Restoring Fluke 731B - Page 1
Forum section: electronics / general

author: opa627bm
author_rank: Regular Contributor
timestamp: November 07, 2022, 06:15:56 am
content: Hi all,I recently got 4 Fluke 731B , 3 are in good shape but one is beating up pretty bad. 1 work right out of the box, I am going to replace the battery with 5 16V 15000uf cap with a 14.4V zener .one unit have the 10K Gain resistor going bad but the reference and other unit works.Here is my few questions:1. Do anyone here have a broken Fluke731B (dead reference) that is willing to sell so I can use the 10K 4.22K pair to restore my current one?2. I have seem some discussion regarding the replacement of the battery, can I putting the zener in parallel with the capacitor ? or I need to add a current limiting resistor in series, those are 16V caps, I am also trying out some 3V (5 in series) super caps. My guess is the capacitor voltage will be regulated by the zener diode, so if everything balanced out, it should only have the charge current passing it (25V / 510 Ohm), so a 500mV zener should be sufficient?Regards,Li

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 07, 2022, 04:48:23 pm
content: Doubt you'll find a parts unit but you never know. I'd just use some Vishay resistors, or wind my own from a spool of manganin wire. I think I used a 500 mW zener and no other limit. Probably only used 4-6 thousand uF. It's a good reference, but nowhere near a 732, so don't be too afraid of using non-exotic parts.

author: bdunham7
author_rank: Super Contributor
timestamp: November 07, 2022, 05:24:47 pm
content: I restored mine with batteries (and I'm going to have to do it again, this time with NiMH) because it is occasionally useful to operate the reference disconnected from mains power. If you always have it plugged in, then I don't think you need that much capacitance, 10mF total is probably plenty.I assume you are referring to R2 and R3. They aren't part of the ref-amp set and I think you can use off-the-shelf resistors. They are WW, so a low-ish tempco is probably implied. The 10K is stated to be 0.05%, but the 4.22k is a 5% part. I think precision metal foil would be good enough for either.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 07, 2022, 09:53:44 pm
content: AFAIK, you can't trickle NiMH indefinitely, though maybe you can float it at a suboptimal voltage. Every single piece of battery-backed test equipment I own has been a problem- 731s, 845 null meter, 419 null meters, Fluke 540b, Julie SCO-106 and HP204 sig gen. They were all a bad idea and getting rid of the batteries was the answer.

author: bdunham7
author_rank: Super Contributor
timestamp: November 07, 2022, 11:24:07 pm
content: Quote from: Conrad Hoffman on November 07, 2022, 09:53:44 pmYou can if you use a low enough current and limit the voltage. It helps to use normal, not LSD cells. Still, I understand the advantages of eliminating them--I've got some HP 403Bs that I'm trying to decide what to do with. It's just that it can be handy to run the 731B on batteries in some cases. I've discovered that they are somewhat susceptible to power line noise.

author: opa627bm
author_rank: Regular Contributor
timestamp: November 07, 2022, 11:45:49 pm
content: I think the problem with most of people is they take out the battery cell and run it on AC only, the problem is since there is no current passing in the charge circuit, it will power the entire reference module and bypass the internal regulator. I think the normal output from the internal regulator is 18V and charge circuit is ~20V, so the diode oring will prioritize the unregulated charge circuit. when you have a battery, it will clamp that node down to 14.4V (will still slowly charing it up) when it is charged beyond 18V, the internal regulator disables and the battery will self discharge untill it when back down then the internal regulator takes over.Overall, it is not a very good design but since most of unit I have is 10 years older than me, I will assume they did what they could back then.if you want to run on internal regulator only, just add a 14.4V zener with a small cap, this will disable the charge circuit .

author: bdunham7
author_rank: Super Contributor
timestamp: November 08, 2022, 12:10:07 am
content: I don't think it works that way with the 731B. I've posted a schematic and you can see that the battery is isolated by diodes from every part of the regulator circuit if there is line power. Without line power, the battery feeds the circuit directly through CR8. The battery does supply a lower voltage, but it's presence or absence does not alter the voltage when line-powered. As long as C5 isn't dried out, the 731B should work fine with the battery absent.

author: opa627bm
author_rank: Regular Contributor
timestamp: November 08, 2022, 03:43:12 am
content: Quote from: bdunham7 on November 08, 2022, 12:10:07 amWhat I have seen is the output voltage from the regulator is about 17.5V, When battery is connected, the charge terminal will be clamped around 14~15V. If battery is absent, no current is flowing through 510ohm resistor, to charge the battery, and normal current draw from the reference module is about 5mA, so it will still over power the internal regulator (about 20V output) (Vdrop on the 510 ohm is about 2.5V and there are 2 diode drops, the transformer is about 24V )Not sure if anything is out of spec on my unit.

author: bdunham7
author_rank: Super Contributor
timestamp: November 08, 2022, 05:06:45 am
content: I tested mine and found results similar to yours--with the battery connected there was ~27V across C5 and ~17V output from the regulator. On battery there was ~14V at the output (CR8/CR9 junction) and on line power with no battery present, there was ~30V across C5 and 21.4V output from the regulator. I measured 18.06V across CR11 under both line-power conditions. However, the 10V output of the 731B did not change under any of those conditions, so I assume it is intended to operate properly even with significant variance in the input voltage.I've never paid any attention to the power supply of the 731B, although I've heard of people improving them. I think the mystery player here is the battery gauge, but I haven't quite figured out how it is involved.