Fluke 720A Kelvin Varley Divider –  Repair, Calibration, Linearity Validation - Page 1
Forum section: electronics / general

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 26, 2021, 03:56:40 pm
content: Purchase and first checkI purchased a 720A from another volt-nuts on eevblog, as ‘working’.The seller demonstrated that all switches were ok, and the basic divider functionality was given.What I had seen so far in the net were instruments with broken switches, or defective precision resistors (open or with internal short circuits), and instruments sold for parts. In this case, it was not checked upfront, whether the instrument was fully functional, i.e., if an autocalibration could be done successfully.The instrument is from about 1984, i.e., 37 years old. So, it was very probable, that several of the precision wire wound resistors had drifted out of their trimming ranges.The exterior of the case showed quite some signs of age also. I removed all stickers, and the dirt with isopropanol, cleaned all knobs with detergent water and a soft brush and replaced the broken knob with one from my Fluke 845A.The interior showed the usual mix of green PWW resistors, and the mica ww ones:Somebody had been already been inside and made an ugly repair on resistor of B8, Deck B.He slightly increased R1062 by soldering a Thin Film resistor in series to R1061, 4k75, shown here for B3:On the top side, there is a big solder blob which melted the trim pot a little bit, on the bottom side the corresponding solder pad had been burnt and probably damaged, and there were several bad solder joints.I only made proper solder connection and cleaned this area from the burn stains as far as possible with isopropanol. To avoid any unintentional contamination, I wore all the time white gloves.Then I checked the positions of all seven decks if they all make correct contact.Afterwards I wired it up for self – calibration, i.e. 10V or 20V provided by my Fluke 5442A DC volt calibrator, and a Fluke 845A for zero, and using two low thermal e.m.f. PTFE & spade lug cables with shield.At first, I did the basic internal calibration of the Wheatstone Bridges and Deck C resistors.(*)Then I found out, that two resistors of Deck A could not be trimmed any more, R303 and R310. Their nominal values had initially been trimmed by individual fixed resistors to sum up to 9901 Ohm.Their zero variable trimmers (A2, A9) of 98.8 ... 99.3 Ohm were stuck at -30µV, which is equivalent to missing -6ppm. The trimming range of Deck A is +/- 22ppm, so both resistors drifted about +28ppm, which is pretty reasonable for its 37 years age.On Deck B, trimming pot B3 of R408 was stuck at +40µV, that’s +16ppm here, and this one obviously drifted about -68ppm, as the trim range is +/-52ppm for this deck. The nominal value is 9901.5 Ohm.Also, zero aligning for all 11 switch positions was inconsistent and unstable, when re-checking the zero.(*)For such old instruments, the basic alignment procedure might be required, especially in case you can't zero the bridge.First follow then the procedure in chapter 'Maintenance', 4-25 Bridge Calibration, p. 4-14 - p. 4-18, and then proceed with the normal auto-adjustment of Deck-A and -B

