Fluke 87V displaying dashes only - Page 1
Forum section: electronics / repair

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 03, 2021, 05:01:30 pm
content: Hello everyone!I have recently gotten my hands on a broken Fluke 87V (rev.9)Unfortunately it's currently unusable, as selecting any position with the dial results in the multimeter booting up and displaying the F87 logo, however after that it's only 4 dashes. (I'm linking a video showcasing the fault here ->https://streamable.com/0c5k3fas well as attaching some pics of the board, just in case you spot something I didn't.)Holding the AutoHOLD button while turning the DMM turns all LCD segments on just fine, so I assume it isn't an issue with the LCD display itself.Of course I have tried checking the fuses, replacing the battery, cleaning the PCB with isopropyl, different probes, all to no avail.I can't see any corrosion(under the rotary dial as well), cracked/cold joints(at least to naked eye), nor any obviously blown components anywhere on the board either - no visual clues so farHas anyone ever seen anything like this with a F87V? I Would like to fix it, but no idea where to even start troubleshooting this other than the stuff I've already tried.Of course I have contacted Fluke about this, and was told to contact my local Fluke repair point, but I'm not quite ready to give up (and pay someone else to repair it instead) just yet.Oh, and I haven't found anything in the manual about this sort of behavior either.Any ideas what causes this? Or how to fix it up? I'm out of ideas

author: Manul
author_rank: Super Contributor
timestamp: July 03, 2021, 06:52:25 pm
content: Just a guess, maybe ADC is not responding and firmware is stuck waiting forever for conversion to finish? ADC is LTC2435-1. You may scope the SPI to see what is going on there.Also, what is that on the traces in the middle?

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 03, 2021, 09:02:37 pm
content: Heya! Thanks for the reply and tip!Furthermore, you might have (not 100% sure yet) caught the issue on your first guess.After reading your comment I've probed the three SPI pins on the ADC with my logic analyzer & another multimeter and found that:state | SCK | SDO | CS-------|------------|------------|----------off : | floating | floating | floatingon : | LOW | floating | HIGHSome explanation:State is what the rotary dial is currently set to, off being the off position, and on being any (it was same for all) other.The LOW and HIGH (3v3 logic) states i feel like are self-explanatory. The "floating" means that the pin was sitting near 0V, but not quite pulled down - my other (working) multimeter was picking up slight voltage fluctuations within milivolt ranges that usually appear when the probes are left floating in the air, which makes me assume those pins weren't in proper "LOW" states. This would also make sense with the logic analyzer channels not being pulled down by those pins. But that's just my speculation, do correct me if I'm wrong about this.The state of the pins did not change with time.Hmm, fried ADC? What do you think?I feel like this is already a huge leap forwardThanks!P.S. Oh, and regarding the anomaly on the traces in the picture - it's not there anymore, must have been a dust particle.

author: Manul
author_rank: Super Contributor
timestamp: July 03, 2021, 09:35:09 pm
content: Looks ok, but MCU is not reading the ADC at all. No CS low, no clock. ADC is probably fine. The question is what causes the MCU to get stuck. Multimeter often experience some mechanical stress during use, maybe some trace, via or solder joint is broken.Found service manual on the internet. Schematics down in the end.

author: Miti
author_rank: Super Contributor
timestamp: July 04, 2021, 01:52:48 am
content: Post a picture with the switch and it’s contacts. Is there any contact missing or misplaced (if that’s even possible with this meter)?

author: bdunham7
author_rank: Super Contributor
timestamp: July 04, 2021, 02:44:25 am
content: Phone-bricked?https://youtu.be/-13Q-UsXKQM

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 04, 2021, 09:23:30 am
content: The contacts under the dial seem good.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 04, 2021, 09:30:53 am
content: It does in fact behave like the DMM in the vid at 5 minutes in, however (unfortunately) it seems to be permanently stuck instead of recovering. Any way to troubleshoot this further?

author: Manul
author_rank: Super Contributor
timestamp: July 04, 2021, 01:44:11 pm
content: Check rotary switch circuit and its output. Check that the voltages supplied are correct ( -2.5, +0.8 ) and then check how is SWPOS value at different positions. Does it match calculated value of the resistor dicider in that position?Quote

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 04, 2021, 08:17:00 pm
content: Here are the voltages of TP20/pin 5 of the microcontroller at different switch positions:off 0vVac 336mVVdc 0.6Vtemp. 0.9Vohms 1.1Vdiode 1.4VmA 1.8VuA 2.2VThe voltage at "-2.5V" is 0V on my board, and SWPWR is 3.3V. I suppose that's not good(?)

author: Manul
author_rank: Super Contributor
timestamp: July 04, 2021, 08:54:41 pm
content: Quote from: Multiple Cheese Slices on July 04, 2021, 08:17:00 pmAre you sure that you used circuit ground as a ground and not -2.5V?

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 04, 2021, 09:15:10 pm
content: I have put the black probe on the negative battery terminal and the red probe on the via left of R22/ & solder joint left of R61. Is this the correct way?

author: Manul
author_rank: Super Contributor
timestamp: July 04, 2021, 09:30:06 pm
content: Quote from: Multiple Cheese Slices on July 04, 2021, 09:15:10 pmWell, no, it is not correct, because if you observe schematic of power supplies, you will see that battery negative is actually "-2.5V" in the circuit. And "ground" is a point 2.5V above the battery negative. But that does not change nothing, your measurement results seem ok, except that they are shifted by 2.5V.If we assume that MCU is ok (which is of course not guaranteed), then logically the problem should be some signal which is input to that MCU. So it waits for something to be ready and it is not ready, something like that. You could use schematic, determine the inputs, determine their function and look for something suspicious. I'm getting out of ideas here, except slow, methodical testing.By the way, have you tested, that SWPOS signal actually reaches the MCU?

