Fluke 8845A (like 8846A) 6.5 digit Multimeter showing "overload" at all times

[t-17]

Hi all,

I have here a Fluke 8845A (6.5 digit bench multimeter) in good cosmetic shape, with a blown 440mA input fuse and probably some defects in the ADC. I do not know what happened to it earlier. However, I can see from the flux residue on the PCB that someone was already in the unit and apparently, changed rectifier diodes for the 3.3V rail of the analog part.

The unit is built around two Altera FPGAs, a LM399 reference, some Fluke ceramic resistor networks (I guess from their looks), as well as some fancy Opamps/ comparators by LT and AD, and a RMS voltage converter AD637.

What I have checked so far:

• All voltages are fine on the digital part (+1.5V for the FPGA core and +3.3V for I/O I guess, as well as +60V for the VFD).
• All voltages in the analog part are also fine (+1.5V again for the FPGA core, +3.3V I/O, symmetric +/- 5V, +/- 15V, +/- 20V)
• All clocks are running.
• Optical link between digital/analog part is working (checked by inserting a piece of paper inbetween-- Fluke is not using an integrated opto-coupler but went with a IR LED/LDR combination of some sort). When a piece of paper is inserted, the "measurement update" indicator in the display stops blinking.
• At least the LAN interface is working fine.
• Display and front panel in general are working fine.
• No errors are reported. However, the unit does not appear to have any reasonable self-test (checked the manual, did not find any).
• Unit boots up normally.

Observations:

• The 1.5V regulator for the analog part is getting very hot, but that is probably due to the fact that it's being designed in without a heatsink, and it generates 1.5V from the 4.8V winding of the main transformer.
• The Analog Devices RMS converter is also getting somewhat hot.
• There are slight burn marks on the PCB near the 3.3V rectifier diodes on the PCB, which are presumably used for generating the FPGA I/O voltage. The diodes have been replaced from what I can tell, and the rail is stable.
• The unit reports "overload" no matter what mode is selected (ACV, DCV, ACI, DCI, ...)
• When in continuity/ diode mode, it reports always "OPEN".

Any hints are very much appreciated. I'll report new findings into this thread. If it helps, I can also post images.

Side note
I have decided to start a new thread for this nice multimeter. I have already posted in another thread about the same piece of equipment, but this was to offer help for an entirely different problem (bad display): https://www.eevblog.com/forum/repair/fluke-8845a-repair/msg979347/#msg979347

[ManateeMafia]

Have you tried manual range selection? Also, single trigger mode vs continuous trigger.

I would start with 10v dc range and try to follow a test input signal or short the inputs. It could be a defective op-amp and that can be verified by checking inputs and outputs. Not easy without schematics.

Good luck.

[t-17]

Yes, I have already tried manual range selection. It works, but the result is the same. Single trigger works as well, but this will be mostly useful for checking the opamps. There are also a lot of analog switches/ multiplexers in there. I'll have to check them, too.

I'll try to check all of the ICs in the 10V range. This will be my next step.

If anybody could provide me with voltages/ signals at certain test points of the main pcb of a working unit, please contact me. This would help me a lot.

[Kleinstein]

There are two relatively easy to check points:
The reference voltages (should have test-points for the 7.x and maybe 10-12 V).
The current flowing in the ohms range.

Without a good selftest and schematics, the repair might get difficult. At least it take quite some time to draw a rough schematics from the board. If there is something wrong with the FPGA program, you might be lost getting replacements.

[t-17]

The reference voltages are all okay. (+/-7V and there is one +10V test point, also okay).
I have not measured the current flowing in the ohms range yet. I wanted to concentrate on one problem at a time, but maybe I'm missing something. Can you elaborate?

I'm making (slow) progress in tracing the signal. There is one opamp I'll have another look at. It's showing strange voltages when I apply 4V DC at the inputs in the 10V range (all opamps before are fine). I'm also trying to figure out how a multi-slope integrating ADC works. There are 4 transistors and 4 voltages regulators all 3-pin SOT-343 or something similar with various voltages at the pins. The strange thing is, although there are various positive voltages which appear to make sense (like +20V, +10V) on the negative side it's all off and I measure strange values. I suspect something is wrong there, too.

What I am pretty sure of is that the FPGA is ok. There are active digital signals between it and its SRAM and at the optocoupler. Also, the integrator is showing activity when I manually trigger it or leave it free-running. I am 99% sure it's a problem in the analog part.