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 26, 2021, 03:57:28 pm
content: To rule out that there were no contact errors, or any leakage problems causing these deviations, I measured all precision resistors of Deck A in the oil tank, and of Deck B, and all of their corresponding raw trim resistors. A 3458A with 4W Ohm and Offset Compensation or similar is needed to make precise absolute measurements.I append all measurements in the Excel table for your convenience:https://www.eevblog.com/forum/metrology/fluke-720a-kelvin-varley-divider-repair-calibration-linearity-validation/?action=dlattach;attach=1360625It’s clearly visible, that the resistors under suspect plus their fixed trimming resistors all were off by about the anticipated amount, compared to the other resistors.R403 + R1063 were also off, but this switch position had been ‘repaired’ by someone else already, as described.The 37-year drifts are evident in the table, as practically all resistors drifted in both directions away from their default value. Some years later, there might be the necessity to re-trim some others also. Candidates are already identified by this table.Deck A resistors were trimmed 0.5 Ohm downwards each by paralleling a 39k or 47k thin film resistor to R1039 and R1011.This was the least invasive way, does not change the trimming range and the overall T.C. of the resistor assembly.For Deck B it was necessary to increase the value by about 0.6 Ohm. Best way is to de-solder one of the cables going to the switch of Deck B and solder a 0.56 Ohm thin film resistor in series.That does not look nice either, but it’s much less harming the solder joints, and you don’t need to de-solder and bend the mica resistors.I needed to clean all switch contacts of Deck B due to the unstable zero, by means of isopropanol and cotton swabs.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 26, 2021, 03:58:25 pm
content: Calibration was now possible, consistent, and stable, so both Decks A, B could be trimmed for a zero of < 0.2µV.For this process, take notice of the errata sheet for the 720A manual. #3 is important, in one step “B” decade has to be set to Blank, not “C”.For linearity and performance validation, I connected a precise 10V from my 5442A in external sensing mode, by 4 Kelvin sensed wires, so that the voltage was very stable, no voltage drop occurred, and there were no contact resistance variations at the jacks. (The interior voltage drop can mostly be ignored.)The 3458A was connected via another low e.m.f. cable to the output and using its superior INL of typ. 0.02ppm I measured Deck A and Deck B linearity.I calculated the INL of input, i.e., related to F.S. value of each deck, 10V for Deck A and 1V for Deck B, as well as the INL of output, i.e., deviation of 3458A from best fit line in relation to the output value.I made six quick and dirty measurements, w/o averaging the readings to have more consistent values. The variations are on the order of a few parts in 10-8, anyhow, see xlsx sheet.Even at this situation, you can already see, how good the linearity of both decks is, and this measurement was already a real challenge for the 3458As linearity specification.The specification of 0.1ppm of input is clearly undercut, the ~ 0.5 ppm for Deck B as well, and the INL of output calculation reveals that it’s also possible to make a precise 10:1 transfer @ < 0.5ppm, in contrast to the INL-of-output specification, which would allow only a 10:1 transfer of about 1ppm.When I calculate the error budget for the 0.1ppm alignment of the resistors, I also find a maximum error of 0.18ppm for a 10:1 transfer.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 26, 2021, 03:59:10 pm
content: The 720A might be used to validate or to calibrate the 22V and the 275V ranges of my 5442A, which requires precise transfers from 2:1 down to 10:1 from 10V. The 3458A is used as a 2nd instrument for a precise transfer validation.HighVoltage generously lends me his 752A for more precise 10:1 and 100:1 transfers.Then the 720A is assumed to be stable enough over time and temperature to perform differential noise/stability tests of divided down 10V reference versus the 7.15V from an LTZ1000, or the 6.668V from an ADR1000. That way I hope to do Three-Cornered-Hat statistics with unequal sources.At last, I would like to make further challenges of linearity between the 720A and the 3458A.I would also like to check the (missing) error calculation for the 720A, (compare to manual of 752A) as I have the assumption, that the original specification from 1969 was not properly calculated, underestimating the possibilities of the 720A. For a description of this subject, see also hp journal April 1989, about INL, DNL, and verification of the 3458A against ratio standards.

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 26, 2021, 03:59:55 pm
content: The 720A is still an extremely precise ratio standard as a complement to the 3458A.Most instruments on the 2nd market very probably suffer from this resistance drift, or even worse error patterns. That’s the main reason I can imagine, why they show up on the market, anyhow.With some deep analysis of the errors it’s often possible to bring the instrument back to its specifications. I hope this description will help others.Frank

author: TiN
author_rank: Super Contributor
timestamp: December 26, 2021, 06:39:06 pm
content: Nice work! I have a 720 with removed trim mica resistors which need work.Did you verify INL of your 3458A (used as reference here for 720A test?) before?It would be also interesting to see how 720A behaves with power dissipation, once signals above 200V applied and how long would it take to warm-up.

author: 1audio
author_rank: Frequent Contributor
timestamp: December 27, 2021, 03:45:42 am
content: I find this really interesting. I have two vintage 7 dial KVD's; a JRL 106/7 and an ESI RV-722. I have not intercompared them or checked against anything else not having anything else nearly able to verify their performance. Having just obtained a suitable null detector I'll see about checking them. Not sure what can be learned from two independent ancient KVD's however something interesting may come from this.The JRL manual suggest a simpler verification that the "NBS" (195?) was using then that could validate a 7 dial KVD (for $45 !!). I could not find anything on how they do that test so far. Any references?