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 04, 2021, 09:53:35 pm
content: Ah, I see, in that case my bad about the measurements. Good thing it's only shifted.Yes, the SWPOS does reach the pin5 on the MCU.I'll do as You've said. I'll post updates if I spot anything unusual.Oh, and just in case the fault was caused by GSM (since this fluke is pre-rev11), what would be the actual part getting damaged? the MCU? Would there be any way of making sure whether that is the case or not?Thanks!

author: Manul
author_rank: Super Contributor
timestamp: July 04, 2021, 10:08:12 pm
content: Can they get permament damage from GSM? I think no, but I don't know for sure. Can not help on this.One final idea is to check all power supplies according to schematic (+2.5, +0.8, -2.5). Also make sure that MCU ADC is powered and has correct reference voltage (check AVCC, VEREF-, VEREF+).

author: Manul
author_rank: Super Contributor
timestamp: July 04, 2021, 10:30:44 pm
content: If you are a patient person, you could actually do a full list of every MCU pin voltage/signal (1, 2, 3 ... and so on), except maybe LCD SEGx and COMx pins. Better do with scope, so you don't confuse varying signals with some average voltage. Use circuit ground as ground. If you would do such a list then forum members can think about it. Also it could help others too, who will find this thread later looking to solve their problem.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 05, 2021, 12:59:08 pm
content: Hello!Before I move on to checking each MCU pin one by one, I've decided to check the voltage regulator outputs. My ground reference was the battery (-).I've found that the output of:U8 is 0-250mV (is bad - the output seems to be random), expected 2.5V, as shown in the schematic. The input is 5V, as it should be.The output has no continuity to ground, so I don't think it's a short (?)U6 is 3V3 (good)U10 is 5V (also good)I suppose this would indicate a faulty LT1790 (U8, 2.5V) voltage regulator. What are your thoughts?Thanks in advance for your help

author: giosif
author_rank: Frequent Contributor
timestamp: July 05, 2021, 01:21:50 pm
content: Reading the data sheet for LT1790, it seems to be a voltage reference and not a voltage regulator.Based on that, I expect the reference to be able to source much smaller current than a voltage regulator would (reading the data sheet, it appears it could source about 5-10 mA).Where I'm going with this is maybe, even is there is no dead short on the output, the load of the circuit is high enough that it brings the output of the reference down.Probably the quickest way to confirm if U8 is good is to de-solder it and check it out of circuit.HTH

author: Manul
author_rank: Super Contributor
timestamp: July 05, 2021, 01:45:06 pm
content: That is a bit ironic, because when you were checking rotary switch, you did not choose the circuit ground as reference. If you did choose it, then you would spot this power supply issue.I aggree that it is likely LT1790 is just loaded too much. Might be also solder crack or something.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 05, 2021, 02:17:25 pm
content: I've tested the voltage reference by itself like you've suggested, giosif. When supplying it with 5V, the output is still just a few milivolts and varies, I suppose that's one issue found.Also, yes, my bad about picking the wrong circuit ground earlier. I admit it, ManulI'll order a new one and see if the DMM works after replacing it. I'll update you as soon as I can, of course!

author: Manul
author_rank: Super Contributor
timestamp: July 05, 2021, 04:41:07 pm
content: Maybe that LT1790 is bad, but frankly speaking this is not a test, because there are no bypass capacitors and the behaviour is unpredictable. LT1790 hooked like this will likely oscillate like crazy in the region of Mhz, and not produce any meaningful output as a regulator.

author: giosif
author_rank: Frequent Contributor
timestamp: July 05, 2021, 06:56:08 pm
content: Would you have a power supply capable of providing 2.5V limited at a few mA?If so, maybe you can hook it up in place of the voltage reference and try powering on the meter.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 06, 2021, 06:21:39 pm
content: Giosif, I have a few LM317 's (adjustable linear voltage regulators) which could be set to 2.5V. However I have no way of limiting the maximum current they could supply to a range of few mA; Would hooking one up risk frying anything else? The brand new LT1790 voltage references should be arriving tomorrow if everything goes right, so if a regular voltage regulator shouldn't be in there for whatever reason, we won't wait long for the proper replacement.Manul, I have re-done the tests with capacitors of capacities close to those shown here:https://www.analog.com/en/products/lt1790.html, and the output is still the same. Thanks for pointing that out tho!