For now, I have a suspicious looking opamp to investigate, so I'm quite satisfied for the moment

For reference, if somebody ever has this thing on the table (note: based on observations/ measurements, 99% certain):
DCV range selection/ input divider (by 1, 10, 100, 1000): Z1/U37 (ceramic laser-trimmed resistor network/ multiplexer DG444)
1st opamp: U36 (LF353)
2nd opamp: U35 (AD8510) (either dead in my unit, or has a special purpose-- strange voltages, need to double-check)
integrator: C131/U49 (AD8510)
comparator: U50 (LT1394)
FPGA: ALTERA Cyclone I EP1C12 (FPGA used in the analog part of the PCB)

I'll be updating this with new findings. Hopefully, this is useful to somebody one day.

[Kleinstein]

Measuring the Ohms current indirectly does a kind of check in the reference voltage. No real need to measure if the ref. voltage are there.

For the typical mulit-slope ADCs, there also should be a -10 V or similar reference. Just using +-7 V would be unusual, though not impossible. Strange negative voltages might be a problem. Usually many of the OPs should get something like +-15 V.

The usual configuration is an switchable 1:100 divider for the input- switching via relay of high voltage MOSFETs. Than the input amplifier with amplification of 1, 10  and often 100 - switching via DG444 would make sense. One should be able to test this input part with low frequency AC.

[t-17]

I made an interesting discovery. Two opamps (U35: AD8510 and U36: LF353) have supply voltages which change and shift with the input voltage applied to the input terminals. Before I continue with the search for the root cause, is there any application where it makes even remotely sense to design opamps in with this kind of behavior (variable supply voltages)? I don't think so. I suspect there is a problem with the supply. If I leave the input terminals unconnected, these opamps have about -5.5V on the negative terminal and around 5.4V on the positive terminal. With 4V DC on the input terminals, I have -1.5V and ~9.3V. When I disconnect the inputs, the voltages seems to undershoot a bit (V- for example then has -5.8V slowly coming closer to -5.5V, what is reached after half a minute or so). All other opamps in this section are all supplied with +/-15V.

If I measure the resistance to the voltage regulators (from the opamps supply pin to the voltage regulator), I measure about 8Mohms what does not make any sense to me. The connection between the opamps (the two nets V+ and V-) are low-impedance and seem to be intact. Is it possible that some trace or component has gone bad for some reason and the two opamps now do not have a proper supply any more?

I traced these nets to two transistors (Q31 and Q32) in the vicinity which have one pin directly connected to -20V (Q32) and +20V (Q31) and another pin to the V+ net or V- net of the opamps, respectively. I don't know yet what the third pin is for, but maybe this topology makes ring a bell for somebody. Why would these opamps have their rails switched by transistors? Does this make any sense?

I'll keep investigating this thing for the time being.

Thanks so far for your replies! They already helped me a lot!

[Kleinstein]

A supply voltage the follows the input voltage makes some sense, especially if the input voltage range is large compared to the supply of the OP. So it's less a problem with a LF353, but could still be used to reduce input bias / increase the input impedance.

The Keithley 2000 DMM uses this way in the input stage with an LTC1050 AZ OP. There two transistors as constant current sources from a high supply e.g. +-20 V provide current. 2 Zener diodes limit / stabilize the supply of the OP, an the center is dirven by a second OP.

[t-17]

I believe I have finally found something.

TP25 is at about 270mV in my unit. It is driven by an opamp and I believe it should be zero volts. I measured this with manual triggering on while the unit was idle, range 10VDC (manual).

Can somebody verify this with another 8845A/8846A unit?

[voltsandjolts]

Can somebody verify this with another 8845A/8846A unit?

Let me know if you still need this info, I can help this week.
Edit: I assume my 8846 and your 8845 will be similar enough that TP25 is the same point!

[t-17]

Hi,

thanks for the reply! I'd appreciate if you could provide me with this info. Also, when in 10VDC with auto-trigger on (default), I could make use of the waveforms visible at TP28 and TP20.

Thank you in advance!

[voltsandjolts]

Input shorted for all tests.

Manual trigger (idle), 10VDC range (manual):
TP25 +80uV (effectively 0V as you suspected it should be)
TP20 3.636V
TP28 -70uV (effectively 0V)
All clean and stable DC

When you press MAN trig button there is a burst of activity on these test points, similar to AUTO mode scope captures below.

Auto trigger, 10VDC range (manual):
See attached pics
CH1=TP20
CH2=TP25
CH3=TP28

TP28 seems to provide negative pulses which start/stop the acquisition cycle.
TP20 square wave is around 15kHz with varying duty cycle


I will keep the meter open for a couple of days if you need some more info.

