Fluke 732a repair - Page 1
Forum section: electronics / repair

author: nikonoid
author_rank: Regular Contributor
timestamp: October 22, 2017, 05:12:39 am
content: I had been repairing Fluke 732a Voltage Reference that I purchased in "for parts or repair" state. This is a bit of a worklog/teardown, covering repair and 12v battery upgrade, for anyone who is interested. Click photos for high resolution versions (hosted by TiN).It appears that sometime in the past the batteries in this Fluke 732a failed, leaked and caused some cascading failures. The unit came to me without batteries and with some parts of battery compartment missing.After initial warm up:10V read 10.00008 (8ppm out)1.018V read 1.017998 (2ppm out)1V read 1.000111 (111ppm out!)1V reading was off beyond what could be adjusted. Resistors responsible for division of 10V reference down to 1V are located inside of the oven. I attempted to partially disassemble the oven to look for 1V problem. I wanted to see if I can fix it, but half way though the disassembly things got complicated enough that I was not sure I would be able to assemble everything back the way it was. There is a lot is loose individual wires on the inside. The foam over last 30 years hardened and chances of me being able to put loose wires back into hardened foam were about zero.Here are some teardown photos. Top and bottom covers are off:Top shield is off, revealing foam. The top piece of foam got fused with side foam making it very difficult to remove it without crashing. I used Stanley Blade to separate foam. Same blade was used (whole width of it) as pry bar of sorts to lift the top foam plank:Inside you can see another layer of foam along with special card with links to adjust 10V reference in 5ppm steps. I switched the link to lower reading by 5ppm:Underneath second foam plank there was an oven assembly:I removed its cover and pulled it out a bit:At this moment I decided to stop and concentrate on the rest of the unit. After all if resistor/resistors responsible for 1V output had drifted by 111ppm in 30 years, there is no guarantee they stop drifting once I add additional adjustment network trying to bring 1V in range. By the way, it was actually very difficult to assemble the unit back from this point.This is regulator PCB Assembly:Note the charring underneath of two large resistors. I guess it was the failing battery that caused them to overheat. It appeared that resistors were replaced. I made sure they are working and also desoldered a small polystyrene cap next to them to check it. Cap was Ok. I cleaned up the worst of charring from the board. I could not remove all of the compromised material without totally destroying the PCB. The large electrolytic cap appeared physically lumpy and not even cylindrical. It was of cause replaced. One of two transistors on the bottom of this board had hardware missing that was designed to tie it to bottom shield for heat dissipation. I bough and replaced nylon non-conductive washers and corresponding screws. I kept old thermal sill pads. I hope that is OK.Battery compartment was missing one metal panel on the side. Thankfully I have a small milling machine and I was able to make an aluminum cross plank that is now holding batteries in place and together:also decided to convert the unit to using two 12V batteries instead of four 6V batteries. I ordered two of: "EnerSys Genuine NP5-12 Genesis NP Series 12V 5Ah" from Amazon. They seem like the same brand and kind that Fluke is using, just much cheaper from Amazon. I fully charged and load tested both batteries @ 250mA and one was 2% less than nominal capacity and another 5% more. Close enough for me. Switching wires and connectors for using 2 batteries instead of four was completely trivial. Now terminal location for new batteries came out to be different than existing holes. Milling machine to the rescue again. Since I made new holes a bit smaller than old, I used kapton tape around them to prevent me from accidentally shoring the batteries while installing/removing them. See yellow insulation in photo:Two 12V battaries are a bit smaller than four 6V, so I had to create few spacers to keep batteries from rattling.Battery compartment includes a small lamp that apparently is not used as a lamp, but as some sort of regulator.The PCB next to the lamp showed discoloration, but lamp itself appeared ok when I removed and checked it:Power supply circuit appeared fine. I just changed two electrolytic capacitors and replaced Schaffner time delay bomb of an EMI filter, and replaced it by Delta 06GEEG3E. I hope it is sufficient:Now Fluke 732a had been powered and running for few weeks. It would not stabilize for first two weeks, but now looks stable. I will be monitoring it for months to come and still want to do a study of thermal behavior of internal boards that appeared charred to make sure the problem is not continuing.Is here is a member in a derivable distance from NJ with a calibrated Fluke 732 or something similar? I would really want to synchronize my 732 to theirs. Please let me know by a PM or something.Questions, comments, and suggestions are welcome.

