IS that you FLUKE ... IS THIS ME.... - Page 1
Forum section: electronics / general

author: opa627bm
author_rank: Regular Contributor
timestamp: September 17, 2022, 07:20:30 am
content: I bought a used Fluke 731A on ebay and found out the battery exploded at some point and the pervious owner was trying to fix it but accidentally dropped the solder at back side resistor.So I bought 2 7.2V battery pack, clean up the entire box and use the nail polish to fix the resistor.But I am getting very high drift at 10V when either powered by AC or battery.you can see one of the data log is the output and the other one is the reference. they both captured by 34470A.I know later 731B have fixed a lot of the problem but this performance is no way fluke can sell it as a product.Is that normal for the 731A owners?

author: alm
author_rank: Super Contributor
timestamp: September 17, 2022, 09:22:28 am
content: Can you also show temperature in the same plot? The change is in the order of 10 ppm, and I believe the 731A (and 731B) were not super low tempco. The good news is that if it's temperature coefficient, then that can be adjusted to some degree. Search the forum on posts on the 731B.I believe the main difference between the 731A and B are the resistors. There was also a change from the TI DH... refamp to the Motorola SZA263, but that occurred after the change to 731B. I have an early 731B in need of repair that still has the TI refamp.

author: HighVoltage
author_rank: Super Contributor
timestamp: September 17, 2022, 09:58:04 am
content: When working correctly, the Fluke 731B is more stable than the 34470A.The 34470A is not a metrology grade multimeter and therefore should not be used to gauge a reference voltage.

author: opa627bm
author_rank: Regular Contributor
timestamp: September 17, 2022, 10:28:20 am
content: Quote from: HighVoltage on September 17, 2022, 09:58:04 amCant afford any 8.5 right now.. the other 7.5 I have is a DMM7500...also , does Solartron Schlumberger 7061 count as a metrology grade DMM?

author: opa627bm
author_rank: Regular Contributor
timestamp: September 17, 2022, 10:36:12 am
content: Quote from: alm on September 17, 2022, 09:22:28 amI dont have the room temp but it is pretty cold today in bay area, the logging was started last night , I think the delta is about 15C the entire day..

author: HighVoltage
author_rank: Super Contributor
timestamp: September 17, 2022, 12:01:22 pm
content: Quote from: opa627bm on September 17, 2022, 10:28:20 amYou mean the Keithley DMM7510?Hook both up in parallel to the Fluke 731B and check the results.Most likely the DMM7510 will give you much better results, unless you have a problem with the 731B

author: alm
author_rank: Super Contributor
timestamp: September 17, 2022, 06:54:19 pm
content: Quote from: opa627bm on September 17, 2022, 10:28:20 amGenerally measuring the delta relative to another voltage reference is the easier and better route, but that requires at least two different references.Quote from: opa627bm on September 17, 2022, 10:36:12 amThe 731A temperature coefficient is specified as 0.5 ppm/°C between 20°C and 30°C, so that could explain up to 7.5 ppm fluctuation over the day. The 34470A is specified as 2 ppm/°C for 10 VDC. So the fluctuation you see can easily be the product of the combined temperature coefficient from the 731A and 34470A. Like I wrote, the 731A can be tuned for temperature coefficient by tuning the current through the reference, but you obviously need something more stable as a reference.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 18, 2022, 07:59:33 pm
content: The official volt of my Secret Underground Lair is maintained by three 731s, two "B" and one "A". I gave up on batteries long ago because they die every few years and the units don't actually change much when powered up and down. I leave them on all the time, but don't worry much if the power goes out now and then. I think I replaced the batteries with a large capacitor and zener diode to sort of emulate a battery. The "A" and "B" are quite different mechanically in terms of shielding and maybe some circuit differences, though I don't remember all the details. I did tweak the current setting resistor on one of the units and was able to improve the tempco quite a bit. Pretty sure that also required tweaking the divider to get the trim range back to where it should be. I tested it by putting it in a cardboard box with a small light bulb to heat it, measuring the output with a 6.5 digit meter (all I've got). I periodically intercompare the three units and they tend to be stable at the ppm level for many years at a time.

author: opa627bm
author_rank: Regular Contributor
timestamp: September 18, 2022, 10:25:53 pm
content: Hi Sir,What is the current driving value you have for the zener diode?During the tuning phase, do you use a trimpot to set it?Also I_ZENER is equal to the bias current from the starting resistor + the internal BJT collector current. which one dominates more?the resistor ladder going to the base of the Internal BJT in the LTFLU reference seems hard to replace with anything since they are fluke branded hand wound resistor, the only thing you can replace is the 10ohm resistors.pretty much all the units I see on internet are reading low (<10V) and mine cannot be adjusted to 10V. even when trim pot is maxed out (0 ohm).In the end I have to play with the LM301HA gain resistor setting to get it back to 10V.Regards,LiAlso, the original LM301HA was bad, and I have to replace it with a QC301HA from a strange company back in the 80s.

