Fluke 731B Voltage Standard Restoration and Mods? - Page 1
Forum section: electronics / projects

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 05, 2020, 06:30:10 am
content: So, to add to all my other projects, I just picked up a Fluke 731B voltage standard..This looks to be an easy one though as at a cursory glance, it just has the usual failed batteries (But they have barely leaked at all, so no real cleanup required) and on hooking it up to my 34461A multimeter, all the outputs are looking relatively within the ballpark of where they need to be (a tiiny bit out, but nothing that a session of holding my tongue at the right position can't fix).So the reason for this thread is, I'm looking for advice on any mods that I should do to this thing while I have it on my bench.I have access to Ni-Cd and Ni-Mh batteries at Akihabara, so there's no problems there. (Or maybe I should upgrade to 18650 lithium cells? Thoughts?)But I have read that the voltage regulation in this unit is a little archaic by modern standards, what would be the possibility and benefit to upgrading the power supply for better regulation and less ripple etc?Maybe even sticking a filtered IEC socket in place of the standard one seeing as I have a bunch sitting here on the shelf? (questionable benefit?)

author: Kleinstein
author_rank: Super Contributor
timestamp: January 05, 2020, 09:42:40 am
content: As long as one can still get NiCd batteries, I would not convert to Li as this could be a mayor limit to transportation. So if at all it would be more like a removable option for in Lab use.With NiMH batteries it may be a good idea to upgrade the charging part, as they are more sensitive to overcharging. Even NiCd cells could get better life with less stress. Modern cells tend to have higher capacity, but may not be as robust as the old ones.

author: Gyro
author_rank: Super Contributor
timestamp: January 05, 2020, 09:56:00 am
content: SLAs? They are happiest being float charged.