author: dj831
author_rank: Regular Contributor
timestamp: December 08, 2017, 03:23:16 pm
content: Hi Nikonoid, great pictures !I've just started yesterday to revive a 732A purchased that summer (it remained unpowered for at least 10 years, but the lab who sold it to me is in Italy, hence few chances that it suffered frost). I've taken some pictures that I may share. Mine is dated 1983 and I have few differences with yours:Main and stangest difference is regulator PCB: I do not have Q14, and R24 is replaced by a short, meaning that heater is switched on by a 3906 (Q13) That would not work if the heater conforms to schematics (i.e. 44). My heater measures ... 1.12k. Strange, but explains why power transistor is not fitted ...All tantalums are orange. My board is Rev.FOn power supply, your diodes are bigger than mines (1N4817)I'm waiting for parts ordered from RS. Things that will be replaced:Batteries, of course. Mine are dated 1987. I will use 4x LC-R064R5P from Panasonic. They have a life expectancy of 6...9 years when used at trickle (that seems a bit optimistic...).Chemical capacitors. I will use Kemet PEG124, rated for 27500 hours @ 105°C. Both 330µF/80V will be replaced by 470µF/100V, and the 220µF/100V will be replaced by same value.Line filter. Perfectly agree with you, these are time bombs- I had once smoke from my PM3350 oscilloscope due to filter.Question: did you performed modifications of your 732A, according to manual errata, i.e. mainly addition of R19 (15k) on power supply and +58°C temperature protection on reference case, since you dismantled upper foam?I did not take the risk to take that foam apart on mineWhat I didn't do too is to replace tantalum capacitors by OSCONs - let's hope that these will not short with time. I plan perhaps to add R19.Any suggestions (especially on oven) are very welcome!

author: HighVoltage
author_rank: Super Contributor
timestamp: December 08, 2017, 04:22:25 pm
content: Nice tear down / repair, thanks for sharing.Does the resistor divider for the 1V may have an oven that is not working right.May be it will recover over time / change back in the right direction?

author: Bill158
author_rank: Regular Contributor
timestamp: December 08, 2017, 05:57:14 pm
content: Quote from: HighVoltage on December 08, 2017, 04:22:25 pmThe dividers for the 1.018V and 1V outputs are on the same PC Board with the reference IC and all of temperature sensitive components of the 732A including the adjustment pots. This PC Board is then inside of the oven assembly.nikonid:What is the resistance of the Thermistor measured at the Oven Temp Thermistor terminals? It should be between around 3,500 to 5,000 ohms. This will tell you if the oven is operating at 45 degrees C.I have several 732As that the 1V and or 1.018V outputs are beyond the adjustment range of the pots. Since these are card wound resistors, low values and hand selected you may be hard put to adjust these. You will have to get inside the oven assembly, which you have already found out is a difficult task to begin with, and then wait until the oven warms up again to find out if you made a correct guess on how much change you needed to make. This whole process could take many tries before you get it correct. If you don't need the 1V output then don't worry about it. Just the voice of experience speaking.BillBill

author: nikonoid
author_rank: Regular Contributor
timestamp: December 12, 2017, 05:37:10 am
content: My thermistor is at 3.656k. It is fairly stable. I did a logging of it before and from my memory it corresponded to about 0.02C fluctuation.My thinking was that since 1V drifted so much but others remained relatively stable it must to do something with one of resistors. I saw that on multiple Dial-a-Source standards I previously repaired, when some precision resistors drifted too much over year. Trimming resistor that is already drifting might be pointless as it will just keep drifting. If I was to replace it, than new resistor will be subject to ageing.Combined with difficulty of opening the oven I decided to leave things as is.HighVoltage: did you possibly mean that there are several heater resistors in the oven and one that is the closest to 1V divider is not performing as should?Is there some specific reason to chance 330µF capacitors to 470µF? My 732a still have some potential noise of +/-1uV. Maybe additional filtering can help that.I did not know about manual errata and suggested additional modifications. Do you have references to where to get them?I also did not adjust the regulator, as I did not have a variac. I have one now, so I may do that and other small changes before getting 732a calibrated.I have a cal lab 10 minutes from me, but they are horrible. Can someone recommend a reputable lab in a derivable vicinity of New Jersey?

