Electronics > Repair

Fluke 87V displaying dashes only

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Multiple Cheese Slices:
Hello everyone!  ;D

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/0c5k3f   as 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 far  :(

Has 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  :-[

Manul:
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?

Multiple Cheese Slices:
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  | floating
on  : | LOW      | floating | HIGH     

Some 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 forward  ^-^ Thanks!


P.S. Oh, and regarding the anomaly on the traces in the picture - it's not there anymore, must have been a dust particle.

Manul:
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.

Miti:
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)?

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