Author Topic: Fluke 731B Repair/Modifications  (Read 9264 times)

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Offline dr.dieselTopic starter

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Fluke 731B Repair/Modifications
« on: November 30, 2016, 01:22:48 am »
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.


Offline ManateeMafia

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Re: Fluke 731B Repair/Modifications
« Reply #1 on: November 30, 2016, 01:58:24 am »
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.
 

Online edpalmer42

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Re: Fluke 731B Repair/Modifications
« Reply #2 on: November 30, 2016, 02:14:37 am »
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.

 

Offline TiN

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Re: Fluke 731B Repair/Modifications
« Reply #3 on: November 30, 2016, 04:27:08 am »
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.
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Online edpalmer42

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Re: Fluke 731B Repair/Modifications
« Reply #4 on: November 30, 2016, 04:51:31 am »
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.

For 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!  :)

 

Offline VintageNut

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Re: Fluke 731B Repair/Modifications
« Reply #5 on: November 30, 2016, 06:37:21 am »
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.
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Offline HighVoltage

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Re: Fluke 731B Repair/Modifications
« Reply #6 on: November 30, 2016, 10:54:38 am »
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.
 
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Offline dr.dieselTopic starter

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Re: Fluke 731B Repair/Modifications
« Reply #7 on: November 30, 2016, 01:38:37 pm »
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.

Yup, that seems reasonable to me as well.  Looks like mine (SN 4150027) dates from late 1986, component dates jive with this.




Offline HighVoltage

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Re: Fluke 731B Repair/Modifications
« Reply #8 on: November 30, 2016, 01:46:33 pm »
Your batteries look the same as in my units - horrible but to be fixed.

I have used these NiCd batteries
http://www.ebay.de/itm/142061266264

 
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Offline dr.dieselTopic starter

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Re: Fluke 731B Repair/Modifications
« Reply #9 on: November 30, 2016, 02:01:38 pm »
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.


Offline MosherIV

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Re: Fluke 731B Repair/Modifications
« Reply #10 on: November 30, 2016, 02:31:31 pm »
Quote
Any reason one couldn't swap to some low drain NiMH?
No, should be fine.

NiMH is most like NiCAD compared to other technologies.
 

Online Alex Nikitin

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Re: Fluke 731B Repair/Modifications
« Reply #11 on: November 30, 2016, 02:35:03 pm »
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) .

Cheers

Alex
 

Offline saturnin

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Re: Fluke 731B Repair/Modifications
« Reply #12 on: November 30, 2016, 04:01:03 pm »
Any reason one couldn't swap to some low drain NiMH?

I 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.

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) .

Cheers
Alex

Alex,

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.

 

Offline Kleinstein

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Re: Fluke 731B Repair/Modifications
« Reply #13 on: November 30, 2016, 04:08:23 pm »
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.
 

Offline MosherIV

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Re: Fluke 731B Repair/Modifications
« Reply #14 on: November 30, 2016, 04:33:00 pm »
Quote
Changing to NiMH makes charging a little difficult. They don't like the slow trickle charge often used with NiCd so well.
I 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/40

http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf
 

Offline saturnin

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Re: Fluke 731B Repair/Modifications
« Reply #15 on: November 30, 2016, 04:56:20 pm »
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.

Yes, 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
 

Online edpalmer42

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Re: Fluke 731B Repair/Modifications
« Reply #16 on: November 30, 2016, 05:08:58 pm »
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.

This 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.  :)

 

Online Alex Nikitin

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Re: Fluke 731B Repair/Modifications
« Reply #17 on: November 30, 2016, 09:44:18 pm »
Alex,

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.

If I remember correctly, I've just disconnected R30 and the wire to the meter (so it could be used for a temperature readout).

Cheers

Alex
 

Offline dr.dieselTopic starter

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Re: Fluke 731B Repair/Modifications
« Reply #18 on: December 03, 2016, 06:00:46 pm »
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.


Offline HighVoltage

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Re: Fluke 731B Repair/Modifications
« Reply #19 on: December 03, 2016, 07:08:20 pm »
Combined with some 2" heat shrink tubing = Fixed.
Looks 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.

this forum is like magic, there is actually no 731 left on ebay  :-DD

A 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.

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Offline dr.dieselTopic starter

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Re: Fluke 731B Repair/Modifications
« Reply #20 on: December 06, 2016, 01:23:31 pm »
Hmmmm, what could these be for:


Offline HighVoltage

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Re: Fluke 731B Repair/Modifications
« Reply #21 on: December 06, 2016, 01:44:56 pm »
sometimes, test and measurement equipment has a tendency to multiply.
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Offline dr.dieselTopic starter

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Re: Fluke 731B Repair/Modifications
« Reply #22 on: December 10, 2016, 04:28:33 pm »
Not near the shape as my first one, but it works!

This one has a SN of 450007, but a manufacture date of 1976.


Offline dr.dieselTopic starter

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Re: Fluke 731B Repair/Modifications
« Reply #23 on: September 25, 2017, 08:07:10 pm »
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, 9.9998050
SN = 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.

Offline HighVoltage

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Re: Fluke 731B Repair/Modifications
« Reply #24 on: September 25, 2017, 09:00:54 pm »
Very interesting, thanks for sharing your serial numbers !

Tomorrow, I have to look up the serial numbers on my four 731B
I was under the impression the black face was the newer model.
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