author: giosif
author_rank: Frequent Contributor
timestamp: July 06, 2021, 07:07:51 pm
content: If the replacement part arrives tomorrow, I don't think it is worth the effort (and maybe some risk as well) of using an LM317.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 07, 2021, 06:26:28 pm
content: Heya!The replacement voltage reference has arrived! I have soldered it in, and another issue came up.After setting the rotary switch from off to any other position, the LT1790 seems to be working properly, however only for (about (i don't have a scope atm to check the exact time)) 1/4 of a second.Looking at the input voltage, U10 seems to be the cause - the 5V regulator powering the LT1790 (among other stuff) is only running for ~1/4 of a second before dropping it's voltage from 5V to ~0.6 - 0.4V. The voltage supplies to the ENABLE pin is constant 3V3.Assembling the DMM back together and powering it on results in a "low battery" message, which makes sense to me, since I assume that this voltage regulator would be one of the first things to stop working with low supply voltage.What are your thoughts? Should I solder the U10 out and test it by itself, or does that sound like an issue caused by something else?There don't seem to be any shorts caused by my attachment of the new voltage reference. Also, U10 isn't getting hot (hot enough to boil IPA).Here's a vid showing the behavior of U10:https://streamable.com/bgudk8

author: Manul
author_rank: Super Contributor
timestamp: July 07, 2021, 07:00:19 pm
content: This does sound like something else. Maybe something turns on after 1/4 of second and causes overcurrent condition. TPS77050 is only 50mA rated and has overcurrent protection.

author: giosif
author_rank: Frequent Contributor
timestamp: July 07, 2021, 09:53:12 pm
content: Ok, so the meter is showing some signs of life. That's good.For the 5V rail, it could be overcurrent but, at the same time, unless I am reading the schematics wrong, the Enable pin on U10 is active Low.And, since you mention the voltage on the En pin is always 3.3V, that would normally mean the output of the regulator should be off.Still, probably worth testing U10 out of circuit, maybe there is a trend here (i.e. multiple supply rails having failed).

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 07, 2021, 10:40:30 pm
content: Actually, your response made me realise I was wrong just now.The 3V3 DISABLE EN isn't actually onallthe time, just when the output voltage of U10 drops low. My bad - I assumed the delay of it turning high was simply the DMM booting up.Somehow I have missed the little line above the EN in the datasheet and therefore the part that the EN pin is negated and supposed to turn the regulator off, and without that assumption it all makes sense now - the regulator is working as it should - it turns on, which in turn turns on the 2.5V reference, which i assume allows the controller to turn on properly, but for whatever reason, it decides that the pin 69 (V5*, leading to the (not)EN on the regulator) should be high - at 3v3.Since I don't have a scope at the moment, I have used a working multimeter to confirm that the voltage values match up with my theory and a logic analyzer to record the waveforms.So, If i'm not incorrect yet agian, for some reason the μC wants the 5V regulator to be off. Question is - Why?Here's what I have found in the service manual about "5V*":U10 is a 5.0-V regulator that generates the +2.5-V (-2.5 V + 5.0 V) power supply. U10 is enabled and disabled via control line V5* from microprocessor U2. C15 bypasses the output of U10.When U10 is enabled, 5 V is supplied to the voltage reference U8 input. U8 is a 2.5-V reference with a buffered output and accurately holds the COMMON input 2.5 V above the -2.5 V power supply or at 0 V. The triangular ground symbol represents this reference level throughout the Meter schematic. C16, C57, C50 and R109 bypass the output of U8 keeping the voltage between COMMON and the -2.5 V power supply noise free and stable. C50 and R109 bypass theoutput of U8 keeping the voltage between COMMON and the -2.5 V power supply noise free and stable. C50 and R109 form a low Q bypass that is directed at electromagnetic interference (EMI).Not really sure what implications this would have (is this *supposed to happen?* If yes, that would mean there is yet another issue somewhere.), but posting it here in case it helps.Oh, and I think it's worth adding that the issue isn't the battery itself - it measures sightly over 9V and is brand new.

author: giosif
author_rank: Frequent Contributor
timestamp: July 08, 2021, 09:18:04 pm
content: Like in most cases with broken devices, it rarely is just one issue, so this doesn't come as a surprise here.I am guessing the reason the microprocessor U2 is disabling the 5V rail is because it thinks the battery is too low and it shuts off the meter (meters can become quite inaccurate when their power supply - i.e. the battery, in this case - is running low and that could pose a safety risk - e.g. the meter might display a voltage measurement of 20V while, in fact, it might be more like 30V and the meter operator might decide to work on that circuit thinking it is safe).Can you measure voltage at TP16 and then at TP22 and let us know the readings?Also, with the battery disconnected from the meter, can you confirm continuity between TP22 and pin 6 of U2?

author: Manul
author_rank: Super Contributor
timestamp: July 09, 2021, 12:13:38 pm
content: I think it is quite clear that MCU ADC reference is wrong. Reference comes from LT1790 basically and should be 2.5V. My guess is that +2.5V (5V regulator output) is excessively sinking into circuit ground (LT1790 output). So all MCU ADC readings are wrong, inluding 9V battery voltage (reads lower, then it actually is).

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 09, 2021, 03:45:06 pm
content: Regarding Giosif's reply;There is continuity between TP22 and pin 6 of U2.The voltage at TP22 when the DMM is on is 1.87V, and TP16 is 9V. I'm using the battery (-) as a ground point.The output of TP22 matches up with my calculations of the voltage divider consisting of R15 and R16.Funnily enough, I think this is a bad sign, since the 1.87V does reach U2 (i have even probed the pin directly), so it's not a matter of a shorted resistor or cracked trace/via. Do correct me if I'm making a wrong assumption here tho.Regarding Manul's reply;Would there be any way of checking that / making sure that is the case? It would in fact explain this behavior.

author: Manul
author_rank: Super Contributor
timestamp: July 09, 2021, 05:58:14 pm
content: Well, if you would connect TP22 to the LT1790 output you would likely bypass low battery alarm and have it running, so you can check voltages. BUT the problem is that if 5V is really sinking into 2.5V (and I'm guessing that this is what damaged the old LT1790) then you could expect more failures if you keep it running like that. Because now it just disables the 5V regulator quickly after starting up, but if you bypass low battery, likely it will not do that.Or there could be another scenario, that LT1790 failed, its ouput got low and something which was powered from +2.5V (from 5V regulator output) got over voltage and is now failed. But this I would say, unlikely.Thermal camera would help here so much... How is ouput of LT1790 when you start it? Does it look, that it exeeds 2.5V for a moment?

