Fluke 731B Repair/Modifications - Page 1
Forum section: electronics / repair

author: dr.diesel
author_rank: Super Contributor
timestamp: November 30, 2016, 01:22:48 am
content: Since it did power up, perhaps more of a modification thread than repair. I thought I read somewhere that the white faced units were newer, maybe slightly better in terms of drift, but now I can't find that info. Anyhow the goal is the best 10V reference I can make it, though the best I've currently got is a K7510 to bench it against. (where are the 3458A Cal Club members in the US, ha ha)Coming up are new batts, possible elimination of the voltage selection switch, maybe replacement of key TC components.Brief overnight test (k7510, 10 NPLC, rolling 10, autocal disabled) of about 12 hours, I will improve temperature measurements in the very near future, currently it's logged by a separate system on the other side of the room.

author: ManateeMafia
author_rank: Frequent Contributor
timestamp: November 30, 2016, 01:58:24 am
content: I think I remember seeing comments on the volt-nuts list. I am not sure which color is better but the 731B is better than the 731A so you have a good starting point.Mods crossed my mind the other day on HighVoltage's thread. I mentioned bypassing the switch but I wonder if a second set of binding posts wouldn't be better like the Fluke 510A. You could move the selected output minus the 10V to the rear and retain the 1V function for the rare instance you need to check a lower range on a meter.I have looked at the LTC4079 as a better charger for the batteries or the UC3906N and convert to SLA. I have the 510A waiting for mods.

author: edpalmer42
author_rank: Super Contributor
timestamp: November 30, 2016, 02:14:37 am
content: Do you really need the batteries at all?I've seen various pieces of old equipment that used the batteries as a big filter to keep out mains ripple. The power supplies are garbage, at least by today's standards. They're pretty much nothing but an unregulated charger for the batteries. I've got two pieces of test equipment and two high quality quartz oscillators in that category. I'm considering throwing out the batteries and power supplies and replacing them by a modern low noise or ultra low noise circuit. The only part I might keep is the transformer.

author: TiN
author_rank: Super Contributor
timestamp: November 30, 2016, 04:27:08 am
content: Batteries here are to keep the reference online and uninterrupted when you moving the box around without mains connection. I'd keep battery power, as it's important to avoid long warmup/stability delays when you move reference around.

author: edpalmer42
author_rank: Super Contributor
timestamp: November 30, 2016, 04:51:31 am
content: Quote from: TiN on November 30, 2016, 04:27:08 amFor my quartz oscillators, moving them around isn't a good idea. They're too touchy.But for things like the 731B, the first mod I'd be looking at is replacing the power supply and NiCd batteries with more modern ideas. Lead-acid would be easiest, NiMh would take a bit of thought, Lithium would take more thought. Maybe follow that with an ultra-low noise linear regulator to buffer changes in battery voltage during charging or off-mains operation.It would be interesting to see if there was a measurable improvement in performance or if the old designs still rule!

author: VintageNut
author_rank: Frequent Contributor
timestamp: November 30, 2016, 06:37:21 am
content: My rack has 2 x 731B and a DMM7510.I suggest that you give the 731B months to stabilize if you recently acquired it.I adjust my references a couple of times to bring them within a few ppms of my calibrated KE2000 that is within 1pmm at 10V. It has been over a year since I touched the adjustment pots.Expect something like 1pmm change in 10V for every 2 deg F change in your lab for the 731B.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 30, 2016, 10:54:38 am
content: Out of my 4 pieces, one has a black front face and that one has the highest serial number, so I concluded that the black face was the newer one but I am not sure.NiCd Batteries are cheap and easy to install, I would recommend to to this, to have the standard always powered up, even if you have to move it.I have not done any TC measurements on mine, that is to be done soon.But one of my units was already modified for better T/C with some new resistors.One of them also was changed to the latest ECO.Will be interesting to see your measurements over temperature.What amazed me most so far, was the stability over time at the same temperature.

author: dr.diesel
author_rank: Super Contributor
timestamp: November 30, 2016, 01:38:37 pm
content: Quote from: HighVoltage on November 30, 2016, 10:54:38 amYup, that seems reasonable to me as well. Looks like mine (SN 4150027) dates from late 1986, component dates jive with this.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 30, 2016, 01:46:33 pm
content: Your batteries look the same as in my units - horrible but to be fixed.I have used these NiCd batterieshttp://www.ebay.de/itm/142061266264

author: dr.diesel
author_rank: Super Contributor
timestamp: November 30, 2016, 02:01:38 pm
content: Any reason one couldn't swap to some low drain NiMH?Here is the result (step changes in the middle) of connecting/disconnecting the battery pack, perhaps the battery charge level slightly influences the output. I will attempt to trend this with a new battery pack starting from fully discharged state, if I can differentiate it from TC as I suspect it charges quite slow.