[t-17]

Thanks a lot, voltsandjolts, this is very helpful! My waveforms look totally different and I suspect that either the CMOS switch (U57) or the opamp (U49) is bad. I'll report back once I know more. Thanks again for your efforts!

[t-17]

Could you provide the waveforms also with 4VDC at the inputs? (This is the voltage I have used so far for testing; I chose this value in particular because it is not present anywhere inside the unit.)

[TiN]

Those are multislope integrator waveform (triangle ramps) and it's comparator output, which is counted by digital logic into code. It will look similar at any input voltage, just "duty cycle" would vary differently.

If you have no ramps, you need to check charge control circuitry which connects various ratios of VREF to integrator. If no digital counter output, but charge ramps there - check comparators.   

[voltsandjolts]

4.00VDC input for all tests.

Manual trigger (idle), 10VDC range (manual):
TP25 +10uV (effectively 0V)
TP20 3.614V (effectively same as before)
TP28 4.00V
All clean and stable DC

Auto trigger, 10VDC range (manual):
See attached pics
CH1=TP20
CH2=TP25
CH3=TP28

TP28 seems to provide negative pulses which start/stop the acquisition cycle. Not sure about that now! Edit: Looks more like the input voltage to be measured which is periodically grounded to cancel internal offsets - differential measurement (?)

[t-17]

Thanks again! This will be all I need for quite a while. I'll now turn back to my TODO list and report back as soon as I find out something new. I will not need any more measurements/ waveforms for the time being, so feel free to put the cover on again. Your help is very much appreciated! 

[voltsandjolts]

Glad to help, good luck with the fault finding.
With all the 'clone' manufacturers around it's no wonder companies don't release schematics anymore but it is a PITA.

[TiN]

Often in bench meters there is autozero function used to remove offset errors on measurements. What it does is simply make measurement with A/D input grounded, then make actual input signal measurement. User's manual should have information how this  works in 884X exactly.

[t-17]

Hi TiN,

sorry for replying so late, but I have indeed overread one of your posts with valuable information. The comparator works-- I have checked the input and output waveforms and they look fine. However, I will replace the opamp used in the integrator, since it has a very large offset (>200mV) when it should be zero and have a look at the waveforms I see at TP25 (Ch1-- attached to this post). Apart from these zero drops I also have irregular (about 3-6 times a second or less) anomalies (the measurement in the attachment shows it horizontally in the center, as I've managed to let my scope trigger on it).

Feel free to comment if this makes ring a bell!

Thank you for your contributions!

[t-17]

I want to share something with you guys. After months of intensive troubleshooting and learning and most importantly, thanks to your help, the instrument is finally back to life! The culprit was indeed the AD8510 at the heart of the instrument. After replacing it, I had immediately reasonable readings. Of course, the unit is out of calibration now, but for me, this is still a huge success.

This is what I've done (please comment if I missed something important):
I've covered everything around the opamp carefully with Kapton tape in order to protect the parts from the heat of my hot air tool. I desoldered the old opamp, cleaned the pads carefully and used "no-clean" solder paste and again the hot air tool to solder the new replacement opamp. I did it this way in order to minimize stress to the PCB and to avoid flux residue. From my understanding, this is probably the most sensitive part of the PCB.

I came back to leave some images and once again say THANK YOU!

The first image shows the unit measuring the 4V DC of my power supply and the second image shows the "problem area". So far, all other modes of operation are working fine as well, so the blown input fuse and the dead opamp seem to be the only problems.

The thread can be marked as "solved" now, I guess

[ManateeMafia]

Congratulations on the repair and thanks for sharing your results.

[voltsandjolts]

  Nice result!

In UK my 8846A is around GBP180 for Fluke calibration or GBP70 for RS cal.
However, the Fluke report is more comprehensive.

[t-17]

My last post was a bit premature, I have to admit. The multimeter had more problems, which I discovered one by one after the central part of the multimeter (the A/D converter) was working again.

The entire ACV/ACI measurement was gone, but came back after replacing U18 (AD673) and U20 (AD8510). This was almost "easy" to debug and solve (RMS output pin was constantly at +15V, and after replacing U18 it did not react to input changes so I traced the input of U18 to the output of U20), but now I'm kind of stuck again with the so far only remaining problem.

Which is: frequency and period measurements. I have found that U16 (LM393) is a dual comparator, which compares *some* AC signal against +0.5V and -0.5V. Its outputs are connected together (logical OR, as they are open collector according to the datasheet). The output measures constantly at 3.3V which is the FPGA I/O voltage. Since this is the only comparator in the vicinity of the entire AC circuitry and I know that it is related since it does the range switches when the signal amplitude changes (and also the manual says somewhere the AC circuitry is used), I suspect that either another opamp or a multiplexer is bad (assuming no damage in passive components as the board looks absolutely clean without any traces of heat and all resistors and capacitors showed reasonable values as far as I could probe them in circuit). AC voltage and current measurements work fine now in addition to the previously repaired DCV/DCI part. I have tested all ranges. Continuity, diode and ohms work fine, too.