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 10, 2021, 11:25:01 am
content: Unfortunately I do not have a thermal camera laying around, however I've sprayed the whole board with isopropyl alcohol (which should evaporate if anything gets hot, indicating where a short may be), but unfortunately nothing seems to heat up enough to cause the IPA to evaporate.Checking the output of LT1790 yet again with my other DMM shows stable 2.5V. I do not have an oscilloscope to record the waveform, but what I can see from the DMM, it's unchanging even in the milivolt range (does not exceed 2.5V). Of course it drops to 0 after the 5V is cut off.

author: Manul
author_rank: Super Contributor
timestamp: July 11, 2021, 04:08:10 pm
content: Then you may try to bypass low battery alarm, but I suggest to monitor current consuption from battery and if it is excesive be ready to turn it off. Maybe try to understand what is getting hot. Normal consumption should probably be less the 10mA. If everything looks good, recheck all supply volatges, ADC reference and basic functionality of meter.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 12, 2021, 01:41:18 pm
content: I've done as you have suggested. Now there is a direct connection between the output of LT1790 (C57) and TP22 (R16/C28) as shown in the picture attached.The multimeter does in fact start up, however it still seems to be freaking out. Here's a vid of what's happening:https://streamable.com/vb4yb2The current consumption from the battery is around 5mA, I have measured the consumption before making the modification to the board and it was around 5mA aswell. Of course it was 5mA for a short period of time before the multimeter went into low battery mode, in which it was around 0.1mA. No surprise nothing is getting hot at those currents. (as you can obviously see, I'm not exactly using a top-of-the-line ammeter, but I've calibrated it to be at least somewhat accurate).Not idea what is causing the 87V to behave in such a way.Any ideas?P.S. I feel like this may be important information - Out of all the modes, one actually seems to work properly - the diode testing. The 87V successfully measures a voltage drop of a rectifying diode, and isn't that far off from my other (two) DMMs. That's a start!

author: Manul
author_rank: Super Contributor
timestamp: July 12, 2021, 02:41:04 pm
content: So 5V, 3.3V, 2.5V are all spot on? Check VEREF-, VEREF+, also look at ADIN and FASTADIN at different meter modes.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 12, 2021, 09:11:57 pm
content: Spot on? Well, not quiteEverything (including the input current( starts with 7mA, drops to 4mA) drops over time to not-good levels.2V5(U8) 2.8V, at start, then gets lower and stabilizes at 1.5V. not good...3V3(U6) is 2.4V and also is slowly lowering.5V(U10) is 3.8V at the beginning, but slowly drops to around 3V over time.I assume the new connection somehow caused everything to misbehave, as the voltages on those were good and stable before.VREF- is between 140mV and 250mV. Seems to change as well.VREF+ 2.7V (but then drops down along with current consumed from the battery)FASTADIN (pin 96) starts off at around 250mV and follows the trend of other measurements and drops to 96mV.and ADIN? It may be just me, but I can't find anything about ADIN in the schematic itself, just some text in a section about "secondary analog circuits" in the service manual. May you point me to the exact spot I'm supposed to measure? Thanks!Those voltages are pretty abnormal. This definitely wasn't an issue before. All measurements were taken relative to battery (-), just like before.

author: Manul
author_rank: Super Contributor
timestamp: July 12, 2021, 09:44:09 pm
content: Something is hugely wrong here. With 7mA total current consumption from battery none of these regulators can be overloaded. If they are not overloaded, but output is too low, that means only one thing - input voltage (battery) is low. Can you take a stable 8-9V source like bench power supply or something?However, looking at 2.5V output (2.8V) it seems that it sinks qiute a lot of current, so much, that it can not keep regulation. So maybe your 7mA is hugely incorrect? You need to sort some things here, because it is not logical at all.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 13, 2021, 01:16:34 pm
content: I have made sure that the ammeter is reasonably close to displaying the real current (example: measuring 9.5mA when calculated 10mA should be flowing in a test setup), so it's likely not the issue.I have also connected the 87V directly to a 9V bench PSU (13A max, no issue with limited current here), and the voltage/current measurement functions were still broken. I have decided to re-take the measurements of voltage on regulators/references output, and now the 2V5 and 3V3 seem to be proper and stable, however 5V is still dropping to 4-3V. Not sure why that is, since nothing changed from the last time I have measured them. (I have tested this with both battery and PSU as the power inputs, makes no difference).Checking the "notEN" pin on the 5V regulator yet again, connecting TP22 to 2V5 does indeed make it stay low all the time, and without the connection it still rises to 3V3 after a fraction of a second, like it did before. So nothing changed here.It would be great to find a proper way of fixing the 5V reg turning off because of "low battery", I assume hooking up 2V5 to TP22 is bound to cause problems in the future. No idea why everything but the diode measurement is behaving strangely as well... Could it be an issue with a broken ADC?