author: MosherIV
author_rank: Super Contributor
timestamp: November 30, 2016, 02:31:31 pm
content: QuoteNo, should be fine.NiMH is most like NiCAD compared to other technologies.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 30, 2016, 02:35:03 pm
content: I run my 731B without a battery, it is powered from the mains more or less 24x7, except an hour or two when moved from my home lab to my work lab and back. I've converted the battery meter into a thermometer measuring the internal temperature in 18-28C range. As my 731B has a rather high temperature coefficient (about 1.5ppm/C) I can calibrate the voltage accurately to about 1ppm just by looking at the needle. The voltage is accurate and very stable if the temperature is stable, so I don't want to interfere with the reference assembly or even attempt to adjust the voltage (it is 10.00000V at 24C measured by 3458A) .CheersAlex

author: saturnin
author_rank: Regular Contributor
timestamp: November 30, 2016, 04:01:03 pm
content: Quote from: dr.diesel on November 30, 2016, 02:01:38 pmI used twelve 2300 mAh NiMH batteries (AA size) and I can confirm they work well. They are properly recharged (well, it takes some time since charging current is only about 20 mA). Benefit is they last about 140 hours - tested.Quote from: Alex Nikitin on November 30, 2016, 02:35:03 pmAlex,have you modified the pre-regulator (Q3, CR10, CR11) in your unit too? I am asking because I found out the pre-regulator is bypassed by CR5, R30, CR8 when a battery is not installed. Then, the voltage at Q1 collector is not fully regulated (it is quite high too - I measured nearly 30V if I remember correctly). Also, the needle in the battery meter is noisy in this case (I mean it makes sound because it is fed by rectified voltage). In operation with battery and powered from the mains, the voltage a Q1 collector does not exceed approx. 16.5V and it is well stabilized...I think operation without battery was not originally supposed.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 30, 2016, 04:08:23 pm
content: Changing to NiMH makes charging a little difficult. They don't like the slow trickle charge often used with NiCd so well. Still the cells are not that expensive.

author: MosherIV
author_rank: Super Contributor
timestamp: November 30, 2016, 04:33:00 pm
content: QuoteI have never come across that. In fact just a quick search on NiMH charging says that for indefinite trickle charging, they suggest C/30 or even C/40http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf

author: saturnin
author_rank: Regular Contributor
timestamp: November 30, 2016, 04:56:20 pm
content: Quote from: Kleinstein on November 30, 2016, 04:08:23 pmYes, you are right I read similar information (e.g.https://na.industrial.panasonic.com/sites/default/pidsa/files/downloads/files/panasonic_nimh_chargemethods.pdf, 2nd page at the bottom), but I guess it will be ok in this particular application:- the charge circuitry in 731B does not feed batteries with steady DC current. They are charged by pulsing current from Graetz bridge. In a part of the cycle they are even being discharged (through R31 and R32)- average charging current is very low: well below 1/100C at 100% capacity (some manufacturers allow even higher trickle current)- I am going to disconnect the unit from the mains from time to time anyway just to discharge/recharge them, since I think it is not good for any type of batteries to be charged all the times without cycling them...- they were really cheap. It is no problem to renew them in let's say two years

author: edpalmer42
author_rank: Super Contributor
timestamp: November 30, 2016, 05:08:58 pm
content: Quote from: dr.diesel on November 30, 2016, 02:01:38 pmThis graph shows why the entire power system should be upgraded. Even though the batteries are ahead of the regulator, the output voltage changes with changes to the regulator's input voltage. These changes will occur when line power is present or absent and with changes to the battery's state of charge. NiCd does have a relatively flat discharge curve but, after all, this is volt-nuts territory.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 30, 2016, 09:44:18 pm
content: Quote from: saturnin on November 30, 2016, 04:01:03 pmIf I remember correctly, I've just disconnected R30 and the wire to the meter (so it could be used for a temperature readout).CheersAlex

author: dr.diesel
author_rank: Super Contributor
timestamp: December 03, 2016, 06:00:46 pm
content: Batteries arrived today, 2550mAh NiMH Panasonic BK-3HCCA4BA Eneloop Pro AAs. Combined with some 2" heat shrink tubing = Fixed.Doesn't appear any further repairs are necessary for some solid before data. Plan is to let this burn in for a month or two then possibly modify, or buy another and modify it if another pops up on the cheap.

author: HighVoltage
author_rank: Super Contributor
timestamp: December 03, 2016, 07:08:20 pm
content: Quote from: dr.diesel on December 03, 2016, 06:00:46 pmLooks really good with the shrink tubing.Instead of using tape to hold the cells together, I have used superglue in the past.Now you have battery capacity for many hours probably, if not days.Quote from: 3roomlab on December 03, 2016, 06:07:38 pmA long time ago, when I first started to look for one, ebay was full of them.Only problem, I could not find one seller that was willing to ship to Germany.Yes, eevblog forum has this effect on ebay, I have noticed this before.

