Re: [volt-nuts] Update on 720A
> My concern is you seemed to be going at this repair like you were working on > an oscilloscope, rather than a super precision instrument. More a case of: Oh my goodness how did this end up in this condition, and is there any chance I can bring it back from the edge. No, I didn't use super precision resistors for the two padding resistors I inserted but I did take care to use metal film resistors with a low thermal coefficient (ppm/C). If the values I've used don't need changing (which I think I will know soon enough), I'll get in some really good ppm/C parts (maybe from Edwin Pettis). Yes, I know that the two 9.896k resistors sets that have gone low will probably remain suspect until they remain stable at their current value for a long while. I wish I could afford a new 720A ... but if wishes were horses, beggars could ride. Aside: I'm still struggling to understand how two sets of four precision 2474 Ohm WW resistors (9896 Ohm) in an oil bath can go low as they have in this instrument, while the remainder appear to be almost dead on specification. Dave ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Update on 720A
David wrote: When I got it, two oil bath resistors were *way* off value (202 and 100 ohms low respectively), and the S2 shunt resistors were about 1.3 ohms high which was far enough off to prevent S2 calibration. There were were also sundry other problems like two open circuits in the final decade, a badly worn trim pot, and sundry wires broken at solder joints. The resistors in a 720A -- even the pad resistors -- do not drift that much by themselves under normal use. Trim pots do not get used enough to wear out under normal operation. If you have some that are worn, the instrument has been abused. This is further indicated by the broken wires. I have *never* seen a broken wire in a 720A. Unfortunately, it seems clear that your 720A was traumatized in its earlier life, and some of the divider resistors were damaged (in addition to other probable damage, based on your reports). There really isn't anything you can do but replace the bad resistors with equal or better parts. Trimming them as you have done is just a temporary band-aid. Resistors that have been traumatized will never allow the instrument to meet its time and temperature drift specifications, and you will be going back inside to replace (and re-replace) pad resistors every time you want to use it. Worse, the problem goes deeper than that. Even replacing the resistors you know are bad will not restore the instrument to its accuracy and stability specs. The techniques you are using to identify the bad resistors will only identify ones that have been grossly damaged -- they will not identify all of the resistors that have been damaged and are preventing the instrument from meeting its specifications. Further, the very act of replacing resistors in a 720A is likely to cause additional damage -- to the resistors you are soldering, the resistors attached to them, and the infrastructure of the instrument. Add to this the fact that only a few qualified replacement resistors will cost more than a properly-working used 720A, and you can see that repairing a traumatized 720A -- if you can do it at all -- is not cost-effective for an amateur. The only 720As I've ever seen that worked correctly after being traumatized had been repaired by Fluke at astronomical cost. I hate to say it, but if you expect to use a 720A to calibrate meters of 6-1/2 digits and up, you will eventually learn that you need to replace the one you have. Best regards, Charles ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Update on 720A
Hi David, It isn't your soldering quality or ability that will degrade the instrument, it is rather the quality of the parts you add, or replace, and what you do to fudge things into sort of working, that will damage/degrade the instrument. Devices like the 720A are firmly in the category of magical devices. Resistors, switches, pots, and even the plating, and the solder alloy are critical to the ultimate performance the 720A can achieve. Fluke worked every angle they could think of to make the 720A a precise and very stable device. If you change a resistor type in a critical place, the whole recipe can fall apart. The resistors they used aren't the sort that will simply drift off value. They are wirewound using a very special alloy, and are essentially good forever. If they have changed, either someone has changed them, or something drastic has happened, like they have been overloaded, or an internal contact has failed. The pots used as trimmers should have only been trimmed a couple of times over the life of the instrument... hardly enough to wear them out... though corrosion is always possible. My concern is you seemed to be going at this repair like you were working on an oscilloscope, rather than a super precision instrument. -Chuck Harris David C. Partridge wrote: > I can't replace the bad resistors in the A decade - they are in the oil bath > and > the manual says that in this case you sent the unit back to Fluke for a > re-build. > These days, I suspect they won't even do re-builds at all (or only at *silly* > money), but make you buy a new one, and may well not supply *any* parts unless > they do the repair (as happened to a friend of mine with another instrument). > > I'm really not convinced that "the very act of replacing the resistors is > likely > to cause additional damage" - I've been around delicate and high-Z > electronics for > a while now and can get in and out in unsoldering pretty handily. I'm also > very > aware of the critical importance of not messing up the conformal coating > which is > there for a reason (and re-sealing it where necessary). > > I accept that if the primary resistors keep on drifting then I am "in a hole" > and > should probably save my pennies for another one. > > The problem is that the only source (that I know of) of used 720As in UK is > either > eBay or very occasionally a cal. lab. that is closing. "Known good" isn't > normally a term that applies to either. > > Dave > > > ___ volt-nuts mailing list -- > volt-nuts@febo.com To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the > instructions there. > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Update on 720A
