Measuring low voltages with old Keithleys

what the hell is that thing doing on my precision granite surface plate with its non-standard threats and it's terrible surface finish I'm a simple machinist is it too much to ask for a proper carbon steel construction but that name that name looks familiar somehow Kai he heat Keithley that's kind of cute it just displays a lot of digits six and a half of them you know funny story back in the days before my metalworking Korea started I had this unquenchable thirst for digits okay it can stay but I think I've got a much better place for it than the granite surface plate welcome home hey guys thank you so much for 100,000 subscribers it's an absolute honor that so many are interested in my silly little presentations to celebrate this milestone I thought I'd bring back a classic that I haven't shown in a while the ever-popular Kiki Kiki theory pair that idea is as always sabotage by ebay sellers falsely advertising their Alt key fleas as defect this adorable model 155 microvolt null detector needed only a new set of batteries and a thorough cleaning because of the old ones from 95 left us a little something yeah before we take it apart any further here's why this ancient meter is interesting in the first place there are a couple of reasons a thermocouple consists of at least two different metals and junctions between them if there's a temperature difference between the junctions a small current ones to flow it's called thermal EMF and it will haunt us for the rest of this video luckily Keithley 155 is so sensitive that it can detect and quantify these usually very small effects it's so sensitive that it can detect to the heat of a normal red laser pointer I've shown something similar before with a certain heat flux meter also an interesting device but I don't like how it tries to lure you into physics by giving you something as absurd as kilocalories per square meter hours just good old microvolt will be fine for me thank you very much these are the microvolt over a thin piece of wire caused by a constant current through it and changed by an ever so gentle stretching that's the fundamental idea behind strain gauges and load cells this chunk can measure forces equivalent to 300 kilogram with the same deformation sensing technique check out the deformation caused by this bold bold there is a little bit more going on in that load cell though than just a sticky taped piece of wire we've got four very equal pieces of wire forming two equal voltage dividers so naturally the two central voltages are also equal no matter what kind of excitation voltage you apply to the top and bottom nodes however when one of the four resistances the one that's actually attached to the metal body shifts ever so slightly so does the voltage between the two central nodes the fact that these slight shifts happen around zero and not around the much higher excitation voltage is useful because our Keithley null detector one can tell by the name is an expert in all things that happen at or around zero the circuit with a four equal resistors is called a Wheatstone bridge and it's all over the place on this temperature stabilized enclosure for example has one with a resistance thermometer instead of a strange gage back in the 60s this unremarkable looking box was one of the best voltage reference as one could buy it accomplished such a world-class performance simply by keeping the internal temperature at 30 degrees C precisely and curious thinkers like me out behind double or triple shielding at a few interlocks to prevent overheating it was filled with these glorified glass batteries aka western cells which in turn were filled with mercury and some electrolyte when kept undisturbed in a perfect environment these are still absolutely supreme voltage references to keep them undisturbed means that you can't just take a voltmeter and measure them though you would allow a small current to flow through the volt meter's internal resistance and that will change your glorified batteries output voltage forever that might be a bit of a deal-breaker for the vault nuts who like to voltmeter the hell out of everything but no problem Heath Lee's got you covered the proper way of utilizing such a high and voltage reference is to dial in a secondary reference with a differential measurement because you are generating equal voltages on both sides of your voltmeter its internal resistance doesn't matter no current will flow either way and because of the outstanding micro volt sensitivity you're really able to hit that sweet spot precisely [Music] Oh 7.1 something-something vault that sounds familiar doesn't it those were a few features of the Keithley model 155 it's still cool let's see if we can figure out how they accomplish that in 1975 I didn't exactly have to carbon date to the five percent resistors in here it's the date code on multiple components construction wise there is not a lot going on some beautiful mica and film caps which will last forever but the original electrolytic sand tantalum 's are good candidates for a timely replacement even though they have survived for forty four years with a perfectly fine ears