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 18, 2021, 04:17:58 pm
content: I feel like this may be helpful information; I have probed the digital pins on the external ADC with my logic analyzer yet again, and there does in fact seem to be communication going on, so that's one thing replacing the 2V5 reference helped. (also, rules out one possible point of failure?)

author: Manul
author_rank: Super Contributor
timestamp: July 21, 2021, 09:54:30 pm
content: Quote from: Multiple Cheese Slices on July 13, 2021, 01:16:34 pmI suggest to forget about low battery for a moment, because it is just a side effect of other things not working properly. Just keep it bypassed for debugging purposes until you will fix the real problem.When measuring current, don't forget that at 20mA range your multimeter likely has around 10 Ohms shunt resistor plus some inductance because of long leads, which drops voltage and also might cause instability with some circuits which rely on having low impedance source (battery) and do not include enough bulk capacitance. So if in doubt, add bulk electrolytic capacitor on the board where battery is supposed to be connected. It will suppress possible oscillations and will lower the impedance.Now about 5V regulator with droping voltage. There is really not much options:1. There is not enough voltage at input2. Current limitting (>50mA ?)3. Cycling of enable signal4. It failed and needs replacementSo you need to setup current measurement, voltage at regulator input and output, also to be sure that it is enabled all the time. It seems to be working and enabled, so either it does not have enough input voltage (check exactly at its input) or it is overloaded.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 24, 2021, 05:00:28 pm
content: I have added a 330μF (is it enough?) electrolytic cap parallel to the battery as you have suggested.Though it appears that the issue causing U10 to have low output was the 9V battery which suddenly started to massively drop voltage with just a few mA of load. (drops to 4V with just (what should be) 9mA (1k resistor across the terminals) of test load), no wonder the voltage regulator didn't produce enough output voltage. This is a recently developed issue though, I have previously checked the input voltage on all the regulators and it was fine. Not sure what made the battery suddenly "die" too, it was new and unused, as far as I know at least. But hey, accidents happen, I may have confused it with some other (used) battery.With a new battery, the output of U10 is 5V as it should be, the 3V3 and 2V5 work fine as well. However the issue seen on the display (constant overload condition) (issues with ADC?) persists, so it appears that the battery/power supply section was not the key culprit.

author: Manul
author_rank: Super Contributor
timestamp: July 24, 2021, 05:38:24 pm
content: I think that diode mode is working, because it is the only mode which is not using autoranging. Because diode mode does not need autoranging, probably it is not using FASTADIN and uses just the dedicated ADC feed from ADIN.I suggest such test: put meter into milliamp mode (almost guaranteed to produce zero, voltage modes may have noise) and measure ADIN and FASTADIN. Maybe you will find that OPA2347 is broken. But be sure to use a GOOD battery and that all power supply voltages are good

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 24, 2021, 10:15:24 pm
content: I have set the 87V to mA mode and made sure yet again that all power rails have proper voltages.I have found that:FASTADIN is at 5mVADIN is at 2.5Vfurthermore, I have probed all the pins on the (OPA2347) dual Op-Amp in case that will help.pin1= 1mV | pin8= 5Vpin2= 1.666V | pin7= 2.505Vpin3= 2.512V | pin6= 2.505Vpin4= 0mV | pin5= 2.441V(I have attached a pic marking all my measurements, hopefully it is intelligible)all measurements are relative to -2.5V (battery (-))

author: Manul
author_rank: Super Contributor
timestamp: July 24, 2021, 11:03:44 pm
content: And again you measured relative to -2.5VAnyway, looks bad. Either R34 is somehow not connected (solder/trace cracked) or OPA2347 is bad. You can buzz pin2 to R34 and then pin1 to R34. Make sure it is really connected. If ok, then try to replace op amp. That could be the end of problems if you are lucky. Because now, op amp output (FASTADIN) seems to be stuck at max scale, so no wonder that software endlessly tries to autorange. It is like constant overload.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 25, 2021, 01:02:28 pm
content: Sorry about the -2.5V again, fortunately it's only offset.There is continuity between R34 and pin1/pin2 (to different sides of the resistor, of course, as shown in the schematic).While desoldering the op-amp for testing, I have accidentally knocked off R34 and R36 into my carpet... considering they're 0603, it's safe to say that I won't ever see them again.I have already replaced R36 with an equivalent value resistor, and will be doing same to R34 as soon as I find a proper one or decide to order it if I can't find it. Although that leaves me with a question - are those resistors important for calibration by any chance? the 200K will be getting replaced with 200K and 100K with 100K of course, but there always will be minor differences. Should I be worried about those at all or is this non-essential for good measurement accuracy?I'll be testing the op-amp one more time, outside of the PCB, and will order a new one if it's definitely fried. The delivery shouldn't take long; I will post an update as soon as I solder the new one in or have any new info.

