TSP #71 - Teardown, Repair & Calibration of a Valhalla 2701C Precision DC Voltage Standard

hi welcome to the signal path this episode is dedicated to this Valhalla scientific I think that's how you pronounce it 2701 see this is a programmable precision DC voltage and current standard now when I bought this on eBay it wasn't it was listed as working actually but unfortunately it doesn't work and I feel that maybe something happened to it during shipping now I don't want to send it back because I really want this unit so I thought that what the heck I didn't actually pay very much money for it so let's maybe try and fix it and this has obviously six knobs in the front and it should be good enough to calibrate six and a half and even seven and a half digit multimeters visit one of the other things I really like about this unit is that it goes all the way up to 1200 volts so it has a 200 millivolt range two more 2120 and 1200 and that's fantastic because it allows you to check and to see how a multimeter for example that has their own app digits and that I can accept all over to a thousand most and a high cute that is now obviously because it's broken we can't test it right away we have to figure out what exactly is going on with it and at the same time most likely once if I am able to fix it it will have to be recalibrated and unfortunately some of these solar stickers in the front are missing although that is a really straightforward minute we've got our output and sense ports right here there's a light in here which will light up once you hit the color limit unfortunately I don't think this has the option for the current sense the current standard because it is a different firmware and I am assuming there's a different bit of hardware change that needs to be made for the current standard to be activated the current standard will go to 100 Mille am now I remember seeing a post on the internet that someone had one of these units that had the firmware that had the the current standard built into it so if the hardware is there and you're only missing the firmware I hope that that person is watching this video and maybe they can share the firmware with us and if it is upgradable I'd love to install that so let's go ahead and plug it in and see what happens and in fact I've done that what happens is that this display doesn't light up at all and only the buttons light up and when I press the buttons I can hear some clicking but I don't see anything is showing up actually never measured the voltage at that point to see if something was coming up but you know what let's just go straight to opening it taking a look at it inside because regardless that's what we're going to have to do oh by the way there is it the keys are here and they there's a switch at the back that's right now is covered with the calibration sticker which meaning that it was calibrated properly at some point and I'm gonna have to take the keys in order to turn this switch at the back when we want to get the calibration if you get to that point but first take it apart and here we are it's open and we can see all the circuitry and check this out tada well all I did was I wiggled the board a little bit and I guess the connection must have been interrupted during shipping and now it's working in I fixed it I'm a genius so now what we need to do is we need to take a look inside just to get an overview of what the circuit look like and what the principle of operation is and then we can go ahead and test it out see if it actually outputs the correct voltage because I tried to make it break again by shifting things around and I couldn't so it's fixed but let's see yeah what happens and I'll make use to measure the output see if it helped it still works because if it's sustained mechanical short who knows what else could be wrong with it and here we are looking from above and you can see the construction really does resemble the 1990 style everything is through-hole the components are very recognizable really widely spread and I've left the unit on right now so that it would warm up and come up to temperature so in case it works properly we can calibrate it I've also left it caitli DMM 7510 on so that I can use that as my calibration and now you can see they quite clearly the different sections and what the purpose of each section is obviously everything that's over here has to do with the output everything over here has to do the power supplies this is all the digital circuitry and there's some missing components here and I'll talk about that in a second this board right over here controls the GPIB and we have two transformers one of the far back one over here and we shall find out soon why that is so if you look at the label on the firmware there's a 27 0 1 C which is a model number and the date date code 12:20 1990 and this is version 3.0 so that's clearly from the 90s but they have a motorola 68000 processor which obviously is going to grab the firmer directly from this UV erasable chip and here there is a non-volatile memory which is going to use to write the calibration data to so this combination over here is just storing calibration and so on everything on the unit is controlled by that main microprocessor now the principle of operation of this unit is fairly straightforward it