Could anybody provide me with waveforms of the comparator IC (U16) pins (output: 1 or 7 and input: 2,3 and 5,6) when measuring an AC signal (1Vpp and >5Vpp)? This would help me a lot.

[TiN]

Good job on repair, but..

The thread can be marked as "solved" now,

It's far from it, my young padavan  . I'd consider solved when meter fully working and calibrated .

Also using hot air gun for SO8 package is sorta overkill in my book. Usually better and safer technique is to put solder blob on one side (4 legs) and melt whole side with chisel tip. Then melt second side same way. And flip side quickly to other side back and forth. After 3-5 seconds opamp is flowing and can be removed easily by tweezer It sounds much complicated than actually is. Try on some dummy boards. This way you don't risk overheating nearest parts..

[t-17]

Thanks TiN, you're absolutely right. 
My bad. I guess I was just blown away after several weeks of troubleshooting when I finally started making progress.

Talking about progress: I have just solved the last problem and (apparently) fixed frequency and period measurements. I was wrong with the suspicion that U16 would have something to do with the frequency measurement. Rather, it seems to be used for autoranging when in ACV mode. The IC I was looking for was U32 (also an LM393). I found it after close inspection of the PCB. I figured that for frequency/period measurements one would want the AC signal normalized to a certain amplitude range first. So the signal should be tapped off somewhere, where this is the case. And I knew from the ACV repair part that the signal has its largest amplitude, and is also within limits at all times, at the output of U20. And in fact, the output of U20 would feed U18, the True RMS converter, and *something else* (the trace splits into two on the TOP side of the board, and one of them mysteriously vanishes into a via). It took me quite a while though, to find U32 sitting next to the power connector of the analog PCB part. I thought it would rather have something to do with power rail regulation. I could not be more wrong

Now, the multimeter also has frequency and period measurements back, BUT: When measuring period or frequency, the flash symbol appears in the display, which usually means "potentially harmful voltage at the input terminals". I guess it may be that there is still something wrong somewhere. Can somebody with a working unit confirm that under normal conditions the flash symbol does not show (when measuring frequency)? Apart from that, all basic measurement functions are back. But I'll perform a complete test, testing every single function right through the user manual to be sure.

I have made a list of ICs and their function, in case somebody wants to repair this beast. Note that the entire AC circuitry is under a metal can and cannot be seen easily as long as it is not removed.

Designator	IC Code	Function
U5	LF356	AC input buffer, only used for higher ranges (at least 750V range, maybe 100V as well-- not checked)
U12	LF356	AC input buffer, input and output are at the same level (no amplification), only used for lower ranges
U14	AD825	AC input buffer, input and output are at the same level (no amplification)
U16	LM393	AC amplitude comparator, 99% positive that it is used for ACV autoranging. Compares against +0.5V and -0.5V.
U18	AD637	True RMS AC -> DC converter, used for True RMS ACV/ACI measurements
U20	AD8510	Input amplifier/ buffer for AC, frequency and period measurements.
U36	LM393	Comparator for frequency and period measurements
U37	DG444	Analog multiplexer used for DCV range switching
U49	AD8510	Main integrator opamp of the ADC

[hbrown]

Just wanted to say thanks! Your investigative work and good reporting saved me a great deal of time. I received an 8846A for free because all the AC ranges (ACV, ACI, frequency) showed overload. I swapped U18 and U20 and its good as new. Just need to verify calibration now.

[coromonadalix]

@ tin  i would absolutely not use your method in this case, it may stress the pcb pads, the hot air removal is the best and fastest non damaging solution for me, i work a lot on pcb's   and hot air is the best.

Tried on defectives pcb the solder blob on one side and lift the other side ....  you have to be fast and precise ...

[TiN]

If that works for you, then already have your solution. Just keep in mind that stress from heat to precision resistor network nearby can cause permanent shift/drift and non-reversible stability issues after using hot air...

[coromonadalix]

kapton can be of some use, but in this case i would provide an heat shield deflector around it.

normally   when pcb's thru holes are installed and soldered like we do in my company, the boards pas thru an pcb oven for at least 30 seconds to 1  minute,  and even precision part have some heat tolerances to do so.