author: beanflying
author_rank: Super Contributor
timestamp: January 05, 2020, 10:07:27 am
content: KISS was the answer I came up with a NiCad powered HP 5000 series counter. I use a heap of LiPos in Planes and Helis but I went with NiMh (D cells). It was a size for size replacement and the NiMh cells handle being floated and from time to time I drop the power to keep them getting some actual use.I went with the stock charge circuit initially but I am going to look at it again with a modern IC solution but as it is working well it is down the list.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 05, 2020, 10:24:04 am
content: I stood in front of the same challenge on all my 731B references and decided to stay with Ni-Cd packs, after I found some very nice packs.https://www.eevblog.com/forum/repair/fluke-731b-dc-reference-standard-repair/msg1073583/#msg1073583BTW, over the years I have owned 6 of these references and only two are without any significant drift.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 05, 2020, 10:44:26 am
content: Yeah, the easy thing is to just stick Ni-Cd back in there seeing as I can buy them off the shelf, there is a good amount of space to increase the capacity though, so that's nice.Looking at the schematic, what would be the best way to make the charging nicer? It looks pretty simple as it is now, so maybe I can add a simple but better charge controller?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 05, 2020, 03:14:08 pm
content: My "volt" consists of three of 'em. I've seen very little change on power up/down so I don't worry much about battery power. NiCads always eventually fail and no other battery technology tolerates trickle charging well. You still need something for regulation so I replace the batteries with a large aluminum electrolytic capacitor (probably a few smaller ones to fit in the space), with a zener across them and a limit resistor. That mimics a battery. I wouldn't change anything else. I did optimize the tempco of one unit to near zero near room temperature by changing the reference current and probably a divider resistor. That unit has proven the best of the three. Mine have been powered up for about 15 years now and have only moved 5-10 ppm over that time.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 07, 2020, 03:03:50 pm
content: That tempco optimization sounds interesting, how did you do that? (I probably won't do it, but I'm interested to see what's involved).As for today's work, I built a new battery pack.I went for 12x 2000mAh AA Ni-Mh cells, which should work fine as the charge rate from the unit should be within the trickle charge ratings of these cells. (Datasheet included below).The connectors I used (Mine were missing or corroded) are Hirose HIF3 series, HIF3-2226SC, which are a very close match to the ones in the unit.https://www.digikey.com/product-detail/en/hirose-electric-co-ltd/HIF3-2226SC/H12171-ND/151052The battery label I made by printing on sticker sheet with my laser printer, then laminating it with packing tape.It works really well and I even use this technique for front panels and stuff. If you need to make a label that is bigger than packing tape, you can use the clear adhesive plastic used to cover books.I also picked up some metal film resistors, and a new electo and tant cap, just in case (Passives are cheap). I'll give stuff a test tomorrow to see what actually needs to be replaced.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 07, 2020, 03:23:01 pm
content: Your 12x 2000mAh AA Ni-Mh cells look nice.It will be interesting if they will work well in the 731B.Please let us know your test results.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 08, 2020, 03:39:58 am
content: Since you're going to be trickle charging forever (or is it a float?), I'd keep it under 10-20 ma, based on a bunch of stuff I've read, maybe less. It might be a good idea to cycle the batteries on a charger/discharger once a year or so.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 08, 2020, 04:23:31 am
content: Yeah, I'll measure the charge current once it's all back in one piece and adjust as needed.This morning I had a poke at the resistors. I found most of the carbon composite resistors were bad as expected. One on the reference regulator board was at exactly 10% out of spec for a 5% part, and the 8.1K on the power supply board was reading 9.2K... Two others were reading right at the limits of their tolerance, so the lot were replaced with 1%, 50ppm metal film resistors.I checked the rest of the resistors (Except the hermetically sealed and precision ones) and they all measure spot on.Also, the capacitor on the PSU board was reading bad on my LCR meter so it was replaced too, better safe than sorry.I currently have the battery pack on charge so tonight I'll reassemble the unit and fire it up and let it settle for a couple days then start tweaking the adjustment pots.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 08, 2020, 09:05:43 am
content: For the battery pack, especially with NiMh I would consider an extra switch, so one can reduce the charging current from normal to a low current to just stop self discharge. So if one used on battery for a long time reduce the current.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 09, 2020, 02:47:37 pm
content: I'll have a think about adding a switch, although by the time they need replacement it'll probably be getting lipo's or something anyway.So I ran it for a day to let it settle then hooked it to my 34461A multimeter for a day with a set of Pomona low EMF leads that I added a shield to, to see how things look.I can get the voltages pretty much spot on to the extent that my 6.5 digit multimeter will show, except the dial is a little out. It seems it's a little lower than what is shown on the counter. I'm hoping I can tweak the position of the counter dial on the shaft of the pot to 're-zero' the mechanical stops so they are properly matched and then have it come into spec. As it is now, I don't quite have enough range on the cal pot to pull it into line.Anyway, for now, a pic of the histogram after a day of running on the 10V output in a non-airconditioned room. I just ran through the calibration again and it seems I probably don't have to adjust the knob etc. It might have come good.

author: HighVoltage
author_rank: Super Contributor
timestamp: January 09, 2020, 11:23:38 pm
content: Nice, that is a fist good confirmation that your unit is stable in the short term.I had some drifting much more !

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 10, 2020, 01:26:50 pm
content: Alrighty, I think I got me a winner winner chicken dinner.I had it running all day on the 1.018v range, check out the results after 100,000 samples in my non temperature controlled room.Doesn't look too bad to me.I'll do the same with the other ranges to make sure it's as spot on as I can get it and I think I might have this unit sorted out. More screenshots to come!

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 10, 2020, 09:06:48 pm
content: When that's done, put it in a cardboard box or picnic cooler with a small light bulb and see how far it moves with temperature. Might be great, might be awful, but then you'll know.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 11, 2020, 03:41:22 pm
content: I'll give that a go, would be interesting to try to see what happens at different temperatures.Below are screenshots for each range setting (Some are reposts of previous images).They all look pretty good except some weirdness on the 1V range. Anyone got any ideas what went there? It was done in a room between around 22.5 - 23.5 degrees, so it shouldn't be too much of an issue with temperature drift.I noticed a handwritten note on the schematic saying that if there is "T.C problems only on 1v range, change R20 - R23", which is listed as a resistor set, part number 391417. I'm hoping they aren't the problem...Let me know if you have any ideas.....

author: iMo
author_rank: Super Contributor
timestamp: January 11, 2020, 05:29:02 pm
content: Off the 1V histogram it seems it jumps ~2.5uV between two values almost 1:1. Bad contact?