I can't replace the bad resistors in the A decade - they are in the oil bath and the manual says that in this case you sent the unit back to Fluke for a re-build. These days, I suspect they won't even do re-builds at all (or only at *silly* money), but make you buy a new one, and may well not supply *any* parts unless they do the repair (as happened to a friend of mine with another instrument). I'm really not convinced that "the very act of replacing the resistors is likely to cause additional damage" - I've been around delicate and high-Z electronics for a while now and can get in and out in unsoldering pretty handily. I'm also very aware of the critical importance of not messing up the conformal coating which is there for a reason (and re-sealing it where necessary). I accept that if the primary resistors keep on drifting then I am "in a hole" and should probably save my pennies for another one. The problem is that the only source (that I know of) of used 720As in UK is either eBay or very occasionally a cal. lab. that is closing. "Known good" isn't normally a term that applies to either. Dave ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Update on 720A
David: I had the same problem as you when I got my used 720A years ago. Both the "A" and "B" decade had a few resistors that had changed over the years. I don't think as bad as you experienced but still they would not balance within the range of the adjustment pots. So I "adjusted" those that needed correcting with either parallel or series resistors as necessary. I used the best tolerance resistors I could get at that time. I figured that the "adjustment" resistors were such a small amount of the resistors I was trying to adjust, that the TC wouldn't be much of a problem. However I did have to go through these "adjustments" several times as I noticed that correcting one would then affect others in the string. This is probably what you are experiencing with needing to "readjust" others again. A few passes in each string finally got the adjustments into a good range and they have stayed there for years now. As far as I am concerned this is all "good enough" for hobbyists or VoltNuts who don't want to spend $5,000 or more to get FLUKE to refurbish your instrument, if FLUKE is even interested in doing the job and has the parts. Notice that some resistors in the "A" and "B" decades are selected at the factory anyhow. If you have the money, then be a "purest". It might be cheaper just to buy a new 720A. As you realize now the 720A input resistance is 100k. So you will get a small voltage drop from the voltage source to the input of the 720A, depending upon the size of the wire you are using. Of course the proper way to insure that there is no drop is to use Kelvin connections from the voltage source to the input of the 720A and then set the voltage source to external Kelvin sensing. What I usually do is to set the ratio dials to .99X and then set the voltage supply to get exactly 10.000 on my 3458A. Then you have the best chance to get an idea of the accuracy of the 720A ratio at .8, .7, .6 and so on. Don't let the 3458A autorange when down to the 0.1 ratio! Then to minimize the effects of the "B" decade, use .89X, .79X down to .09X. The only problem with this method is that you don't get to check the .9 ratio. So what you can do is connect the voltage source to the "1.1" input, set the ratio dials to 1.000 and then set the voltage source to give you 10.000 volts on the DVM. Now you can check the .900, .800 and so on ratios. I am sure that I did the R203 adjustment as described in paragraph 4-25 of the manual. It has been years now. All IMHO of course. You are now down into measurement "mud" so to speak. This is probably why the 3458A development team used the JJA at Loveland to check the overall linearity of the 3458A A/D. The 720A (or equivalent) wasn't good enough at 0.1 ppm! Bill - Original Message - From: "David C. Partridge" To: "'Discussion of precise voltage measurement'" Sent: Tuesday, August 08, 2017 7:39 AM Subject: Re: [volt-nuts] Update on 720A > Chuck, > > I totally get your point that ideally you should use any KVD in null-balance mode. I do note however that you can load it up to 11mA without damage though I've not once gone anywhere near that (worst case insult has been about 0.1mA). However it does also say that to avoid loading errors you should load with an impedance >= 1TeraOhm (10**12). > > Clearly a 3458A on the 10V and lower ranges is merely > 10GOhm and will show loading effects. > > So, yes, I do recognise I was "doing it wrong"! > > I don't believe I've inflicted any damage, and have managed to restore it to a useable state from a fairly sorry condition: When I got it, two oil bath resistors were *way* off