just basically uses PWM so it has a PWM that it uses to generate the output voltage and it has different ranges depending on the switching of the relays so the PWM generated by the microprocessor has a adjustable width for when it's on when it's off as to be expected and that width is in fact the calibration parameter so once we calibrate this unit did the instrument remembers what width of PWM it was using for each setting and then depending on that it will apply that again and the voltage of that PWM that is an analog voltage test generated internally is compared with the reference and that that's how it is adjusted so it has a feedback network that compares that PWM from the digital to analog converter with the reference voltage that is right over here and you can see that the reference is actually a custom part I think it's an LM 299 I believe I have to double check that actually I can check that right now just that I don't give you a wrong piece of information yeah it is an LM 299 in fact but the part number on it is custom and because it's the custom part number I tells me that linear technologies has either been them or aged them or something that prepared them for this particular instrument now this missing component section over here are the components required for the option I t2 that I was just talking about that's the section that has some relays here and output power transistor over here and a couple of other components which then switches the output to provide the current of the hundred and twenty milliamp unfortunately this instrument is not equipped with those components which means that even if I hadn't other firmware without these components I wouldn't be able to run it anywhere which is a shame I mean it's just a single 100 20 milliamp current I believe but it would have been nice to have it maybe it's worth upgrading at some point so they do a PWM not just on the voltage generators time a microprocessor but I also they do the similar thing on the reference as well and there's a chopper amplifier which includes all of that up and that takes care of it but because this goes to 1200 volts obviously needs a way to step stepped up even higher now this portion over here is the output transistors you can see there's a fuse that protects that and these relays will switch depending on the combination of voltage that is necessary but on top of that this secondary transformer here is used to generate very high voltages all the way to 1200 volts and there is a rectifier diode configuration over there right around here which uses as high voltage diodes and now we rectifier voltage to DC now voltage is then distributed among different transistors at the upwards that they all all see a very large voltage and then they can use the you know different currents in those transistors and a different distribution of the voltage across those transistors to give it a voltage that you're looking for so they basically switch the voltage around the way it's distributed along the transistors now to give you up to 1200 volts so the operation is fairly simple and the components are really really old so I'm very curious to see if it works and if the voltage is close to where it should be there is an interesting board that's sitting over here that's spaced above the surface and you can see that it actually occupies two slots and it seems like these two chips would have fit to where those sockets would have been but they have raised this up and they have added this additional circuitry on the board it says couple of fixed board I wonder what that means maybe they had a problem and then they constructed this little addition to solve some bug they had in some hardware problem they had as opposed to respray in the board they've tried to do it this way the capacitors are obviously fairly old their own unfortunately only am eighty five degrees Celsius rated but they all seem to be in fairly good condition use a couple of transistors there either this isn't likely required for the power supply portion perhaps I'm not a hundred percent sure but the components like I said are pretty old but they seem to all look good I don't see any problem there's a couple of barges there's a little bit of a barge if someone has actually burns the corner of that relay let me see if I can get that while they were trying to solder it there you go you can see that the little corner of this relay is missing while they were trying to start that component there and there is another socket for a capacitor that you can see the line just over here this supposed to be one capacitor in this format but they have put two in series that like accommodate they're the same thing so you can now get a fairly good idea of what's going on so the principle is simple now the question is will it work and how will be calibrated and I have found the calibration procedures really straightforward so let's go through that and see if we can get everything else to work I'm sorry this video is probably gonna be shorter than I expected because I was you know I'm going to go through debugging and fixing this problem but so far it works so I don't know what else to do about it we can break it again if you want to but if it's working let's try and go ahead and calibrate it I'm very curious about its performance