author: opa627bm
author_rank: Regular Contributor
timestamp: September 18, 2022, 10:28:23 pm
content: Also, since it is 20mA trickle charging, I tried to use a capacitor but instead of maintain at 14.4V as the standard battery pack, the voltage keeps going up untill it hit the rail lol.So I bought 2 7.2V packs from Amazon, seems the pack can supply the 731A for 5days + without AC.My next plan is for sure, try to tune the zener current, also 3D print some cases for the reference and OPAMP to see if I can improve stability a bit.

author: opa627bm
author_rank: Regular Contributor
timestamp: September 18, 2022, 10:29:27 pm
content: Quote from: alm on September 17, 2022, 06:54:19 pmThat is right, I have noticed that with 34470A and I have a HP DC Transfer standard, but it reads much stable so I think it has something to do with this 731A. This units has been through a lot, so I need to slowly bring it back to life...

author: opa627bm
author_rank: Regular Contributor
timestamp: September 18, 2022, 10:30:16 pm
content: Quote from: HighVoltage on September 17, 2022, 12:01:22 pmYes, I have it now hooked up to a 7510. 34470A was a bad choice,

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 19, 2022, 02:38:29 am
content: It's been years since I worked on mine and I can't remember much. I'll try to open one up in the next day or three and see what I did.

author: bdunham7
author_rank: Super Contributor
timestamp: September 19, 2022, 03:47:59 am
content: Quote from: opa627bm on September 17, 2022, 10:36:12 amYour room ambient (where the instruments are) is 15C? That is really huge--are they outdoors or in a garage? That just won't work well with metrology in general. The 731A/B are not ovenized, not even the references, so they work much better in a controlled environment.As for the DMM, I'd be pretty surprised if the 34470A were exhibiting that much tempco (its reference is heated) but since you have another meter to try give that a go.

author: opa627bm
author_rank: Regular Contributor
timestamp: September 19, 2022, 05:44:00 am
content: Quote from: bdunham7 on September 19, 2022, 03:47:59 amloool it is 15F not C... sorry about that.

author: bdunham7
author_rank: Super Contributor
timestamp: September 19, 2022, 01:04:11 pm
content: Quote from: opa627bm on September 19, 2022, 05:44:00 amOK, but that is still a pretty large swing. Look at it this way--the 731A is generally specified as +/-30ppm IIRC and the 34470A as +/-16ppm, so what you are seeing is probably not out of specification. I say probably because I haven't reviewed the details of the specifications.

author: alm
author_rank: Super Contributor
timestamp: September 19, 2022, 01:27:03 pm
content: I'd convert the temperature swing to degrees Celsius, then multiply that by the temperature coefficient of the 731A and 34470A, and calculate how much change in measured value is to be expected as a result of the temperature swing.The Fluke 731A stability isspecifiedas 10 ppm / month, but this number doesn't mean much without knowing any history. If you knew it was measured as X volt 2 months ago, then you could say the current value should be within X +/- (2 months * 10 ppm/month + 0.5ppm/°C * deltaT), with deltaT as the difference between current ambient temperature and calibration temperature, if the ambient temperature stays between 20°C and 30°C.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 19, 2022, 04:44:08 pm
content: I'd think you'd lose your mind trying to do any sort of precision metrology with a 15 degree swing, be it C or F.The 731 is an old design and I'd expect any of them to have settled down to a very low drift by now. Mine move maybe a ppm or two per year at most.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 21, 2022, 10:11:35 pm
content: I know my manual and notes for my mods is in the lab somewhere, but a fairly thorough search has failed to turn anything up. I'll keep looking.

author: opa627bm
author_rank: Regular Contributor
timestamp: September 27, 2022, 07:49:20 am
content: Hi All,I think I have fixed it.I have replaced the 2 gain resistors near the amplifier with just normal 0.1% 15ppm resistors ( I think the old ones are going out) and TC improves a lot.I also 3D printed a small shell just enough to cover the reference and gain amplifier. I believe that helps a lot too .I also switched to DMM7510 instead of 34470A and I am now getting <1ppm combined drift through out the day, my room temp probably shifts 4 ~ 5 degs during the day.I am getting those weird spiky and realized it is in sync with room light and outdoor AC units.Not sure how to fix that , the 731 was on battery so it is probably on 7510, also 34470A have the same issue (they were hooked up in parallel in high Z mode)Regards,Li

author: bdunham7
author_rank: Super Contributor
timestamp: September 27, 2022, 03:50:01 pm
content: Quote from: opa627bm on September 27, 2022, 07:49:20 amJust FWIW, I have found that large enough conducted EMI will cause my 731B to have a significant (gross, several percent) DC output error. I don't know if running the reference on battery completely resolves this since the meters are line powered.

author: opa627bm
author_rank: Regular Contributor
timestamp: September 27, 2022, 07:39:45 pm
content: Quote from: bdunham7 on September 27, 2022, 03:50:01 pmHi bdunham7,The 731A was on battery (unplugged ) it will run at least 4 days without charging lolRegards,Li