value (202 and 100 ohms low respectively), and the S2 shunt resistors were about 1.3 ohms high which was far enough off to prevent S2 calibration. There were were also sundry other problems like two open circuits in the final decade, a badly worn trim pot, and sundry wires broken at solder joints. > > Ideally I'd like to replace the KVD "Bridge Balance" pot as that's a bit noisy but shudder to think what that might cost if I could even find one. > > Cheers > Dave > -Original Message- > From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Chuck Harris > Sent: 08 August 2017 13:34 > To: Discussion of precise voltage measurement > Subject: Re: [volt-nuts] Update on 720A > > Hi Dave, > > Fundamentally, any input impedance from your 3458 is going to throw your 720A out of wack. the 720A is not supposed to have even picoamps of loading nothing, nada, zip. > > I would suggest that before you damage your 720A further, you read the manual for the 720A and follow the instructions to the letter. An HP3458A is not a substitute for a null detector and a voltage calibrator. > > The calibration of a 720A requires that no current, none, zip, be drawn from the 720A on any port. It can only be done by using a voltage calibrator, and a null detector, as a differential voltme
Re: [volt-nuts] Update on 720A
Hi Dave, The 11ma loading is a threshold where over dissipation will occur, letting the magic smoke out of the box. The 720A won't be physically damaged, but will be far from usefully usable long before that point. No load is the only acceptable loading of a 720A. The damage I am referring to is the damage done by you in your attempt to diagnose and fix the 720A through faulty techniques. A snip here, a solder joint there, and you can quickly, though unintentionally, turn the KVD into a worthlessly unstable piece of junk. I know that the 720A is yours, and you can treat it anyway you please, but if your intention is to bring it back to its design specifications, you are going to have to follow the rules. No shortcuts. The 720A is a bridge device. It works in that magical region where no currents are sourced or sinked by the 720A. Typically, trim pots don't wear out. They are accessed so infrequently that they never get a chance to wear before something else renders them unuseable... usually corrosion. It can probably be saved by a little cleaning. The bridge balance pot may not be so lucky, though. I don't think it needs to be anything very special. -Chuck Harris David C. Partridge wrote: > Chuck, > > I totally get your point that ideally you should use any KVD in null-balance > mode. > I do note however that you can load it up to 11mA without damage though I've > not > once gone anywhere near that (worst case insult has been about 0.1mA). > However > it does also say that to avoid loading errors you should load with an > impedance >= > 1TeraOhm (10**12). > > Clearly a 3458A on the 10V and lower ranges is merely > 10GOhm and will show > loading effects. > > So, yes, I do recognise I was "doing it wrong"! > > I don't believe I've inflicted any damage, and have managed to restore it to a > useable state from a fairly sorry condition: When I got it, two oil bath > resistors were *way* off value (202 and 100 ohms low respectively), and the S2 > shunt resistors were about 1.3 ohms high which was far enough off to prevent > S2 > calibration. There were were also sundry other problems like two open > circuits > in the final decade, a badly worn trim pot, and sundry wires broken at solder > joints. > > Ideally I'd like to replace the KVD "Bridge Balance" pot as that's a bit > noisy but > shudder to think what that might cost if I could even find one. > > Cheers Dave -Original Message- From: volt-nuts > [mailto:volt-nuts-boun...@febo.com] On Behalf Of Chuck Harris Sent: 08 August > 2017 > 13:34 To: Discussion of precise voltage measurement Subject: Re: [volt-nuts] > Update on 720A > > Hi Dave, > > Fundamentally, any input impedance from your 3458 is going to throw your 720A > out > of wack. the 720A is not supposed to have even picoamps of loading > nothing, > nada, zip. > > I would suggest that before you damage your 720A further, you read the manual > for > the 720A and follow the instructions to the letter. An HP3458A is not a > substitute for a null detector and a voltage calibrator. > > The calibration of a 720A requires that no current, none, zip, be drawn from > the > 720A on any port. It can only be done by using a voltage calibrator, and a > null > detector, as a differential voltmeter. > > Remember, and heed my words, *no* current may be drawn from the 720A. > > -Chuck Harris > > David C. Partridge wrote: >> Update: After changing the resistor I added in series with R1008 (A decade >> position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means that R302 is >> slowly increasing in value back towards nominal) and re-calibrating again, >> the A >> decade is now within 1ppm of linear (using the 10V range and nominally 10V >> input). >> >> I note that when I'd done this, with the A decade set to 0.9 (reading >> 9.00V >> on the 3458A), the actual input voltage needed was 10.40 or so. Is that >> to >> be expected? >> >> Two of the positions on the B decade wouldn't quite adjust for a null when >> calibrating it. In this situation I think I have two options: >> >> 1) Add series resistors to compensate for slightly low value 9.898k >> resistors in >> the relevant two positions of the decade and recalibrate. >> >> 2) Adjust R203 to change the bridge balance slightly to the 10K ohm decades >> and >> recalibrate. >> >> Your thoughts on which approach to take is much appreciated. >> >> Thanks Dave >> >> ___ volt-nuts mailing list -- >> volt-nuts@febo.com To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the >> instructions there. >> > ___ volt-nuts mailing list -- > volt-nuts@febo.com To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the > instructions there. > > ___ volt-nuts mailing list -- > volt-nuts@febo.com T