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 27, 2021, 10:49:19 pm
content: Quote from: TiN on December 26, 2021, 06:39:06 pmHello Illya,thank you for the flowers!When I wrote this article, I had your ripped 720A on my mind.Maybe I write an additional chapter with a deeper analysis, how to select these raw trim mica resistors from scratch.In your case, you also have to take the wires to the oil tank for Deck A, as well as all wires from the trimming PCB to the switches into account, several 100mOhm for sure.The resistance of these wires have to be added to achieve the nominal resistances of 9900 Ohm for R312 + R1046, of 9901 Ohm for the other resistors pairs of Deck A, and of 9901.5 Ohm for all resistors of Deck B.Well, I did not check the INL of my 3458A. Of course not, because I don't own a JJA or don't have access to one.The 3458A by design should have the INL / DNL figures as described in hpj 4-89, otherwise it would not be able to make precise internal 10:1 transfers; its linearity is not well or not clearly specified, b.t.w.Anyhow, these linearity measurements give like all such comparisons the sum, or the quadratic sum of both constituents. That's why I call it a challenge for the 3458A.Therefore, the INL can be assigned to both or mainly to one of the instruments. If I would use a 6.5 digit DMM, like the 34401A, or so, I would for sure measure mainly its <1ppm INL.If I repeat this alignment and linearity test several times, and always find that this 0.1ppm INL of Input specification is always undercut, there is some evidence, that this specification is not correctly calculated.Also, if I always find that a 10:1 transfer with the 720A is accurate to <<1ppm, this spec. should also be critically be reviewed. As said, a deeper error budget analysis would be useful.Please, anybody might correct me, if I'm thinking wrong.I'll gain some more insight maybe, when I compare the 720A with the 752A, which currently acclimatizes downstairs..Then I will also do some HV tests with both, but the 720A will only be stressed up to 250V, i.e., 1/16 of its maximum load.I already did some comparisons of the 720A and the 3458A @ 250V, and all values were within about 0.4ppm.I did not take notice of any warm-up effects, but that's a good hint.Frank

author: Dr. Frank
author_rank: Super Contributor
timestamp: December 27, 2021, 11:04:05 pm
content: Quote from: 1audio on December 27, 2021, 03:45:42 amHello,the 720A is a Primary Ratio Standard, because it can be auto-calibrated, i.e. w/o any other ratio reference, and then performs as specified.The two K-V dividers you own do not have this feature and therefore need to be calibrated by primary standards. I briefly looked over the manual of the RV-722, maybe the description there is similar to what is happening inside the 720A.. but for sure it's much less stable an alignment process.If you run both K-Vs against each other, you'll definitely see the sum of both non linearities, like in my experiment of 720A vs. 3458A.For sure very interesting and worthwhile to see, what's the outcome.Some other guys here also proposed another precision linearity test, but I don't remember, what the trick was.Anyhow, there are several older papers about K-V dividers, which I will also read, as a lot of time has passed, and our todays possibilities as volt-nuts are much better, I assume.Frank

author: alm
author_rank: Super Contributor
timestamp: December 28, 2021, 03:16:18 am
content: Quote from: 1audio on December 27, 2021, 03:45:42 amWhat I'd guess they did back then is basically the manual version of what the Fluke 720A does: Take a number of very stable voltage sources or resistors, trim them so they all measure the same, and then measure various series combinations (and optionally also parallel configurations) against the KVD set to the same ratio.Looking at the ESI SR1010 manual, although that's not exactly for the same purpose, might give you some inspiration and an idea of the error analysis.

author: TiN
author_rank: Super Contributor
timestamp: December 28, 2021, 04:02:28 am
content: QuoteYou participated in few volt-nut meet-ups before so I assumed you did check 3458A against another good 3458A perhaps, to gain better confidence. JJA is not the only way available. Also binary voltage dividers claim 0.02ppm INL, such asMIL 8000B.QuoteI've noticed that not all A3 are equal and A3's with larger drift/tempco have higher INL errors than better boards. Also just taking "by design" specs is not really metrological way, compared to doing actual measurement test and monitoring parameter in interest.QuoteAre you concerned of causing shift in resistors at higher voltages, thus limiting only 1/16 of the load instead of rated 1100V?