and then we can close it up but don't worry I have a few other instruments that I just recently bought which are surely broken I have no doubt and so I'm sure there will be some interesting repairs but for now I hope you learned something by looking at this configuration and get a general idea of how this instrument actually works so let's talk a little bit about calibration why is easy to calibrate something like this well we can obviously put it into calibration mode quite easily but every little detail here matters for example what kind of cable do you use to calibrate this is actually a pretty bad cable because what you want is a really low EMF cable and there is recommended or EMF cables that can be used to connect for example here in between they keep the multimeter and the output of this because the thermal effect just a thermocouple effect from the connection of these to the instrument matters when you're trying to calibrate something the housing of seven and a half digits of accuracy as well as the fact that all these cables have dielectric storage so if I were to put a thousand volts on this cable in order to calibrate the thousand volt range and then I let that sit for a little bit and then I disconnected and I try to go back to let's say the 200 millivolt range the dielectric storage of this cable is enough to throw off my measurement from where I calibrated and that can be pretty big it can be if your millivolt and electric storage obviously in this case is going to be stored in the material because this essentially behaves like a capacitor with two plates and the dielectric being mostly this plastic and so it in this the air due to fringing but is that the material the plastic material that has a dielectric storage so you have to take that into account as you ramp up the voltage and try to store it not to mention you know not making a good connection having a resistive ohmic connection and noise on top of all of that being picked up by the cable there's just so many things that can go wrong and then once you calibrate in you know the temperature can change then the instrument temperature can change the instrument itself will age and it will drift and then now you have this standard deviation between the calibration of this and the calibration of the keys leave there's a lot of things that can go wrong it's not easy to calibrate these units perfectly so just keep these little things in mind that you know I can calibrate with this cable which is not a very good one and I actually go and get an EMF cable a good Ian McEwan and you know if the results might be off but nonetheless it would still be interesting to measure and take a look and see you know what is it what does it actually risk a report right now the way it is so let's go ahead and connect this to the Keithley seventy five ten and see what it reads without having got any calibration now they've been on for quite some time so everything is warmed up even though with it actually with a cover at the top off and then calibrating and then putting the cover back on is not a good idea so I'm going to put the cover back on before I start the calibration procedure so that way thermally it is what it's expected to be the heat cannot escape from the top so every little tiny detail matters so let's put the cover back on here we go I've closed it up but let me give you an idea about the performance of this unit so in terms of its resolution on a 200 millivolt range it has a 100 nano board resolution if you go all the way up to 1200 volts then the resolution is only 1 millivolt which makes sense so that's pretty good actually for something built in the 1990s now it's temperature coefficient is also very good for between 0 Celsius to 35 Celsius I actually know it's probably more relevant to to report from 35 Celsius to 50 degrees Celsius change we're looking at 2 part per million on the 20 volt range plus I believe it's a Plus 10 microvolt so that's pretty good and then we also have the actual accuracy when it is calibrated and it's coming out of the factory by up to one year you're looking at about 22 part per million plus 50 micro both for example on the 20 volt range and then you're looking at 30 part per million and 20 micro volt on the 200 millivolt range so again it's pretty good right now I've set it to 5 volts just as a starting point and the range you can see I've selected 20 volt and the output is off because when the light is on the output is off it's kind of anti intuitive I don't like to do the other way around but there it is so if the light is on you're not getting anything so this quiet look at what we have under Keithley oh there's a really really big clear on that and go that can be easily fixed let's look in here and see what we see let's go ahead I'm gonna turn on the output and there we go not too bad 4.0 actually spilled and I thought four point nine nine nine eight and then four five so that's pretty good I have set this up to five power line cycles and averaging at ten times so you can see that it is up you know fairly reasonable let's try going one both down there we go and it's stabilized for a second not bad three point nine nine nine eight nine so it's actually went down by almost exactly one vote that's that's very nice let's go to one volt and let