Re: [volt-nuts] Update on 720A
Chuck, I totally get your point that ideally you should use any KVD in null-balance mode. I do note however that you can load it up to 11mA without damage though I've not once gone anywhere near that (worst case insult has been about 0.1mA). However it does also say that to avoid loading errors you should load with an impedance >= 1TeraOhm (10**12). Clearly a 3458A on the 10V and lower ranges is merely > 10GOhm and will show loading effects. So, yes, I do recognise I was "doing it wrong"! I don't believe I've inflicted any damage, and have managed to restore it to a useable state from a fairly sorry condition: When I got it, two oil bath resistors were *way* off value (202 and 100 ohms low respectively), and the S2 shunt resistors were about 1.3 ohms high which was far enough off to prevent S2 calibration. There were were also sundry other problems like two open circuits in the final decade, a badly worn trim pot, and sundry wires broken at solder joints. Ideally I'd like to replace the KVD "Bridge Balance" pot as that's a bit noisy but shudder to think what that might cost if I could even find one. Cheers Dave -Original Message- From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Chuck Harris Sent: 08 August 2017 13:34 To: Discussion of precise voltage measurement Subject: Re: [volt-nuts] Update on 720A Hi Dave, Fundamentally, any input impedance from your 3458 is going to throw your 720A out of wack. the 720A is not supposed to have even picoamps of loading nothing, nada, zip. I would suggest that before you damage your 720A further, you read the manual for the 720A and follow the instructions to the letter. An HP3458A is not a substitute for a null detector and a voltage calibrator. The calibration of a 720A requires that no current, none, zip, be drawn from the 720A on any port. It can only be done by using a voltage calibrator, and a null detector, as a differential voltmeter. Remember, and heed my words, *no* current may be drawn from the 720A. -Chuck Harris David C. Partridge wrote: > Update: After changing the resistor I added in series with R1008 (A > decade position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means > that R302 is slowly increasing in value back towards nominal) and > re-calibrating again, the A decade is now within 1ppm of linear (using the > 10V range and nominally 10V input). > > I note that when I'd done this, with the A decade set to 0.9 (reading > 9.00V on the 3458A), the actual input voltage needed was 10.40 > or so. Is that to be expected? > > Two of the positions on the B decade wouldn't quite adjust for a null when > calibrating it. In this situation I think I have two options: > > 1) Add series resistors to compensate for slightly low value 9.898k > resistors in the relevant two positions of the decade and recalibrate. > > 2) Adjust R203 to change the bridge balance slightly to the 10K ohm > decades and recalibrate. > > Your thoughts on which approach to take is much appreciated. > > Thanks Dave > > ___ volt-nuts mailing list > -- volt-nuts@febo.com To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the > instructions there. > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Update on 720A
Hi Dave, Fundamentally, any input impedance from your 3458 is going to throw your 720A out of wack. the 720A is not supposed to have even picoamps of loading nothing, nada, zip. I would suggest that before you damage your 720A further, you read the manual for the 720A and follow the instructions to the letter. An HP3458A is not a substitute for a null detector and a voltage calibrator. The calibration of a 720A requires that no current, none, zip, be drawn from the 720A on any port. It can only be done by using a voltage calibrator, and a null detector, as a differential voltmeter. Remember, and heed my words, *no* current may be drawn from the 720A. -Chuck Harris David C. Partridge wrote: > Update: After changing the resistor I added in series with R1008 (A decade > position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means that R302 is > slowly > increasing in value back towards nominal) and re-calibrating again, the A > decade > is now within 1ppm of linear (using the 10V range and nominally 10V input). > > I note that when I'd done this, with the A decade set to 0.9 (reading > 9.00V on > the 3458A), the actual input voltage needed was 10.40 or so. Is that to > be > expected? > > Two of the positions on the B decade wouldn't quite adjust for a null when > calibrating it. In this situation I think I have two options: > > 1) Add series resistors to compensate for slightly low value 9.898k resistors > in > the relevant two positions of the decade and recalibrate. > > 2) Adjust R203 to change the bridge balance slightly to the 10K ohm decades > and > recalibrate. > > Your thoughts on which approach to take is much appreciated. > > Thanks Dave > > ___ volt-nuts mailing list -- > volt-nuts@febo.com To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the > instructions there. > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