author: martinr33
author_rank: Frequent Contributor
timestamp: February 06, 2022, 01:08:29 am
content: A big "THANKS" to Dr. Frank for this fabulous work.I just started in on a 720a, and in the "A" decade found one resistor (on 0.7V) that could not be adjusted to null. The range on the pot in calibration mode is +20uV - +90uV. The other pots were all pretty much dead on as received. And this is at 16C. B and C decades are next.I still haven't figured out how the calibration works on this device, but it sure is an easy process for the result you get.Fluke still has the 720a in its catalog, yours for just under $52,000.First pass, the unit appears to be stable. I am using a digital nanovoltmeter instead of the analog 845a, which gives me a little more visibility into noise.Here are the trims that appear to be off:Position Offset at pot limit, uVA7 10B0 -22B2 -18B4 -18B6 -25BC -9S1 -5The Deck A, Deck B adjustment - top side closeup. This side includes the pot, the high value metal oxide resistor and one of the wafer style wirewound resistors. The other worewound resistor is on the other side fo the board.

author: TiN
author_rank: Super Contributor
timestamp: February 06, 2022, 02:09:56 am
content: Even if you ready to pay $52k, Fluke 720A is a special order only. There are other alternatives today as well, such as binary voltage dividers. Also I think many NMI and high-end labs use KVDs less often now, with availability of programmable quantum standards.

author: martinr33
author_rank: Frequent Contributor
timestamp: February 06, 2022, 05:57:57 pm
content: The top side. Note the new pots fitted to the internal adjustment board. The trim circuit metal film resistors are a different case style to the later units, which used a Dale product.This side is most of the Deck A and Deck B trim components. There is a second wafer style resistor on the underside of this board.The bottom side. The wafer resistor belongs to the Deck A,B trim circuits. The other parts trim for C and D.

author: dl1640
author_rank: Regular Contributor
timestamp: February 07, 2022, 12:21:12 pm
content: A rigorous repair and worth reading, thank you

author: martinr33
author_rank: Frequent Contributor
timestamp: February 11, 2022, 05:07:37 am
content: some updates. The unit is reassembled, and behaving OK, again thanks to Dr. Frank's thorough article.1) The phenolic PCB has some kind of epoxy sealer across the resistors, both sides. This sealer interferes with the solder joints, so the iron has to get hot enough to decompose it. I had to tweak one resistor value.2) On my unit, the bridge balance was 20uV off. I figured this by precisely centering the balance pot, and then measuring the differential across the bridge from the reference voltage to the A0 arm (the A0 arm cannot be trimmed and is the reference for the A and B decades).Fixing this offset by adjusting the reference voltage brought the system to the point where I could zero all the A and B settings.3) The metal 270 degree trimmers are awkward. As a first step, I applied a little Deoxit to the top surface, where there is a metal-on-metal contact. That made the action a lot smoother, but electrically, the wipers are still difficult to set.

author: 1audio
author_rank: Frequent Contributor
timestamp: February 11, 2022, 04:49:54 pm
content: Are those trimmers wirewound? They look like they are. That would definitely limit their resolution.

author: TimFox
author_rank: Super Contributor
timestamp: February 11, 2022, 05:07:08 pm
content: Those trimmers are wirewound and are very popular in gear of that vintage, including -hp- laboratory power supplies.I was surprised to see the limited resolution on low-resistance 1.25" long, 22-turn trimpots due to finite wire diameter: Bourns quotes 1% resolution for 100 ohm series 3057 wire-wound trimpots, improving to 0.31% for 20 k.Wirewound does have somewhat better tempco than the equivalent cermet in the same package.