this thing stabilize one more time there we go so it's actually consistently low but look it changed again by about the same about by I bother if you voice as a shoot it should be so it's very good now we can go ahead and try and calibrate it now I don't want to try the twelve actually what forget it I'm gonna try the twelve hundred range as well so let me out turn the output off here selected twelve hundred right now is it a hundred volts that's fine you can try one hundred volts enable and does it work there we go not bad ninety-nine point nine nine so I should be able to actually go say who's making this buzzing noise now there it is it should be 1000 volts 0.9999 kilowatt so it's yeah nine hundred and ninety-nine point nine volts very nice and I have to say I'm very happy that it actually works I'm sorry that it wasn't broken so we didn't have to fix it but at least now we can go through the calibration procedure now the calibration is done with a key at the back you will put in at the calibration mode and go through it step by step we disable this I as I just spoke about oh there we go yes does this little bit of it we had clicking thing like they say with the output and I think it's just stabilized but I I'm gonna have to wait a little bit for the dielectric storage problem that I told you to completely go away so and then we can come back and go through the calibration procedure all right as per instruction I'm supposed to turn the key and then the calibration procedure should begin let's go ahead and turn this key there you go in fact indeed it does so now it says C at the very end and it is waiting for me to do something I can adjust the output voltage with these three knobs and monitor it on my multimeter and I have to wait about thirty seconds or so for everything to settle so right now we are zeroing out the instrument I'm supposed to turn the knobs until I see basically zero volts on my multimeter up here and obviously right now I'm not so let's go ahead and give it a try see in fact if I can make it zero let's take one step there you go that's a little bit Oh that's because it's negative of course it's below zero so I have to go the other way one two let's see it's it moving in the right direction indeed it is three four five there you go it's getting there actually let me let me make a big adjustment how's that oh Jesus Christ I went way way out the other side alright one more try indeed it does take a while for it to settle okay still let's see two more clicks there you go we're getting there so it really is a low-cal actually so now let me see if I do one two three of these there's smaller ones I like it yeah we're getting it there we go I hope this is not horribly boring for you guys but at least let me show you what happens with the first one there we go oh we're so close to zero oh we overshot it in the positive now oh this is such a headache there you go it's almost there I'm getting there so I'm gonna play around with this and eventually well you know what let's just just bringing my hand close to it this cave was really really bad yeah it's not the greatest but anyway it's getting there so let me go through it and I'll periodically show you along the way so into all the ranges now it's jump to trying to calibrate a point one volt output I'd be 200 millivolt range so I'm gonna do exactly the same thing and you can see right now it's ninety nine point nine nine millivolt I'm gonna use this buffer to kind of get an idea for the averages and help me go in the right direction so I have to basically do the same thing increase the output a little bit at a time let me see if I can get that to go too you know it's not the the most exciting thing in the world I've ever done but anyway looks like it's moving in the right direction so I'm just gonna continue on and see what happens and here's the final stage of the calibration I think and they're just doing the calibration of the 1000 volt I I think I've got it fairly close to a thousand ones right now the average is one point zero zero zero zero zero two so yeah it's pretty close to one kilovolt I'm fairly happy with it so let's end the calibration procedure by pressing this one more time it says and Carol okay so now we should be able to switch the car switch back and do some of our own measurements and here we are after calibration at the 100 millivolt range I'm a little bit over I think I may have overshot that in my calibration I was rushing a little bit but anyway I think it's better than he was before so let's go to a 2 volt range now it's set to 1 volt I'm gonna wait for it to stabilize a bit and it's definitely better than it was before and actually stabilizing even better oh it's almost there so there you go let me get rid of this old average and see what I was just giving us just averaging about nine hundred ninety-nine point nine nine eight middle bolt which is pretty good I think I should have probably increase it a little bit but I think I'm fairly happy we let us go to the two modes range wait for it to stabilize again and we're looking at yeah we're looking again at a good value here nine point nine nine nine eight definitely closer than what it was before so oh there you go you just we just hit one of the values so I think it's it's promising so let me go ahead