When i remove smt parts, it take under 10 secs to remove them ..... sometimes even under 5 secs, heathing only the part with the right nozzle and nothing around it with or without kapton protection move or shift ... it really depend of your technique

[analogRF]

Thanks TiN, you're absolutely right. 
My bad. I guess I was just blown away after several weeks of troubleshooting when I finally started making progress.

Talking about progress: I have just solved the last problem and (apparently) fixed frequency and period measurements. I was wrong with the suspicion that U16 would have something to do with the frequency measurement. Rather, it seems to be used for autoranging when in ACV mode. The IC I was looking for was U32 (also an LM393). I found it after close inspection of the PCB. I figured that for frequency/period measurements one would want the AC signal normalized to a certain amplitude range first. So the signal should be tapped off somewhere, where this is the case. And I knew from the ACV repair part that the signal has its largest amplitude, and is also within limits at all times, at the output of U20. And in fact, the output of U20 would feed U18, the True RMS converter, and *something else* (the trace splits into two on the TOP side of the board, and one of them mysteriously vanishes into a via). It took me quite a while though, to find U32 sitting next to the power connector of the analog PCB part. I thought it would rather have something to do with power rail regulation. I could not be more wrong

Now, the multimeter also has frequency and period measurements back, BUT: When measuring period or frequency, the flash symbol appears in the display, which usually means "potentially harmful voltage at the input terminals". I guess it may be that there is still something wrong somewhere. Can somebody with a working unit confirm that under normal conditions the flash symbol does not show (when measuring frequency)? Apart from that, all basic measurement functions are back. But I'll perform a complete test, testing every single function right through the user manual to be sure.

I have made a list of ICs and their function, in case somebody wants to repair this beast. Note that the entire AC circuitry is under a metal can and cannot be seen easily as long as it is not removed.

Designator	IC Code	Function
U5	LF356	AC input buffer, only used for higher ranges (at least 750V range, maybe 100V as well-- not checked)
U12	LF356	AC input buffer, input and output are at the same level (no amplification), only used for lower ranges
U14	AD825	AC input buffer, input and output are at the same level (no amplification)
U16	LM393	AC amplitude comparator, 99% positive that it is used for ACV autoranging. Compares against +0.5V and -0.5V.
U18	AD637	True RMS AC -> DC converter, used for True RMS ACV/ACI measurements
U20	AD8510	Input amplifier/ buffer for AC, frequency and period measurements.
U36	LM393	Comparator for frequency and period measurements
U37	DG444	Analog multiplexer used for DCV range switching
U49	AD8510	Main integrator opamp of the ADC

I know this is a very old thread but did you find anything else wrong with the unit with regard to the flashing symbol you mentioned?

[t-17]

I know this is a very old thread but did you find anything else wrong with the unit with regard to the flashing symbol you mentioned?

First of all, sorry for the late reply. I just want to confirm that even after all these years, the unit is still operating fine. So besides the flash symbol, there is nothing wrong with it. Also, I think I've seen it appearing in brand new units as well. So it does not seem to be a malfunction, but rather a firmware issue (perhaps only with older units like mine)?

[Miti]

So besides the flash symbol, there is nothing wrong with it. Also, I think I've seen it appearing in brand new units as well. So it does not seem to be a malfunction, but rather a firmware issue (perhaps only with older units like mine)?

It shows up on mine as well so I don't think it is an issue but rather a feature. When I've got it it had an inguard/outguard comm issue and the culprit was an IR LED. I replaced it with one from an old DVD remote control and it still works after 4 - 5 years.

[aronake]

Hi all,

I have a Fluke 8845a that have developed some issues.

DC voltage and current work all fine. This issues are with anything AC.

Frequency measurement:
- Works for "easy" waveforms like square wave it measure correct.
- For a little bit more difficult to measure waveforms like Sine need to be above 7khz for lower voltages (below 1V) to measure correct. Above 1V, after the relay kick into higher voltage range it do not measure correct.
- When measure wrong in frequency mode, typically 20% too high and numbers jumping around. But can be much worse than that.

AC Voltage:
- before this issue happened it was very accurate. Now using DDMcheck, it shows 0.1 V low.

AC current 400mA port:
- No current on terminals and it still shows 5 mA
- At around 15 mA is start to show correct numbers.

AC current 10A port:
- I don not have any strong AC current source enough to try, but with noting connected, it 0.5A.

Any suggestion from the expert panel here where to start looking for the error?

[Kleinstein]

There is a shared AC/DC converter for both the current and voltage readings.  So if zero current results in a non zero reading, I would expect a similar thing with AC voltage - it this happening ?

Testing is usually easier with the voltage ranges.