author: martinr33
author_rank: Frequent Contributor
timestamp: February 11, 2022, 05:44:30 pm
content: I messed up. Originally, I created a spreadsheet that recorded the setting limits of the pots, in terms of microvolts +/- zero.So a centered A trimpot has a range of +/- 53uV, and B trimpots +/-133 uV.Many of the B trimpots were well off center, so much so that they could not zero, or would be painfully close to the end of the pot.I calculated the parallel resistors needed across the 100R ballast to correct them, and applied them to the worst 6 ranges on the B pots. (the A pots were mostly OK, except for one).I used dirt cheap steel leg resistors, not my favorite for a precision instrument. So the next day I went to our local surplus store and picked up the values in precision resistors. And forgot that I had not removed the steel leg parts.So when I rebalanced the null with its measured 20uV offset, everything came in. I couldn't figure that out, but now I know...More to come when I clean up my spreadsheet.

author: donlisms
author_rank: Frequent Contributor
timestamp: February 18, 2022, 02:11:05 am
content: The technique is to put the trimmer in parallel with a much lower resistance. So if I understand this correctly, with a 5k trimmer in series with a 4.75k fix resistor (to set a lower limit in the parallel resistance), all in parallel across a 100 ohm resistor, the adjustment range is from 97.93814 ohms to 98.998477 ohms. The resolution of the trimmer is increased by a factor of about 50 because of the ratio of the trimmer to the resistor it's paralleled across, or rather a factor of about 100 because it's only about half of the total parallel resistance (ballpark). Then 1% in the trimmer gives a resolution of 100ppm of the adjustment, which is about 1/100 of the total series resistance of around 10k, so there's 1ppm. To the extent the trimmer's resolution is better than 1% (probably "yes!"), the adjustment resolution is better than 1ppm.What surprises me is how long ago these principles were uncovered and used to make extremely high resolution measurements, given... no amplifiers and no bits!Precision Resistors and their Measurement, James L Thomas, NBS circular 470, October 8, 1948Methods of Measuring Electrical Resistance, Edwin F Northrup, copyright 1912

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 18, 2022, 07:51:53 am
content: Quote from: donlisms on February 18, 2022, 02:11:05 amVerbal descriptions of the scheme are so hard.... Three resistors in parallel one of which is a trimmer? What does the second big parallel resistor do?

author: alm
author_rank: Super Contributor
timestamp: February 18, 2022, 06:48:48 pm
content: The way I understand the description is like way the adjustment for the A/B/C dials is done in the Fluke 720A. The resistor (R1002) in series with the pot (R1001) sets the lower value of the adjustment range, and the much lower value parallel resistor (R1003) limits the upper value. Leading to a fairly narrow adjustment range between R1002 // R1003 = 98.83 Ohm and (R1001 + R1002) // R1003 = 99.26 Ohm. They then add a small series resistor (R1004) to center the adjustment range around 100 Ohm.