author: Manul
author_rank: Super Contributor
timestamp: July 25, 2021, 01:15:56 pm
content: I often find some SMD resistors and capacitors to be slightly ferromagnetic, so you may try a strong neodymium magnet on the carpet. These resistors should not be critical, any 1% part shoud be ok. But speaking about calibration, the 2.5V reference is quite critical. So there is no guarantee that meter will be exactly as it was. Not sure, you will see I guess.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 25, 2021, 01:24:32 pm
content: Yes, I know about them being ferromagnetic. Thanks for the tip tho! I have actually tried looking for them for more time than I'm willing to admit. Sometimes stuff seems to teleport into an another dimension when you drop it, you know what I mean?Anyway, It's good that they're not critical. Worst case scenario - I'll send the multimeter to get calibrated once it's up and running properly.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 27, 2021, 02:56:36 pm
content: Hello! Both good and bad (but mostly good) news.Replacing the op-amp has fixed all modes except Vac and mA/A(ac)/ (which I will talk about in one sec). The measurements aren't that far off from my other DMM, however the 87V should probably get calibrated anyway to be able to show its potential; It' supposed to be accurate after all.That is in fact great! Since, well, the DMM is working almost perfectly fine. Almost being the key word - unfortunately there's something weird going on with Vac and A/mA(ac) measurement functions.Firstly - Vac - it seems to be okay when in the 600mV range (the auto default when probes aren't touching anything, tho unfortunately I do not have a way to test the measurements in this range) however when taking a measurement of a voltage in any other range, the overload condition occurs. Example - 7V, 32V and 230V AC all return overload. This happens regardless of the range being selected manually or by the auto-range function.One anomaly is when measuring 18Vac - for some reason it returns 5Vac. I'm 100% sure I am in fact probing a 18Vac source, and that it doesn't drop to 5V when being measured. I have double-checked it with my other DMM.Secondly - also related to AC - the current measurements.the DC current measurements seem to be working fine and the measured values are reasonably close to the theoretical (calculated) ones. AC, however, always returns something close to 2-3mA, despite definitely way more (should be ~86mA, as calculated prior to testing) flowing through (judging by the resistor heating up alone).Not sure if this is one issue or two separate ones which coincidentally both have something to do with AC. Nothing seems wrong with the input sections at the first look, but I'll be taking a closer look just in case. Not sure what to check besides that, what are your thoughts?EDIT: Oh! And I forgot to mention: replacing the op-amp somehow fixed the battery issue. The wire is no longer required.

author: Manul
author_rank: Super Contributor
timestamp: July 27, 2021, 03:58:33 pm
content: This is quite weird, LT1790 2.5V reference, then OPA2347 op amp, which is not even exposed much to environment (electrically speaking), and now something more again. It is suspicious how there can be so many seemingly unrelated failures. Maybe this meter got struck by lightning or something.You need to study schematic a bit. Hope this AC failure is not main chip failure (U1). Maybe it is RMS converter.I would probably put it to VAC, short the probes (to be sure input is zero), then measure ADIN at different ranges selected manualy. If some ranges gives weird ADIN voltages, then I would try to follow the circuit backwards and see where is the failure.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 27, 2021, 10:04:05 pm
content: This is in fact really strange about the meter having so many faults. Unfortunately I have no idea how this came to be. Who knows, maybe it was lightning. Who knowsThis time I have remembered to probe relative to the circuit ground (2.5V ref output).Actually, ADIN is at -2.8mV when in Vac mode and not changing at all, regardless of the range. This holds true for DC as well, which AFAIK has no problems with ranging/autoranging with the difference of ADIN being at -0.7mV in DC mode, +4mV for ohms etc.This does not seem to differ based on shorted or open inputs.Now about U7 - AD737J, the RMS-to-DC converter, (note that all measurements from here on are taken relative to battery (-), since that seems to be the ground of this IC). it is getting a stable 5V power supply, as it should, so that can be ruled out.I've taken some measurements of the output(pin 6) and Cc(pin1) (not probing pin2, since it is tied directly to 2V5(reference output)) at different AC inputs on the probes, here are the results:probes state: | Output pin state: | Cc pin state:-----------------------------------------------|---------------------------------------|-------------------probes shorted | 2.5V (steady) | 1.43V (steady)not hooked up to voltage source | 2.5V (steady) | 1.425V (steady)18Vac | 2.225V (steady) | 1.97-2.17 (somewhat fluctuating, but by just a few mV)32Vac | 2.1-2.5V (fluctuating) | 1.2-1.8V (fluctuating)230Vac | 2.1-2.5V (fluctuating) | 1.2-1.28V (fluctuating)All voltages of the output and Cc pins are measured in DC mode (with my other DMM). When measuring them (with my other DMM(stating this just to avoid confusion)) in AC mode, they all return 0V.It seems that there is corelation between fluctuating DC voltages on the input/output and the overload condition.Also to me it looks like there are issues before the RMS converter as well - on the CC pin - the voltage fluctuations on it seem to corelate to the output(pin6).Honestly speaking, I'm not sure how to read what input voltages should produce what output voltages and what they would "mean", so I'm not sure if the 18Vac translating into 5V occurs here or before/after. Here is a link in case it would help:https://www.analog.com/media/en/technical-documentation/data-sheets/AD737.pdfInput(pin2)/COMMON measure 2.511Vdc (as it should be), and the PDN is 3V (as it should be(i think, judging by the datasheet))

author: Manul
author_rank: Super Contributor
timestamp: July 28, 2021, 04:30:45 pm
content: Quote from: Multiple Cheese Slices on July 27, 2021, 10:04:05 pmI'm not promoting violence, but someone needs to beat you with a shovel to make you forget that battery negativeIt is not the ground of that IC. In fact, nowhere in the circuit it is a ground. There is power supply section: battery and linear voltage regulators, you may use battery negative as a reference when talking about them if you wish. But after the regulators, circuit ground is clearly drawn as +2.5V reference output (that triangle symbol) and everything else in the schematic uses that. All the signal paths, everything. Voltages in the schematic written relative to that, voltages in the text are written relative to that. Espeacially when we talk about analog signal paths where op amps and other stuff are using that +2.5/-2.5V split supply with virtual ground in the middle acting as common, it is really unnecessary mental stressing and unintuitive to read numbers relative to -2.5V and not the ground.So with no input ADIN in VAC mode is always -2.8mV at all ranges? Seems quite normal, that is very close to zero, just a small offset. And what the meter shows in those different ranges with no input? Near zero, overloading or what?