author: dr.diesel
author_rank: Super Contributor
timestamp: December 06, 2016, 01:23:31 pm
content: Hmmmm, what could these be for:

author: HighVoltage
author_rank: Super Contributor
timestamp: December 06, 2016, 01:44:56 pm
content: sometimes, test and measurement equipment has a tendency to multiply.

author: dr.diesel
author_rank: Super Contributor
timestamp: December 10, 2016, 04:28:33 pm
content: Not near the shape as my first one, but it works!This one has a SN of 450007, but a manufacture date of 1976.

author: dr.diesel
author_rank: Super Contributor
timestamp: September 25, 2017, 08:07:10 pm
content: I ended up buying a couple more of these over the past ~6 months or so when they popped up cheap, just now getting around to looking at them. Listed below is the SN vs date (newest component date I could find) vs front panel color vs today's 10v reading:SN = 4150027, 1986, white faced, 9.9998328 (9.9998030, read by our friend @CalMachine in Feb of 2017, ENI Cal Labs, 25C)SN = 3075014, 1982, white faced, 9.9999200 (New NiCad date of 2004 sticker inside)SN = 450007, 1976. black faced, 9.9998050SN = 84405, 1974, black faced, 9.9999182 (731A)All 4 had no Cal stickers, so who knows how long it's been for any of them. Read via in Cal 3458A, 200NPLC, mental average of LSD bobble, 24C lab.

author: HighVoltage
author_rank: Super Contributor
timestamp: September 25, 2017, 09:00:54 pm
content: Very interesting, thanks for sharing your serial numbers !Tomorrow, I have to look up the serial numbers on my four 731BI was under the impression the black face was the newer model.

author: branadic
author_rank: Super Contributor
timestamp: December 21, 2020, 04:36:16 pm
content: I came across thisa while ago, which I found pretty interesting and watched it recently again, as someone left his F731B for modification and characterization on my desk.Modification:Added some small connector on the backhttps://www.binder-connector.com/de/produkte/subminiatur-steckverbinder/snap-in-steckverbinder-ip-40and installed a 5k NTC to the lid of the reference.Characterization:I currently have it in my thermal chamber to measure its t.c., so I wonder if someone can share results for comparison? In the slope from 23°C to 15°C I can see some -3.5ppm/-8K = 0.4375ppm/K, but profile is not yet finished.-branadic-

author: bdunham7
author_rank: Super Contributor
timestamp: December 21, 2020, 04:53:16 pm
content: Quote from: branadic on December 21, 2020, 04:36:16 pmI have a late-model one like that with a white face and I've cleaned it up and made a battery pack for it. I don't have the means to accurately characterize it, but it does have a noticeable tempco and it is negative and appears to be on the same order as yours, although I have no accurate data for you. I'd be interested in seeing how yours is from 20C to 26C. I can keep my room +/- 3C from the standard cal temp of 23C, but not +/- 1C. I considered putting a hole for a temp probe in it, but the way you've shown it looks good. One issue is that it isn't in one large, heavy aluminum box so there could easily be temperature gradients--so I don't know how well the units temperature variance will be measured by the probe.

author: branadic
author_rank: Super Contributor
timestamp: December 22, 2020, 08:38:23 am
content: Profile finished overnight. Attached are the results I obtained. Not the shape and curve I expected. By the looks of it zero t.c. is at ~28°C, while the parabola is quite narrow. It proves that t.c. for this unit is <0.5ppm/K for temperatures <28°C.The position of the NTC on top of the lid seems to be appropriate.-branadic-

author: bdunham7
author_rank: Super Contributor
timestamp: December 22, 2020, 04:10:13 pm
content: Quote from: branadic on December 22, 2020, 08:38:23 amIt sure looks like it--the voltage follows the NTC at least 2X closer than the ambient. Very nice!Now all you need is a small heater to keep it at 28C.

author: branadic
author_rank: Super Contributor
timestamp: December 22, 2020, 11:43:39 pm
content: Comparison with two F7000 give stable and consistant readings. So the owner of this F731B can be lucky to have a stable and low noise reference, although the t.c. is not perfect. But with the NTC upgrade he can compensate for that.+-F731BF7000#1F7000#1F731BF7000#2F731BF731BF7000#2F7000#1F7000#2F7000#2F7000#1-branadic-

author: TERRA Operative
author_rank: Super Contributor
timestamp: December 23, 2020, 03:16:54 pm
content: That video is mine, hopefully I didn't make too many mistakes.It has been on continuously since that video and is currently stable at 10.000054V @ 23oC.Maybe you'll find something interesting in my thread:https://www.eevblog.com/forum/projects/fluke-731b-restoration-and-mods/I replaced all the carbon composite resistors with 50ppm metal film, they were all basically out of spec and their temp coefficient is horrible.low-ish temp coefficient metal film resistors are cheap nowdays, and every little bit helps I guess.