author: martinr33
author_rank: Frequent Contributor
timestamp: February 19, 2022, 05:03:58 am
content: OK, here's my first attempt at describing how this tricky box works.Although a divider looks like a resistance box, the construction is completely different. In a resistance box, each decade consists of resistors of 1/10 of the value of the next higher decade.The overall resistance in a resistance box is the sum of all resistors in series, and the box has a wide span for the internal decade resistors - ohms to megohms.In a simple stacked divider, every resistor can be the same value.Each range takes the voltage output from the previous divider, and divides it again. However, this approach gives a wildly varying source resistance and therefore does not work.Hence, the Kelvin-Varley design, shown below.source:https://en.wikipedia.org/wiki/Kelvin%E2%80%93Varley_dividerEach range has two connections, which move together up and down the range, spanning two resistors.The bottom connection defines the value of each digit in the range (this is an important point).Therefore, the diagram above shows a setting of .2073Each range has 11 resistors - not 10, as you might expect, However, the range switch picks off two of the resistors, and puts them in parallel with the entire next range (this is the other important point)So for each position in the 10k resistor range:- two resistors in series = 20k (between the 0.2V and 0.4V taps, for a dial setting of 0.2V) - in parallel with 20k for the entire next range of 2k resistors in series- gives an actual resistance of 10k, effectively compensating for the "extra" series resistor, and keeping the resistance constant no matter the dial setting.Therefore, the first range is divided into 10 equal parts.Also, each range has to be 1/5 of the resistance value of the previous range for this to work as shown here.And note that the last range only needs 10 resistors.This design approach keeps the total resistance of the divider constant.Then, there is a trick in commercial systems.The system is easier to manage if there are fewer resistance values.Take a look at the KVD-700 schematic from IET, below.IET places a parallel resistance across the dual wiper, meaning that they can now use the same resistance value across multiple decades.This approach dramatically simplifies manufacturing and inventory, as all resistors have to be matched for temperature coefficient and relative value.Source:https://www.ietlabs.com/pdf/Manuals/KVD-700_im.pdfIn this image, the divider is set to 0.1212186.The challenge is, for the divider to be accurate, all the resistors have to have the same value to fractions of a ppm.Therefore, every high range resistor in the Fluke (and IET version) 720A has its own matching circuit. It looks like ths:This is the schematic for the adjustable resistor that represents the 0.8 to 0.9 voltage step in the highest range in the Fluke 720A.Source: Fluke 720A ManualR304 - the super stable resistor in the oil can. This is by far the bulk of 10k total for this section of the divider, selected for TC that matches the rest of this decade.R1016 - a mica wafer resistor of a few ohms. It brings the selected resistor to the nominal value for the range (remember, all resistors have to have the same value, not the same exact value - but the TC has to be the same).R1015 - a mica wafer nominal 100R resistor, in parallel with the trimmer network. In this decade, the value of the trimmer network ranges from 99.26R to 98.83R across 20 turns of the pot.R1013 - the wirewound Bourns Trimpot.R1014 - a giant metal oxide resistor, 50 ppm. I did find them in a military surplus supplier for $50 - $75 each. You can get much better resistors for a lot less money nowadays.There is also a trimmer circuit for the parallel pot, looks the same. These are the S1, S2 and (internal) S3 adjustments, for the first three ranges.In the 720a, the top two ranges have 20-turn Bourns Trimpots, The third range uses wirewound single turn pots.

author: RoadDog
author_rank: Regular Contributor
timestamp: February 19, 2022, 05:52:22 am
content: Quote from: martinr33 on February 19, 2022, 05:03:58 amTwenty turn and four turn pots I thought. Are yours only 10 and 1? I imagine it would be difficult to balance with fewer turns. Hard enough with 20 turns on the A & B decades.

author: martinr33
author_rank: Frequent Contributor
timestamp: February 19, 2022, 07:05:51 am
content: Good catch, they are 20 turn Trimpots for the A and B decks - but still very fussy.The third set of trimmers (accessible only inside the unit) is standard single turn Fluke wirewounds. They benefit nicely from some Deoxit and cleaning, but are still tricky to set.These are full-size wirewound pots, and suffer from both wire contact friction and friction in the metal case. I am sure that they are expensive parts, but the construction looks cheap.On my unit, the two bridge balance single turn trimmers have been replaced with 20-turn units.

author: RoadDog
author_rank: Regular Contributor
timestamp: February 19, 2022, 07:11:17 am
content: Have you turned em to the ends both ways a few times to clean the wipers? Might help.

author: martinr33
author_rank: Frequent Contributor
timestamp: February 19, 2022, 07:26:17 am
content: Many times! More so with the single turn ones, of course. I also ran the pots from end to end to measure the range and centering.You can add Deoxit to the wiper "bearing" from the top side of the pot, which is the rear of the shell, but the wiper itself is somewhat inaccessible. I used spray Deoxit, and then cleaned the board up afterwards. I also masked off the rest of the unit with cleanroom wipes while making this mess.The single turns ended up being smooth, but still tricky to set. The 20-turn Trimpots are sealed, but are mostly smooth. A couple of them have dead spots (evidenced by a voltage spike), but they are unimportant and seem to make no difference in operation.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2022, 09:13:31 am
content: Quote from: martinr33 on February 19, 2022, 05:03:58 amThe parallel resistor has an additional reason: this avoids getting very small resistors for the lowest deecades and thus makes the switches less critical.There is still a downside with the parallel resitors, as more resistance is added to the output. So it is good to have the 1:5 steps for the initial 2 decades or so and only later use the same resistance.Not all the resistors need to be matched to the ppm level. The lower decades are more forgiving and thus not in the oil tank. The parallel resistors also allows some tweaking, so that matching between the decades can be adjusted independetly.