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 11, 2020, 05:55:46 pm
content: That's what I am thinking.Maybe I bumped it and the leads got wiggled a bit. I have applied deoxit and cleaned everything up, reseated the cables and now I'm running another test overnight with the aircon set to 23 degrees as specified in the manual.If it comes good, I think maybe the leads weren't pushed all the way in and got bumped.

author: iMo
author_rank: Super Contributor
timestamp: January 11, 2020, 06:01:38 pm
content: Trimmers (!) and sockets and connectors are not good for a "metrological" gadget. Too many there in the box, though..

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: January 11, 2020, 07:24:46 pm
content: My Fluke 731B came from the Washington Naval Yard, Washington, D.C., Eastern Standards Laboratory, vintage 1976. As received, the battery pack had been removed, the unit operated without batteries for some years as I had not opened the unit. A few years ago I opened the unit, with the exception of the batteries, it was in excellent condition. I replaced the battery pack with NiMh, 3.3AH batteries, made a slight change in the series resistor charge circuit and also made some minor changes to the op amp circuit to reduce the noise a little bit at the output. The noise specification in the manual appears to be quite conservative.It is important to note that the battery circuit aids in reducing noise in the power supply and voltage outputs. Without batteries, significantly more noise is passed on to the voltage reference circuits. If you don’t require battery operation (output noise is slightly better on battery power), a zener diode of the correct voltage paralleled with an electrolytic of moderate size will produce similar results to the batteries on line operation.My unit has undergone several calibrations in the past few years to determine long term stability as my unit came without any cal data unfortunately. Initial calibrations were done at 6 month intervals and then at one year intervals. There has been very little drift over these measurements, the 1V output was adjusted once the last time, it was 100µV low, it appears that R19 may have developed a weak spot where the wiper was sitting, it was noted there was some slight variability at that setting (probably worn spot). It will be rechecked at the next calibration and replaced if needed.These are the last set of readings for last year’s calibration; note that the 1.018V and 1.019V outputs were not readjusted from the previous calibration.20C ==> 9.9999940 / 1.0000005 / 1.01806359 / 1.01910223 VDC23C ==> 10.0000065 / 1.0000027 / 1.01807099 / 1.01910386 VDCYou will note that there is a small variation in the 10V output with temperature of 1.25PPM equaling a TCV of 0.41667PPM/°C, not too shabby for an unregulated temperature device. Al voltages were stable over time while at calibration. It was also noted that noise in the readouts were limited to 2-4 counts.The 731B is also pretty much immune to being cycled on and off (for hours), it has repeatedly returned to the same value after being on for an hour or so, the cal lab did this at my request and those are the findings, any changes were too small to be significant, hidden in the noise.

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 12, 2020, 06:18:03 am
content: What were the changes you made in the opamp circuit?I have the results in on the 1V scale and it's looking good, so it seems I had a temporary PEBKAC error...Next step is to try at a higher and lower temperature in each range to see what the drift is like.

author: dietert1
author_rank: Super Contributor
timestamp: January 12, 2020, 09:24:08 am
content: A 1.25 ppm/K TC is low enough to be difficult to measure. From my own LTFLU DIY experiments i know that to get below 1 ppm/K one should put one of the devices into a thermal chamber and carefully measure the temperature of the other device. How big of a mod is it to mount a quality thermistor close to the reference inside the 731B? Then you can always subtract the TC effect in case it matters.Regards, Dieter

author: TERRA Operative
author_rank: Super Contributor
timestamp: February 08, 2020, 01:35:48 pm
content: It wouldn't be too hard to stick a thermistor in there somewhere. I have it working well enough for now, and I have other projects staring at me from the shelves in my room, so I'll call this one as done for now until I get back to it.Uploaded a video to youtube about it too.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 08, 2020, 04:54:08 pm
content: I've got two 731B and one 731A. I never realized that the design of the 731A is quite inferior. It has no voltage regulator and is just a single board with no floating shield. In spite of that, it was possible to optimize the tempco and the unit was my favorite of the three, save for a lot of ripple on the output. Surprisingly, that doesn't seem to bother most measurements and null setups. I'm talking a few hundred uV of ripple, even with a large cap and zener replacing the batteries. I'm presently installing a small circuit board where the batteries were, with an separate bridge, filter and LM317 regulator. Hopefully by getting all AC current loops out of common with all DC loops, the ripple can be reduced.Anyway, nice job on the 731B!