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 28, 2021, 06:18:44 pm
content: On most handheld DMM's the easy to access "internal" ground (and thus sensible reference for measurements when troubleshooting) is the COM banana socket.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 28, 2021, 06:23:15 pm
content: I'm sorry about the battery negative, I'll borrow a shovel from a friend and try to fix the issue myself first.(No, but really, I will 100% remember from now on. I got kinda confused; The COM pin kind of slipped my mind since it had no description in the pin funciton descriptions other than "common". This is the actual signal ground, yes?)When the DMM is switched to the Vac mode, it starts off showing 56mV and then drops down.When it settles and is no longer changing, The 87V is showing:1.6mV AC in the auto/manual 600mV range,0.016V in 6V range,0.16V in the 60V range,1.7V in the 600V range,and 2V in the 1kV range.Strange thing is that I cannot get it to display 0 no matter if the probes are shorted. I wonder if that's just miscalibration or part of the problem.Quote from: shakalnokturn on July 28, 2021, 06:18:44 pmAh, cool trick! There is in fact direct continuity between the COM socket and the circuit ground in the 87V. Thanks for that!

author: Manul
author_rank: Super Contributor
timestamp: July 28, 2021, 07:04:40 pm
content: Well, the offset is not big, seems around 0.2-0.3%. Probably calibration, because 2.5V reference was changed. Anyway, it is quite close to zero with no input, but when actual AC volatge is measured it goes way off, yes? Except 600mV range?About that RMS converter. Theoretically it should output negative RMS of input AC voltage. So if you measure AC input voltage of lets say 100mV (RMS), then output should be around -100mV DC.By the way, you can make a voltage divider to use with 18V AC you have. Maybe potentiometer. You can produce different voltage to see how it reacts. Like 100mV or 500mV, 3V and so on.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 28, 2021, 10:06:21 pm
content: Actually, now that I've checked using the potentiometer voltage divider, the 600mV range looks pretty rekt aswell.Here are some measurements that I have taken using both my (known to be working) multimeter and the fluke.(Yes yes, the shovel has helped; this is realtive to circuit ground/GND banana plug)Probes input:F87V display:RMS AD737J inputAD737J DC output430mVac6.8mVac0.22Vac-4.6mVdc880mVac31mVac0.44Vac-19.3mVdc1550mVac360mVac0.77Vac-188mVdc1840mVac566mVac0.93Vac-283mVdc2500mVacOverload0.1 to 1Vac-100 to -300mVdc3333mVacOverload0.1 to 1Vac-50 to -380mVdcA mistake I have done earlier was measuring the RMS-DC converter input as DC. (oops), Here it is in AC now. It was around -1.3Vdc all the time, so I figured something was wrong.Quote from: Manul on July 28, 2021, 07:04:40 pmSo, the outputs are waaay off from the inputs. Does this indicate a broken AD737J, or is it possible that the issue lies elsewhere?I don't know how to check whether the AD737J RMS input voltages are proper, though. The input circuitry seems pretty complicated to my noob eyes.Well, as always - what do you think?

author: Manul
author_rank: Super Contributor
timestamp: July 28, 2021, 11:03:15 pm
content: AD737 input voltage looks exactly proportional to meter input (divided by 2, which seems reasonable). So probably AC AMP, gain (range) selection (U1) works ok and multiplexer (U12) is ok. Now the AD737 output is correlated, but proportion is weird and not linear. So either AD737 failed or its output is overloaded by something. Output goes to U12 B0. Output overloading seems unlikely, but possible I guess. If you could isolate it from B0 and it still makes same wrong output, then it must be damaged. Or just change it and hope for the best.Good news is that RMS converter output looks to be sampled correctly by the meter, because in 600mV range input volatge is divided by 2, so 600mV AC becomes 300mV AC, which should produce -300mV DC and that -300mV DC seems to produce 600mV on the meter display (I'm using your measurement that -283mVdc RMS converter output produced 566mVac reading on the display). So it seems all good after the RMS converter.

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 29, 2021, 09:38:21 pm
content: I've taken some more measurements after disconnecting the output pin from the rest of the circuit. The input/output isn't all that different than when the output signal was going somewhere, and considering that everything before and after the RMS converter seems fine, this must be it.Vac inputRMS input pinDC output pin1550mVac777mVac-204mVac888mVac444mVac-14.4mVdc446mVac238mVac-4mVdcAs I've said, not much difference between connected/disconnected output, as you can see from this and the previous table.I've looked around nearby electronic part sellers, but none see to have the AD737J in stock. The delivery will take a little longer this time, hopefully it's the last one!Thank you yet again for your support. I will update you with news as soon as I have any!

author: Manul
author_rank: Super Contributor
timestamp: July 29, 2021, 10:19:46 pm
content: DigiKey has it, but you will pay some for shiping, also part itself is not the cheapest. Hope it will not be wasted. I mean, this repair is getting ridiculous. When I think about it, I can hardly comprehend how it all might have happened. Does not look like damage from meter input overload. Reverse battery connection should be clamped by diode CR3. Definetly very interesting case...