author: RoadDog
author_rank: Regular Contributor
timestamp: February 19, 2022, 01:53:26 pm
content: I figured. Thought I'd ask though. These things seem to get a little finicky when old even in a good environment.Pretty sure you can get replacements for the 20 turn pots if any of them give you too much trouble. I was looking around at parts to build a divider and ran across some Bourns on Digikey that looked like an exact replacement. Spendy though. Too rich for my silly little divider I want to build.

author: julian1
author_rank: Frequent Contributor
timestamp: February 25, 2022, 06:44:56 am
content: Attempting the self-cal at 2-24 from the manual ;Using 34401a DMM (10G high-Z input) as makeshift null detector.The 34401a has been nulled with input short, to remove 5uV offset from cabling etc.- connect 34401a to 720 bridge detector.- connected linear DC source 10V for 720a bridge power.- set decade readout to .0 0 0 0 0 0 0- function set to Cal A.- turn on DC source.- the manual says, 'adjust the bridge balance control for a null indication'however the 720a bridge balance adjustment range is from 149 -166 uV.There's no chance of hitting null .Are deck A resistors required to be within tolerance in order to hit the initial zero (ie before adjusting trimpots) ?Or is there something wrong with my procedure?

author: martinr33
author_rank: Frequent Contributor
timestamp: February 25, 2022, 08:42:37 am
content: There’s another balance pot in the unit, under the top cover. This is the coarse adjustment for the top right leg of the bridge. You should be able to recenter the bridge null voltage with this pot. Set the front pot to midrange first.You may also need to do some cleaning on the single-turn pots.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 25, 2022, 09:58:34 am
content: Quote from: julian1 on February 25, 2022, 06:44:56 amFor such old instruments, the basic bridge and Deck-C alignment procedure is required, as obviously you can't even bring the bridge to zero.Please follow this procedure in chapter Maintenance, 4-25 Bridge Calibration, p. 4-14 - p. 4-18Frank

author: martinr33
author_rank: Frequent Contributor
timestamp: March 01, 2022, 07:22:20 am
content: I've attached the spreadsheet that I used for sorting out my 720a.This process worked for me, with the instrument's adjustments all closer to the center of the ranges. Note that this is all subservient to the adjustments that Dr. Frank made, and the manual. The big difference here is that we are also centering the instrument to push drift problems further into the future.1) Get the balance pot physically centered, then null the output using the bridge balance pots (on that adjustment board on the top side of the unit). You must make this adjustment for the A.B deck pot and the C deck pot.2) Perform the C deck adjustments as described in the manual. Shoot for midrange on the pots, and tweak the bridge balance pot if you need to do so. Put a little deoxit on the pots to get them to move smoothly, and run them end to end.3) Keeping the balance pot centered (you adjusted it for the A/B deck in the first step), switch to the A deck measure the range on all of the trimpots. This measuring also helps exercise the trimpots.4) Figure out which trim circuits you want to adjust. Put parallel resistors as you see in Dr. Frank's pictures. I used mostly 20K.5) Repeat for B deck.Tweaking the open frame pots is very tricky if you do not apply that drop of deoxit to the casework. I sprayed "a little" deoxit into the pots, and then had a bunch of cleanup to do.One further note. The net result of my process is to shift all of the drift across all parts into the "bridge balance" potential. In other words, I created a new fixed point for the bridge voltage by driving the balance pot off center. I did not measure how far, but it will be tens of microvolts. But by doing this, I restore a wide adjustment range to all the other devices.The 720A is a very impressive design, at least to me.

author: martinr33
author_rank: Frequent Contributor
timestamp: March 25, 2022, 05:56:52 pm
content: Here's a photo of the insides of a Fluke 720a currently on eBay, from seller outback6 (who gets credit for the photo).Note the trimmer resistors attached to the C deck. Unfortunately, the seller did not open the other side of the unit for photos.Without a documented procedure, tweaking these resistors is tricky but not difficult. It does take a while to figure out all the details.