and 70s was made... Straightforward and to the point with no shortcuts, and no fluf. Right to Repair was honored, even expected, with full service information supplied with purchase. Nice! Wish I owned this fine gadget.

[MLeoDaalder]

I just realized, dialing in the resistance to find the value of the resistor is exactly the same as using gauge blocks to measure the length of something.

[IanScottJohnston]

Looking at my small 80's era portable wheatstone bridge, looks at Dave's video, looks back at portable bridge and chucks it over

[czarodzi]

Test equipment in use, whiteboard theory, and a tear-down, this one's got it all!

[andersvandegevel]

The high value resistors and the meter movement had date codes, ranging from '68 to '71. The "penetrators" on that rotary switch were actually the switch contacts; flat side contact, pointy side solder terminal.

[flymypg]

A neat trick with crappy sensors is to put a pair of them in the legs of a bridge, with only one exposed to what is being sensed. At the start of my career I used this approach to quickly evaluate "novel physics" sensors, where the rules were: "To a first approximation, a Novel Physics sensor is a..." Where what would follow would be 1) "thermometer", 2) "microphone", 3) "hydrometer", and so on. Such sensors can have the "signal of interest" buried under multiple decades of "stuff", the goal of sensor development being to determine if the desired signal can be cleanly and reliably isolated and used.

The hard part was at the very start, simply finding the nominal sensor operational characteristics, starting with voltage and current bias, which needed wide-ranging bridges. We'd use nulling meters to start, then electrometers/VTVMs to obtain precise nulls. What worked in the physicists' lab often failed when moved to my bench, meaning there was lots of starting over, sometimes resulting in my part of the project being put on hold while the physicists took another stab at it.

If a candidate sensor passed the "quick & dirty" bridge test, we'd go on to validate and calibrate the performance via a massive amount of data acquisition and statistical analysis (SVD, PCA, etc.), the end result of which would yield a sensor driving and compensation circuit along with a calibration procedure.

My job started with working with the physicists in their lab, assisting their development of the "proof of concept" lab bench setup, done in their shielded room on an optical bench next to racks of lab equipment. I next had to replicate that system performance (as best I could) in a prototype enclosure on my engineering lab bench. I was the software/firmware engineer, but I previously worked as a tech on nuclear instrumentation and lab calibration, so I had the best precision assembly and solder qualifications in the building, and good metrology skills.

I miss those days.

[Leo-pdww]

Nice. Reminds me of our old Schering Bridges. The hands-on method with the dials helps tremendously with understanding what you're doing. Nowadays you press a button and get a result.

[nigozeroichi]

I recently got interested in electronics, I bought a piece of equipment from a shop cheap because it looked cool and what better way to lean than to tinker with stuff, no manual and not a whole lot of info on the web but I haven't done an exhaustive search, RFL 829D ac/dc calibration standard.
After watching this I know a little more about how it works, seeing the big transformers in my unit and the huge red jewel light "High Voltage" and the dial below going up to 2000 volts, I'll wait till I get more info or find a manual before I plug it in ⚡🤯

[TheOldgeezah]

I have to admire the quality of construction of this fine old piece of kit.

[marsa]

The soldering is flawless so is the engineering and the overall assembly. What a beauty.

[patim]

Bridges are so cool, I have a whole collection. Most from General Radio but also ESI and Wayne Kerr. My HP generates 1000V and can measure peta-ohm and I have a GR bridge that can generate 300V and has a zero meter that is 1 uV full scale. It can measure down to aF resolution. It once was the mother capacitance bridge from GR 's own cal-lab. If you want to know more about bridges, Henry Hall is the man that designed them for GR during 40 years. He is still active, I think he is by now in the 90's. You can reach him via the GR group or via IET. He has scanned all his GR documentation. You can find that at the IET site.

Your Keithley is a very nice instrument.

[KeritechElectronics]

A true thing of beauty and a joy for ever indeed! And that's no fluke. Absolutely loving it,
empty inside, but these ceramic standoffs and wafer switches surely are a looker. Wire looming too; that's the way I laced them wires in my amps since mid 2000s. I love the way the old gear was built... tube or not.

BTW, one of my tech tips if anyone is interested:
A Wheatstone bridge circuit and principle of operation can be used for testing ganged potentiometers (most often, stereo volume/tone pots). You just connect both ends of a DMM or even better, a positive/negative analog voltmeter to the wipers, and wire the resistive paths of both sections in parallel. Connect power (e.g. 12VDC wall wart) between left and right ends. A ganged pot with ideally consistent sections will measure zero throughout the entire length/rotation range. Typical pots tend to have inconsistencies, especially close to the extreme positions, leading to imbalance between channels. Stepped attenuators built with high precision resistors have this problem solved by design.

[maybehuman]

This belongs in a museum Dave, not in anyone's lab. Even in the 70s this was highly specialized. It's a work of art. Equipment like this was used to make the 1969 Moon Landing possible.

[daveturner]

Physics lessons in the 60s about resistors. An instrument consisting of a meter long etched rule with a resistance wire and a knife edged brass pointer a galvanometer (centre pointing ammeter) a known resistor and the DUT. Effectively the wiper being South. Junction between DUT & Known North.

That takes me back.

[fredflickinger]

Where art meets engineering! Always great to see this level of craftsmanship!

[owaisahmed]

i asked you on twitter about what is it and what does it do. now i got the 30 minutes video on it. when it comes to electronic you are my favourite person in it

[thomasw]

The format with the whiteboard explaining stuff is brilliant and very entertaining. Love it.

[frogandspanner]

The brand of bridge we had at school in the '60s was Wayne Kerr. You can imagine the ribaldry from us 15-year-old boys!