The frequency reading usually only uses a simple comparator for the input. So low slope signals are always a bit tricky to measure.
To high a reading could be some ringing or interference causing multiple counts in some cases, especially with a low slope, so the input is longer near the sensitive range.

There is a chance the input amplifier may be oscillating for some reason. This could explain trouble with the frequency measurement and the non zero reading. Such an oscillation may be even visible to the input in a lower voltage (e.g. 1 V) AC range.
Another suspect would be hum on the supply voltages.   So as usual: check the supplies.

[t-17]

I agree with Kleinstein. First check all voltages, then inspect anything related to the AC path. Check the component list I've made. It could also be a problem with one of the many opamps, probably early in the AC signal path since anything AC related is having issues. Perhaps several parts have issues.

I'd check all signals around these parts for stability (there should be no oscillations), then also check if voltages around the opamps are okay and behave as one would expect.

[aronake]

There is a shared AC/DC converter for both the current and voltage readings.  So if zero current results in a non zero reading, I would expect a similar thing with AC voltage - it this happening ?

Testing is usually easier with the voltage ranges.

The frequency reading usually only uses a simple comparator for the input. So low slope signals are always a bit tricky to measure.
To high a reading could be some ringing or interference causing multiple counts in some cases, especially with a low slope, so the input is longer near the sensitive range.

There is a chance the input amplifier may be oscillating for some reason. This could explain trouble with the frequency measurement and the non zero reading. Such an oscillation may be even visible to the input in a lower voltage (e.g. 1 V) AC range.
Another suspect would be hum on the supply voltages.   So as usual: check the supplies.

Thanks for comments!

AC Voltage was also knocked out of calibration when this fault occurred. Not as bad as current though. With shorted probes it show 15 mV, with 5V test reference at 100 Hz it shows 4.88V and with open leads it shows 0.28 V.

So yes, AC measurement for both current and volt is wrong. And frequency. But DCV DCI and resistance is well within specs.

Before this incident the meter had no problem picking if the frequency signals in sine etc. My fluke 87V also can easily do it. So also something that got impacted with this error.

The error is also dependent on what range. So jump around when switching range. Particularly when the relay is clicking for new range.

[aronake]

I agree with Kleinstein. First check all voltages, then inspect anything related to the AC path. Check the component list I've made. It could also be a problem with one of the many opamps, probably early in the AC signal path since anything AC related is having issues. Perhaps several parts have issues.

I'd check all signals around these parts for stability (there should be no oscillations), then also check if voltages around the opamps are okay and behave as one would expect.

Thanks. Will do so.

Any though why different ranges show different error with same input signal?

[Kleinstein]

The AC input takes some time (e.g. 1-5 seconds ?) to settling when switching range - that is normal with the analog RMS-DC converter.
For testing it would be better to use manual ranging - autoranging only add complications not needed, as it may not be clear which range is used and there may be constant up/down jumping.

Different ranges use different amplifcation setting at the front end an can thus be effected different. Especially the higher voltage (e.g. > 1 V) ranges will have some "divider", which often is a different input path.
The voltage and open input case is of limited use, as it can easily pic up some hum.
DCV and DVI have a different input parts. The problem is likely in the AC input amplifier or RMS to DC converter. After the RMS converter the signal usially joins the DC path somewhere.

15 mV with a short is about as bad as the current readings.
The deviation seems to be combination of some offset/background and a scale factor that is a little low. So different range may be effected differently.

Without knowing the circuit configuration it is a bit tricky to localize the error more.
After checking the supplies one could look at the input to the AD637 - ideally with a scope, to see if the problem is more with the amplifier or the converter itself and maybe the connected filter.

[aronake]

The AC input takes some time (e.g. 1-5 seconds ?) to settling when switching range - that is normal with the analog RMS-DC converter.
For testing it would be better to use manual ranging - autoranging only add complications not needed, as it may not be clear which range is used and there may be constant up/down jumping.

Different ranges use different amplifcation setting at the front end an can thus be effected different. Especially the higher voltage (e.g. > 1 V) ranges will have some "divider", which often is a different input path.
The voltage and open input case is of limited use, as it can easily pic up some hum.
DCV and DVI have a different input parts. The problem is likely in the AC input amplifier or RMS to DC converter. After the RMS converter the signal usially joins the DC path somewhere.

15 mV with a short is about as bad as the current readings.
The deviation seems to be combination of some offset/background and a scale factor that is a little low. So different range may be effected differently.

Without knowing the circuit configuration it is a bit tricky to localize the error more.
After checking the supplies one could look at the input to the AD637 - ideally with a scope, to see if the problem is more with the amplifier or the converter itself and maybe the connected filter.