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: July 30, 2021, 07:43:23 pm
content: I have no idea how this happened as well. Unfortunately we'll likely never know; I have received it in this state, and only because it was in this state too. Nobody would want to get rid of a working one, aye?But yeah, it is strange how so many things seem to have failed. It is possible that someone has attempted to repair it earlier since some screws were missing, but I really don't know more than that.A thought came to my head. What if by any chance it got damaged by the weird jumping/fluctuating input? The input is always like this when the overload condition is occurring.And so, I went to investigate - I was planning on supplying the meter with an overloading voltage and probing the AC signal before/after the U1 & the multiplexer and maybe some other stuff depending on what I would find happening with the U1/multiplexer. However now all I was able to find was stable (not fluctuating like with the overload conditions) ac voltage on the RMS converter input. After some testing, it turns out that this only happens when the output pin from the RMS converter is connected to the circuit.I was assuming there was something wrong with the input section before the AD737 as well (because of the weird inputs), but would that be ruling it out?P.S. Oh, and one more thing - what's the actual difference between AD737JRZ and AD737JRZ-5? The only thing I can find about it in the datasheet is that the JRZ-5 is "tested with supply voltages of +-2.5V DC". What does this mean? Will AD737JRZ also work?

author: Manul
author_rank: Super Contributor
timestamp: July 30, 2021, 11:38:15 pm
content: Input to RMS converter is very unlikely to ever cause damage, because amplitude is naturally limited by supply rails of previous amp stage, so it can jump as much as it wants, but +-2.5V peak will be roughly the limit which is ok for AD737. Also I guess the multiplexer has clamping diodes. In my opinion, serious over voltage will fry the U1 long before RMS converter.AC input voltage is stable, because meter does not know that there is any voltage at all if RMS converter output is not connected. Better don't do such experiments, especially with high voltages, just wait for new part to arrive. Jumping input was because of range switching or some protection kicking in (not sure, have not studied all schematics).Service manual has (full?) part list. For example, U7 is AD737JR-5. U8 is LT1790ACS6-2.5 (these have different grades too).Tested at +-2.5 means that it meets specifications at that supply voltage. Other parts are guaranteed and specified only for higher supply voltages. So yes, you need AD737JR-5.

author: srmahaffy
author_rank: Contributor
timestamp: August 14, 2021, 05:58:49 pm
content: Hi, are you continuing to troubleshoot your 87V with the dash problem? I just purchased one on eBay with the same issue, that was missing the external selection knob, hoping the related circuit was why the “all dashes” . After opening it up, and doing some troubleshooting the switch circuit looks okay.Please let me know if your continuing your “dash” repair, and do you have a schematic/ service manual for the 87V. I don’t see that on the fluke website.Thanks in advance!

author: 3db
author_rank: Frequent Contributor
timestamp: August 20, 2021, 09:41:31 am
content: Quote from: srmahaffy on August 14, 2021, 05:58:49 pmLook at page one of this thread Manul posts a link to the manual.Getting the manual and reading this thread may also help you fix your meter.Regards3db

author: Multiple Cheese Slices
author_rank: Contributor
timestamp: August 20, 2021, 08:31:56 pm
content: Oh boy, it sure has been a journey.I am happy to announce that it's over though. But it has been lots of fun!To get the most important (technical) things out of the way first - I have replaced the AD737JR-5, and now the Vac and A/mAac modes seem to be working fine. That means It's fully functional (though I haven't checked all the function buttons, haha)Now, for the less important stuff (you can skip this paragraph if you're not curious about non-technical stuff) you might be wondering what took me so long (this was definitely not planned). Looking at the shipping/component costs, I have decided to look for alternative ways of obtaining that part cheaply. I found out that Analog Devices had a free sample program in which the company would send customers free components and pay the shipping bill. I assumed it wouldn't take more than a few days, a week at maximum to arrive. Oh, how wrong I was... the procedure on Analog's part took around 12 days to get my order confirmed and packed up. Later came the fedex courier which extended the wait up until today. But hey, I can't complain too much - I'm not the one to look a gift horse in the mouth - it was all delivered for free.I guess I should get the multimeter calibrated now if I want to use it. After the horrors it has seen, I think it would be perfectly in place. There is one metrology laboratory in Poland which claims to calibrate fluke equipment (here's their website if you're wondering:https://anmar-pl.com/)). I'll likely try to get a little more information about them, see if they are legitimate, and if yes, send the 87V to them for calibration. It can't be that expensive, right?To srmahaffy,QuoteWell, as you already know, my troubles have ceased for now. Seeing that your 87V has the same symptoms, I recommend checking the thing that have been found to be faulty in my case. Hopefully the culprit of your case is one of them, this would make things much easier.Anyways, all the DMM modes now fully work. (at least I think so ^_^). It really isn't possible to convey how much I owe you people, especially Manul and Giosif - this really wouldn't be possible without you. I don't know how I can ever repay you for the amount of your dedication to help solve my issues. You are amazing;Thank you.

author: srmahaffy
author_rank: Contributor
timestamp: August 22, 2021, 06:02:31 pm
content: Hi, Multiple Cheese Slices, I’m so glad you got your meter working! I will start a new post for my repair journey. eevblog is a great resource! Thank you everyone so for.

author: Manul
author_rank: Super Contributor
timestamp: August 22, 2021, 09:22:56 pm
content: Congrats on fixing that thing. I would just say to make sure that you bought the correct "A" grade 2.5V reference before doing calibration. A very strange multi-point failure, but with happy ending.