and change one digit here I can try and change the least significant digit I don't know if you will see it actually let me see hey go yep indeed you can see it there you go so it actually did change there so we'll actually move the average up so I was off a tiny bit in my calibration but I'm pretty happy with it let's go to five volts let me actually go to five volts exactly and let's see what it says and good so easy before calibration it was a little bit under five nights a little over five but it's still five point zero oh maybe not five point zero zero zero zero sorry yeah I'm pretty pretty happy with the result that we got here and it looks like that all the knobs work and everything works and the calibration is fairly successful again I watched it you gonna meet back better cables anywhere but there is also definitely very good and you look at the average right now the average I'm getting right now from the measurement at the very bottom oh it's just so many nines I mean it's like five point zero zero well now it's four point nine nine nine nine nine nine I mean it's just it's basically almost Bangor very happy quite nice very excited to be able to get this unit up and running nice to say about the output change to the twelve hundred range let's go to actually mmm well it doesn't actually go to twelve hundred it goes just below to a hundred so that's the highest voltage you can give us oh shoot that's actually overload on my uh my meter there so let's go down to eleven hundred no it doesn't interesting let's try actually exactly a thousand volts I was under the impression that the Keithley had no problem with this they're back to a thousand it's there for the delay there there you go it's charging up doesn't seem to like it very much let's disable that come on it does this really unusual thing so when I enable the 1200 obviously if you're on one volt and you just click on the thousand volt it's smart it doesn't just jump and give you a thousand volt actually disables the output automatic before you you know sometimes when I click it I guess he has to charge and it doesn't do it in enough time so it thinks that something is wrong so the microprocessor interrupts and cancels the button and then you have to press again it is all I assume these are all safety features so you don't accidentally destroy something that this is connected to but I think overall it it works quite well considering how old this unit is if I were to clear this and you can see that we were actually under a thousand volts unfortunate little bit but again I should be able to clear that and give it a yeah definitely the knobs definitely works something we call it I'm gonna go and change the least significant digit here by one you can see that it definitely does exactly that one more yeah so each I mean it works perfectly right now which means that the two least significant counts off I have to recalibrate it but anyway I'm sorry that this video was cut short I hope he wasn't boring but that nonetheless I think it was pretty cool to Cal this and then take a look at it and have a working unit so now this can be my standard is it like a transfer of standards from the Keithley DM 7510 to this now which we can use for a future for testing out you know another instrument to see how accurate this it's pretty interesting and just for fun what happens if you put a thousand volts into our siglent well let's find out I just turn it on so obviously it's not warmed up yet but there we go let's wait for this bad boy and yeah actually there the signal measures under because this is exactly the same setting you can see I'm at 1,000.00 - and I'm interestingly enough oh maybe maybe not maybe it's just has to warm up with it before it reaches Cal but yeah it's pretty pretty close know what let me close this up and actually while we're at it let's try it on a different multimeter do I have anything close by let me see I scheduled to use the rygel all right same thing on the rival so nail this bad boy put a thousand volts into that and I just turned that one on - interesting that they both actually read under which makes me question a little bit our measurement that we did with the acutely but I am pretty sure the Keithley is definitely in Carroll I think these guys I need to warm up and I'm sure that in fact if you look you see I think it's slowly it's slow yeah definitely it's counting up yeah it has to come to temperature so it's fine it looks very good anyway so I hope you enjoyed it if you did please let me know and I actually wanted to upgrade this and possibly change the voltage reference to maybe a more modern one but then I said something we can hopefully start a discussion in the forum and on YouTube and so on and see if you have some ideas until next time

Channel: The Signal Path

Views: 22,281

Keywords: Voltage Reference, Linear Technology Voltage Reference, Valhalla 2701C, Repair, Teardown, Keithley, DMM7510, Dielectric Storage, Nanovolt, PWM Reference Voltage Generator

Id: e-UwxRhYLS8

Length: 26min 32sec (1592 seconds)

Published: Mon Jan 11 2016

Reddit Comments

[u/[deleted]]

Nice video, as usual! I'm a big fan of your channel.

Looks like your voltage standard wasn't quite ready to meet Odin. :)

Will you be releasing a video on your network analyzer repair project?