author: Bill158
author_rank: Regular Contributor
timestamp: December 12, 2017, 03:25:26 pm
content: Quote from: nikonoid on December 12, 2017, 05:37:10 amnikoniodAttached is a pdf with all of the mods that I have ever seen from FLUKE for the 732A. It discusses repairs to the 1V and 1.018V outputs as well as others that may already be in your 732A. I got this from KO4BBwww.ko4bb.com.I have adopted a different way of setting R-10 which controls the voltage at which the battery charging circuit switches from constant current to constant voltage. I connect a DVM across the battery pack at the rear connector (J10) where you would normally have an external battery to extend battery operation time for shipping. I then unplug the 732A, wait for a few seconds and then plug it back in. The "Btry CHG" indicator should come on and then observe the trip point where it changes from constant I to constant V on the DVM and the voltage begins to drop. I then adjust R10 until I get a trip point of around 29 volts. Don't adjust this if it is close to 29 V +/- 1V. If the constant I trip point is too high it will fry the batteries because it will never trip! Then I wait for a few hours or more and adjust R20 for a voltage of 27.2 volts which is the constant V set point to keep the batteries charged. If you set the constant V too high this will fry the batteries also. I still don't understand the adjustment procedure that FLUKE uses. It is just too complicated.Bill

author: perdrix
author_rank: Frequent Contributor
timestamp: January 17, 2018, 06:53:07 pm
content: Mine is also missing Q14 with a short for R24 so Q13 is also driving the heater (about 1.1k) on its own. So it looks like this was modified by Fluke at some point, however I can't find any manuals that refer to this change. Quite the shame really.The thermistor on mine as reading 4.417kOhms today is that within expected margins?CheersDave

author: Bill158
author_rank: Regular Contributor
timestamp: January 18, 2018, 04:01:21 pm
content: Quote from: perdrix on January 17, 2018, 06:53:07 pmDave:I have seen some that don't have Q14 installed. I have not seen anything about this change or if the S/N range is consistent for this change. Because assemblies may have been swapped between units it is almost impossible to determine this. Fluke has a habit of making changes without documenting them except for internal use and they won't tell you when you call. Frustrating to say the least.My thermistors run between 3.4k and 4.5k over my various 732A instruments. Fluke used to say it should be 4k to 5k but then later manuals say 3k to 5k so go figure what is going on. The important thing is that the reading is stable month to month. This tells you that the internal temperature is stable. This is for the 732A. The 732B is different and no stability is specified or values stated. My 732B runs from about 38.9k to 39.2k depending upon room temperature. Because there is no spec on this I just don't worry about the readings. About the only thing you get from this is that the oven is working.Bill

author: quarks
author_rank: Frequent Contributor
timestamp: January 20, 2018, 11:00:57 am
content: bookmarkthanks a lot for sharing

author: perdrix
author_rank: Frequent Contributor
timestamp: January 21, 2018, 01:14:31 am
content: The missing Q14 mystery is explained if you look in the later version of the 732A manual which can be downloaded from ko4bb.com.The one you want is called "Fluke 732A_AN Instruction Manual".The later 732A builds had an extra board called the A8 preheater board which is inside the oven area. This has the final drive transistors (two of them) for the heater elements.HtHDave Partridge

author: perdrix
author_rank: Frequent Contributor
timestamp: January 21, 2018, 05:27:20 pm
content: Should anyone be interested I have redrafted the schematic for the Reference Circuit on the A5 board using "normal conventions" so it should be easier to understand.I've included the components that are hosted on the A4 board for clarity.I have attached a PDF file of the circuit.I hope this will be helpful.CheersDave Partridge