our I've never seen a multi-turn potentiometers like this before it's sort of a planetary reduction gearbox situation but with ball bearings here's the circuit in an extra simple simplification the input signal goes through some attenuation because the meter is supposed to be able to measure up to one thousand volt next there's a little oscillator that's constantly connecting and disconnecting the attenuated input signal from a capacitor caps only let AC voltages pass so the newly-created AC waveform whose amplitude is proportional to the input voltage it's amplified and again passed through a cap that strips away the amplifiers unwanted DC offset that otherwise would have easily overpowered our microvolt signal now we've got an amplified input voltage dependent offset voltage free AC waveform that just needs to be demodulated based on the easily available oscillator state and that's the 1975 interpretation of a chopper amplifier whether or not we get to see a more modern version depends entirely on how quickly we reach the 100k subscribers it will be close because this thing has been listed as defect correctly for once-beautiful a stuck power switch a constant overflow display missing segments and a missing millivolt input cable so this is a model 1/8 one nano voltmeter built in 1980 that means it's only 5 years younger than the previous micro volt meter but it's about 3 orders of magnitude more sensitive had Keithley released more and more sensitive volt meters every 5 years they would already have to be further than Yocto volt the most sensitive volt meters today can measure only Pico volts however and I think even in this relatively modest device we'll get to see some of the physical non-negotiables that hinder further progress so this beauty is made up from four PCBs the main bot is boring power supplies optical isolation and a microcontroller and associated components it's not worthy that there's no precision analog stuff no PTFE standoffs and no guard traces on here that way a little bit of universal snake oil spray won't do harm it may not save the power switch forever but at least it rejuvenates it and lets me proceed until I find a proper replacement the front panel bot is connected astonishingly poorly with this press fit ribbon cable terminal I have no scientific evidence to support my claim but I'm going to say it anyway that sucks see that's clearly not my fault I'm going to replace that with a precision IC socket but alas it was not what caused to the missing segment the real culprit it becomes obvious when looking at the schematics since all of the C segments and the 200 milli volt LED are missing I'm going to make a wild guess and say that Q 208 is responsible dead or alive such a general-purpose PNP is worth next to nothing so I'm just going to snip it to make desoldering safer and easier want to know what else has made desoldering much easier for me Skillshare oh wait no not today surely I can't be the only one struggling with these obnoxious little rests of solder I just sit there in the centre of holes refusing to be wicked up from either side it doesn't feel like it's worth firing up a specialty soldering tool for but since I got an air compressor I keep finding more and more purposes for it it's also great for the cleaning afterwards before having access to an infinite amount of compressed air I used to feebly try and soak up as much IPA and dissolved flux as possible in pieces of tissue paper so primitive now I can just blast it all the way without leaving any fibers hanging on component legs and I can even get to the stuff that's hiding underneath parts and that's a working front panel as easy as that now we've got to figure out why it insists on displaying a flow all the time I was really hoping for this to become a triumphant edutaining repair project but the problem literally started sending out SOS smoke signals while I was setting up well we've got an overheating voltage regulator and that's usually caused by an overheating consumer somewhere in this case a shorted tantalum cap with the little tantalum protuberance maybe ductility test says no that's just a drop of solder I'll order replacements for all four of these suicidal blobs but for now an SMD patch will have to do that just fixed the board number three the Nano volt preamp and the overflow issue along with it while we are there let's check it out I hope you remember our friend thermal EMF from earlier to exist it needs a metal difference and a temperature difference so the design focused in this assembly is going to be avoiding at least one of the two because they can't really match cable and connect our temperatures to that of the internal junctions they have to use equal metals for those and because you can't really solder that well or make semiconductors from copper they are trying to equalize the temperature of the entire sensitive area the extraordinary cleanliness for low leakage currents that was so important in other Keith Lee's we've seen is not really that critical here um I mean it's probably beneficial around those film caps though because of them holding their charges perfectly is crucial for this implementation