Hi, thanks for comments!

Driving voltages looks stable and nice. Will still swap all electrolytes.

Yes, AC input path ICs or the RMS to DC converter clearly the suspicious items. I put an order of all chips in should have them here in a couple of days.

[Kleinstein]

With the AC part one can usually pin-point the defect better than just a curde swap all caps and chips. When changing OPs one may need to do a new adjustment of the frequency flatness. The AD637 is a relatively expensive chip. One can usually at least check at the input of the AD637, if the signal there is OK. This would tell if the problem is with the amplifier or the AD637 and maybe behind this.

It is not so much about the cost of the chips, but the indirect changes and possible stress to the PCB, that one should test more to narrow down the defect more to individual OPs / CMOS switches.

Another way for checking would be to apply a test signal AC signal and then look at the output of the OPs in the area in the different ranges. This would help to know which chip is used in which range. This can help to get a crude idea of the circuit and signal paths.
If some OP is oscillating the defect could be as simple as a cold solder joint / broken (litterally mechanical) small capacitor.

[aronake]

With the AC part one can usually pin-point the defect better than just a curde swap all caps and chips. When changing OPs one may need to do a new adjustment of the frequency flatness. The AD637 is a relatively expensive chip. One can usually at least check at the input of the AD637, if the signal there is OK. This would tell if the problem is with the amplifier or the AD637 and maybe behind this.

It is not so much about the cost of the chips, but the indirect changes and possible stress to the PCB, that one should test more to narrow down the defect more to individual OPs / CMOS switches.

Another way for checking would be to apply a test signal AC signal and then look at the output of the OPs in the area in the different ranges. This would help to know which chip is used in which range. This can help to get a crude idea of the circuit and signal paths.
If some OP is oscillating the defect could be as simple as a cold solder joint / broken (litterally mechanical) small capacitor.

Thanks for more comments! Much appreciated! I bought most chips, just in case, so i don't end up figuring out i need something I don't have when i am repairing. The AD637, as you wrote, the most expensive.  Around 30 USD form Mouser. Then around 20 USD altogether for one or two of the other potential suspects. Then a new set of electrolytes as those that are now in the meter have been around for 15 years or so.

So with the extremely good mapping of purpose of chips in this thread, I am quite hopeful to pinpoint the problem and swap out the correct chip only.

Again, thanks for comments. Will be out traveling for some time, so may first get to dig into this in some time. Will report back on this, and if I do any other findings.

[aronake]

Hi repair wizards!

I now been looking more into my 8845a.

I discovered that the negative 15 V supply line is negative 7 V and the mosfet controlling is much hotter than the 15 V positive mosfet. All other voltages seems correct.

This makes me suspect that there is some short circuit or some other issue that draw way too much current from negative 15 V.

I have checked negative 15V in input pins on OP amps etc that use negative 15 to confirm that problem is everywhere where negative 15V is used, which it is.

I checked if any component hot, but none except the voltage regulators and power mosfets.

if it would be possible to measure current this could be a way to trace the problem, but since everything soldered on a PCB it is very difficult to measure currents.

Or could it be the regulation circuit for the mosfet that have issues? A LM358M is used. I cant find any reference voltage from any zenerdiod thought. Are there any OP amp + mosfet voltage regulators that do not use any zenerdiod (or similar as voltage regerence)?

Any suggestions on what to check next?

[Kleinstein]

One point to check is if the supply is DC or oscillating.

In quite some older circuits they use one supply (e.g. the positive) as a reference for the opposite sign reference.

Most if the OPs would be using something like a +-15 V supply, so this would be the same current for the positive and negative side.
It would however make a difference of the output of an OP is shorted to ground or similar.

Another point to check is the voltage on the input side of the regulator - with a bad fitler cap one could have excessive ripple there and this can make the voltage drop at least on average.

7 V is still not a dead short - a short from a MLCC is nothing so unusual. With extra series resistors this would usually give quite hot resistors, possibly even burnt ones.  This would also cause a much lower voltage for the effectged part (e.g OP).

[aronake]

One point to check is if the supply is DC or oscillating.

In quite some older circuits they use one supply (e.g. the positive) as a reference for the opposite sign reference.

Most if the OPs would be using something like a +-15 V supply, so this would be the same current for the positive and negative side.
It would however make a difference of the output of an OP is shorted to ground or similar.

Another point to check is the voltage on the input side of the regulator - with a bad fitler cap one could have excessive ripple there and this can make the voltage drop at least on average.