author: dj831
author_rank: Regular Contributor
timestamp: January 23, 2018, 10:38:41 am
content: Many thanks Dave four your info on that missing Q14Since I had not the correct SM, it was a mystery - now solved. Nice to see I grab later revisionI still have not powered mine, but at least, sensitive components were replaced (capacitors and Schaefnner filter, plus batteries two times - I've been caught with placement of leads with Panasonic batteries). Just need now to set voltage battery charger, and repair my Solartron 7071 and 7075 to track voltage drifts (purchased on eBay a while now) ...

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 12, 2019, 08:45:00 pm
content: My standard has a similar problem. The 1B output has the largest deviation. About 40 ppm. Drift yet I do not know.It is very interesting to see what is inside the oven. Maybe someone has photos?And in my device I do not really understand where the output marked in the picture should be connected. There is a soldering spot and wire residues on this connector. BUT that's where they should go, I do not quite understandTell me who knows?

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 12, 2019, 10:20:30 pm
content: I was impatient and looked myselfBoth of these large connectors go to the case ... and the soldering that is visible comes from the wire that simply connected the two connectors.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 23, 2019, 03:21:29 pm
content: Replacing 6V batteries with 12V.1. Cut out the holes.2. We remove excess wires.3. Replace the terminals from F1 to F2.4. Install a simple analog charger balancer.5. Replace the standard connector with mini XLR6. Connect the midpoint of the batteries to the connector through the resistor.7. It looks like you need to adjust the charge voltage. But I have not had time to figure it out yet.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 25, 2019, 08:10:03 pm
content: My suspicions came true.Battery voltage has been overestimated.I had to configureA3.R20: final voltage on the battery 27V (was 28.2)A3.R10: disconnect voltage of the current charge 27.9V (was 31.9)If turn off the AC power, then I observe a 257 mA battery discharge current.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: September 25, 2019, 09:55:36 pm
content: this will help You:https://www.eevblog.com/forum/metrology/fluke-732a-fluke-752a/msg1278667/#msg1278667

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 26, 2019, 08:33:58 am
content: Quote from: doktor pyta on September 25, 2019, 09:55:36 pmThank!!I did just that. I tuned on a working battery by turning the power cord on and off. I wish I had seen this before.The only thing that I could not get is a flashing indicator. If you reduce the voltage of the battery (I replaced the battery with a power source), then at approximately 22-23 V, oven turns off while the zener remains powered up. Consumption drops to 13 mA. The In Cal indicator goes off. But I do not see a blink.Under what conditions does blinking turn on? And what part of the circuit is responsible for this?

author: jfphp
author_rank: Regular Contributor
timestamp: September 26, 2019, 04:16:39 pm
content: The dozen of 732A I have seen had trouble either with the 1,018V or the 1V (not both) and a fully functional one was rare (3 or 4). It is a drift of the divider and Fluke sold a --costly -- kit to repair it (in the yellow pages of the manual)

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 26, 2019, 04:24:47 pm
content: Quote from: jfphp on September 26, 2019, 04:16:39 pmWhat was the behavior of these outputs? Did they continue to drift or did they stop?Do you have this manual?

author: jfphp
author_rank: Regular Contributor
timestamp: September 26, 2019, 06:17:29 pm
content: http://ftb.ko4bb.com/manuals/86.238.172.165/Fluke_732A_manual_Errata_Issue_3_1985_18pp.pdfp.10 to 13drift continues

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 26, 2019, 06:50:15 pm
content: Quote from: jfphp on September 26, 2019, 06:17:29 pmThank.I think now it’s not realistic to buy a set of resistors

author: Kleinstein
author_rank: Super Contributor
timestamp: September 26, 2019, 08:08:39 pm
content: Most modern high-end DMMs can use the 10 V reference. So the importance of the 1 V and 1.018 V ranges is not that high anymore.The 2 divider for the 1 V and 1.018 V ranges are according to the instructions independent. So if only one of them is not working, one still has the other low voltage range.Changing the resistors is not an easy thing and may disturb the rest if not very carefull. So unless there is really a need for both ranges, I would not bother fixing a single broken 1 V or 1.018 v range. I could consider the undisturbed drift history more valuable than having the 1.018 V range in specs.