of a chopper amplifier here too they are alternating between applying the input voltage and zero to the preamp but this time instead of further processing the signal as if it was AC they are storing the zero offset voltage on the large film capacitor in such a way that it reverse offsets future readings that way Keithley won 8-1 is able to continuously auto zero and to subtract its own internal offset voltages from the displayed result in the readily available service manual there are numerous pages where they are talking about johnson noise in film resistors copper leg - Jeff heads and this little container of thermal compound I'd love to say that that's too much to cover in such a short video but the truth of the matter is that I don't understand all of it and I - just be spreading fake news if I kept on talking about this stuff so I would suggest that we start playing around with his newly repaired nano voltmeter until I did my homework and I'm able to show you a DIY Pico vault neuroscience squid Oh almost forgot last but not least the analog board is cool the voltage reference is an LM 399 of a nice - Zener diode giving this thing an absolutely respectable stability even 40 years later but why does anybody need a nano voltmeter in the first place well material science metallurgy in particular also everything related to superconductivity and the name is giving it away nanotechnology of course like carbon nanotubes and stuff I right now can really only demonstrate a few things related to the metrology side of things not even that I don't even have the correct input cable for the thing hmm but maybe I can make one this military connector fits but it uses different metal pins from the custom ones that Keithley must have ordered back then the result is an unacceptable offset voltage even with a direct shot I'll just shove my test subjects in there with a low toothpicks as artificial pins freely hanging PVC isolated via copper wires can perform nicely but just by changing their relative position slightly I got a drastically different reading a PVC isolated twisted pair prevents that but it does seem to generate a few nano volt here and they are the best ordinary cable for nano volt purposes I found in my arsenal is called r3 316 it's a PTFE isolated silver-plated copper coax it seems readily available and cheap even I tried to pause all vital functions for a moment so as to not disturb this measurement but I just had to move and ruin it here damn it let me just solder a terrible Spade connector and a normal screw terminal to it with the absolute worst role of solder I've ever used wait what oh I think thermal EMF is acting in its favor right now gotta let it recover completely from the soldering yeah that's pretty bad because of the multitude of metal transitions in their homeowner makes a range of low thermal EMF connectors those are made from directly gold-plated copper they aren't cheap but they do precisely what they claimed originally I wanted to replace my solar Tron input connector with a few of them because at the time I didn't have that input cable either but now I do courtesy of a generous ee-vie block for a member who gave that to me for free thanks doc it's a beautiful Swiss mate fish our connector with four shielded Teflon isolated silver-plated copper cables that doesn't sound like a source of a lot of unwanted nano volts but since I'm now able to test that I will so Letran isn't really such a specialized nano volt performer so it can have noise and just general inaccuracy in its least significant digits but theoretically this digit corresponds to this one so choosing the best kind of input connector will yield the best kind of measurements I thought I'd do this test surrounded by a lot of grounded and temperature stable steel homeowners low thermal EMF stuff is reading something between 20 and 30 nano volt with an uncalibrated meter whose noise specification happens to be 30 nano volt peak-to-peak precisely I'm measuring solar trans input cable from one end through both connectors and presumably multiple solder joints all the way to the front to rear switch so it's a bit of an unfair comparison but still a decisive result so coming up soon new so Letran and Keithley input connectors an LTZ 1000 based voltage reference for solar Tron it has been powered on continuously ever since I made that video and just more copper for everything in general I guess and that's it for this 20 minute marathon thank you for watching you

Channel: Marco Reps

Views: 98,441

Rating: 4.9663601 out of 5

Keywords: keithley, nanovolt, microvolt, uvolt, 181, 155, null, detector, voltemer, solartron, defect, repair, vintage, diy, battery, chopper, amplifier, thermocouple, emf, thermal, laser, arduino, raspberry, pi, circuit, open, source, µv, load, cell, strain, gauge, force, wheatstone, bridge, electronic, marco, reps, copper, standard, guildline, enclosure, weston, voltage, reference, ltz1000, schematic, sensitivity, fluke, tektronix, desoldering, soldering, esr, audiophile, hifi, teflon, ptfe, squid, josephson, jfet, preamp, capacitor, cable

Id: BPnQeYuuAv4

Length: 19min 56sec (1196 seconds)

Published: Sun Jun 09 2019