7 V is still not a dead short - a short from a MLCC is nothing so unusual. With extra series resistors this would usually give quite hot resistors, possibly even burnt ones.  This would also cause a much lower voltage for the effected part (e.g OP).

Awesome! Thanks a lot! Found it! Cause of the -7 volt was a somewhat shorted tantalum capacitor between ground and the output of the power regulating mosfet. Removed it, and now -15V! Replacement on the way. This cap is located just next to the hot power mosfet, so at first i didnt find it, as i didnt want to touch to close to this mosfet as it was burning hot.

Now DC is working properly, AC volt and AC current still off. Will do some tracing of AC signals and see what i find. Maybe just putting back a working tantalum capacitor may help.

Thanks again!!!

[aronake]

Kleinstein, t-17,

You are true superstars! Thanks to you I now have a fully functional 8845a, and learnt a load about electronics repair! Hats off! And it even measures more accurate now than before.

Showed out that removing the -15V 22uf tantalum capacitor brought DC measurements back to spec. Soldering on a new also brought AC measurements back to spec!

I also have 100 USD worth of AD637, OP amps etc that I bought and never had use for. But they may come to use some other time.

I thought I had checked the power rails before first, before looking into more complicated stuff, but apparently not.

Its a really great feeling to manage to repair something like this!

Again, thanks!!!

[yildi]

Hello all together,

I also have a Fluke 8845A that no longer works properly. To be exact, only the 400 mA current input is not working. The 10 A input and all other functions work. I checked the fuses, of course, they are both fine!

Since my unit is still sealed, I have not opened it yet. First I wanted to do some research, but unfortunately, I haven't found anything about this exact problem yet. Has anyone of you ever had such a problem? It would also help me a lot to have a high-resolution picture of the board to be able to estimate how promising a self-repair is. Maybe someone has tips and/or a good picture of the PCB.

Thanks a lot for your help!

EDIT: With "the 400 mA current input is not working" I mean that the displayed value is in the uA range and changing due to noise. However, when connecting a current source the displayed value does not change. In fact, no current from an external current source is flowing into the 8845A.

[bdunham7]

https://xdevs.com/fix/f8846a/

Are all the low current ranges inop or just the actual 400mA range?

[yildi]

None of the manually adjustable ranges of the 400 mA connector work. Thanks for the link, I will check that out!

[bdunham7]

None of the manually adjustable ranges of the 400 mA connector work. Thanks for the link, I will check that out!

OK, just be aware that there are multiple shunts involved then, and if the fuse is actually good (I'd triple check!) and hasn't been replaced with a wrong one sometime in the past, you hopefully will have a straightforward wiring or connection problem.

[yildi]

Hi again,

I finally found some time to analyze my Fluke 8845A again. As before, the 400 mA current inputs do not work in either AC or DC mode. All other functions are running perfectly.

Here's what I checked so far:

• The fuses are fine
• The relays all switch correctly

Referring to the attached image I did the following: First, in 10 A DC current mode, I measured the resistance from the 10 A input jack to various soldering joints (marked in blue in this image). As can be seen, a number of points are connected with the jack (again: the 10 A input is working fine).

Then I changed to the 400 mA DC current mode and measured again the resistances from the 400 mA input jack to various soldering joints on the PCB (marked in red). As you can see, only this inductor (L9) is connected but nothing else! In particular, there is no connection to the fuse, which from my point of view must be wrong.

Therefore I suspected the relays or their drivers to be faulty. But as I wrote above, I have already checked them. Depending on the mode, these relays (K2 and K3) switch properly.

At the moment I don't really know what to do. What would you check next? Or does anyone know to which part of the circuit the inductor L9 should be connected?

[bdunham7]

At the moment I don't really know what to do. What would you check next? Or does anyone know to which part of the circuit the inductor L9 should be connected?

Your problem is likely to be with the front/rear switch.  I would first try switching to the rear inputs and trying the rear 400mA inputs.  Then remove the front/rear switch and have a look. 

[yildi]

That is a good hint, thanks! The rear connector is also not working and I cannot find any soldering joint on the pcb that is connected with the rear/front inputs. It seems that I have to unmount the complete circuit board out of the housing in order to reach the bottom's switch mechanism.

[Remdale]

The entire ACV/ACI measurement was gone, but came back after replacing U18 (AD673) and U20 (AD8510).

Hi. I got mine with the overload problem. But replacement of U18 and U20 didn't help. What could be the other possible reason?

[aronake]

Ian Scott Johnston is making some Fluke 884Xa work here. May be helpful for anyone trying to fix theirs.

[Remdale]

Great one, thanks for this! Waiting for part 2