author: jfphp
author_rank: Regular Contributor
timestamp: September 26, 2019, 09:21:03 pm
content: I fully agree with your opinion.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 26, 2019, 10:52:36 pm
content: Quote from: Kleinstein on September 26, 2019, 08:08:39 pmYes, I will.Can you tell me more about the blinking LED at low power?

author: Bill158
author_rank: Regular Contributor
timestamp: September 27, 2019, 04:16:02 pm
content: Can you tell me more about the blinking LED at low power?I have 6 of these F732A units and I have never see the "IN CAL" led blink. There is no circuitry in there to allow this to happen. The "IN CAL" led is latched on by shorting the LO to the RESET point. If the battery voltage drops below around 22 volts then the latch shuts off and the led cannot be turned on until you do the RESET. I set my "float" voltage across the batteries to be about 27.3 volts, as best as I can. BUT you have to wait for at least 24 hours after installing new batteries so that they can " top out" and get their full charge. I also set my trip point from constant current (charging) to constant voltage (floating) of the charging circuit to be 29.2 volts. This adjustment is very difficult to obtain accurately. So I just try to get it as close as possible, usually between 28.7 to 29.5 volts. But again this is with a battery pack that has been at constant voltage float for at least 24 hours. If you set the trip point to 31 volts there is a good chance that it will NEVER trip over from constant current to constant voltage and you will FRY the new batteries! DON'T USE THE PROCEDURE IN THE MANUAL TO SET THE TRIP POINT! It doesn't work. I just watch a DMM closely, pull the 732A AC power plug, plug it in again and try to capture the trip point visually, but the DMM I use has a MAX statistics math function so I just set the PLCs to 1 and then let the math do the job for me. The charging circuit will NOT go into constant current if you have pulled the battery pack and and then reinserted it. You then have to pull the AC power plug and then insert it to get into constant current mode.I have one 732A where the 1.018 or 1 volt output is not correctable because the adjustment is out of range. I just don't worry about this because the 10 volt output is the critical one and that is the one you want to use. Going into the oven assembly is just NOT recommended unless you absolutely have to fix something! You are dealing with ribbon cables which can break off from the PC Board easily. I would sure stay away from that. I have gone into one to fix a problem, but I was VERY CAREFUL about what I was doing. If you need either of the lower voltage outputs use a Kelvin-Varley divider, F720A or equivalent, to get those voltages.Bill

author: TheSteve
author_rank: Unknown
timestamp: September 27, 2019, 04:38:17 pm
content: The 732B will flash the "low bat" LED, I've never seen any flashing from a 732A.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 28, 2019, 09:46:35 am
content: Quote from: Bill158 on September 27, 2019, 04:16:02 pmThank you for confirming. I seem to have mixed up with 732B

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 28, 2019, 10:02:56 am
content: But no .... here is a quote from the instructions:

author: Bill158
author_rank: Regular Contributor
timestamp: September 28, 2019, 08:50:41 pm
content: Quote from: MegaVolt on September 28, 2019, 10:02:56 amMy bad for thinking we were discussing the "IN CAL" LED. Now I see that we are discussing the "BTRY CHG" LED.How interesting. This paragraph comes from the "Acceptance Test" section 4-30 after doing repairs. Since all of my 732As were used and many did not even have batteries installed or completely dead with no chance of being charged I never read that section. I always just began by installing new batteries before even trying to power up the 732A. Once new batteries are installed the "BTRY CHG" LED will come on, after the battery switch is set to "ON" and then the AC power is removed and then connected. Then the CI/CV circuit will be in the CI state until the battery voltage reaches 29 volts (if the circuit was correctly adjusted in the past) and then go into the CV state. This paragraph (4-30 1c) makes no sense because once the CI state starts the batteries will be charging for a while IF they were low in the first place. About the only time I can imagine where this would happen is if the batteries have gone bad and won't hold a charge because one cell or more cells have a high impedance. Then the voltage across the batteries would go up to the trip point rapidly where the supply would switch over to CV. But then the CV mode would be enough to keep the pack at 27.2 volts. This doesn't explain why the LED would blink. But I have also seen where the battery pack cannot ever reach the CI to CV switch voltage, because one or more cells have an internal short, and it stays in the CI mode and then fries the remaining batteries, the whole pack becomes too hot to touch and batteries begin to swell and then burst and spill their contents all over the place. I have had this happen more than once with batteries that had passed the monthly check (paragraph 2-47). Bottom line here is why did FLUKE think the blinking could happen? I can't imagine a situation that would cause this in a AC power state. I am wide open to an explanation.Bill

author: MegaVolt
author_rank: Frequent Contributor
timestamp: October 16, 2019, 10:06:05 pm
content: I watched the standby voltage on the battery and found a direct temperature dependence.Although the theory claims that the dependence should be the opposite.What am I wrong about?I looked at the circuit and it was designed just like that. Did older batteries require a direct dependency? And the new inverse dependency?Can this be easily fixed?

author: czgut
author_rank: Contributor
timestamp: November 19, 2019, 04:48:10 am
content: In My 732A 1 V also drifted +25 ppm out of nominal value. In my case it seems to drift continuously about +6ppm/8years.From my measurements it seems, that resistance of 9kOhm Resistor in 1V divider (9kOm + 1kOm) is drifting about -1 ppm/y . 1 kOhm seems to be stable to <2ppm/8years.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: April 28, 2022, 05:58:46 pm
content: Excuse me for hijacking this dated thread.Given that the Fluke 732A and 752A share the same enclosure and feet, I decided to replicate the voltage standard feet so that I could produce them for my voltage divider.Attached is a CAD file for the purpose of 3D printing feet for either unit. They have been printed with a Figure 4 by 3D Systems and tested. Note that the foot replicate is not an exact match because I got tired of dealing with the draft angles on every conceivable surface. They will be suitable replacements regardless. Draft angles are only necessary for plastic injection moulding and serve no purpose for additive manufacturing, to which more members will have access.I hope this is of use to our community. Please feel free to contact me if there are changes and suggestions to the existing design that you would like to share.https://xdevs.com/leigh/fluke_7X2A_-_parts/MP18/Enjoy.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: June 04, 2023, 07:04:56 pm
content: Does anyone know what the oven insulation foam in made of? I would like to try and replace it in a damaged enclosure. The material is extremely brittle and falling apart.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: June 04, 2023, 09:49:59 pm
content: It looks like two compound rigid polyurethane foam.I once used this type for thermal insulation: 'EKO PRODUR PM 4032'.

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: September 15, 2023, 11:53:57 pm
content: Attached is a reverse engineering STP file of an insulated anchor found throughout the Fluke 732A and perhaps other Fluke equipment. For example, it isolates parts of the battery module from the rest of the chassis (pictures for reference). I noticed that a lot of them break due to aging! Note that I have not yet printed these parts. I may share additional details depending on the results.Please let me know if any updates are required so I can upload the next revision here. Thank you.Enjoy!

author: leighcorrigall
author_rank: Frequent Contributor
timestamp: September 18, 2023, 07:38:36 pm
content: The 732A foam can degrade if the environment is harsh. One of them in my possession was exposed to high humidity for a long time during operation. Not a good combination.I have reverse-engineered the foam for the ovenized assembly. In my case, I had IKO Enerfoil Sheathing (polyisocyanurate foam) cut with a bandsaw, according to the attached draft. I did not go with the original shelves to assemble the walls. Instead, I applied a simple approach to installing the front and back walls and sides inside the case quickly. The bottom and top are the final parts to assemble. If one wants to, one can take my draft dimensions and redesign how the walls interface.