Fluke 87III broken Current Input

[Cody Turner OKC]

Hi everybody, so I had a previous post in the beginners section about this meter and I didnt have proper fuses and how I got the meter ETC,
https://eevblog.com/forum/beginners/fluke-dmm-really-need-help/

So once I got he fuses, I tried to measure current on both the mA and A range and going from 0 to 400mA read 0-19 or so mA on the screen, going from 0-3A (in the A jack) the display read from 0-192mA consistently, I am very young but I had a few people at a local ham meeting place help me with there power supply etc,

They all said its out of calibration, but the volts ohm diode capacitance etc all read perfectly fine and great! So i wasnt convinced.

so I looked over the entire board for hours, and finally noticed that the bridge diode and one diode beside it have tiny burn marks, and the bridge is shorted from every terminal, on diode reading i get 0.005v drop with my other cheapo meter, and resistance i get 3 ohms consistently from every terminal to any other BUT my leads measure between 2.8 and 2.9ohm shorted and theirs no REL on the cheap meter, but I cant see how it would effect the reading on the A range..

When I get home in about a hour I will post clear pictures of the front and back PCB, I can only find service manual for the 87 (not III) so i am not sure if its different but after studieng it for hours now I cant see how the diodes protection all being shorted could effect the A range,, or any other components in the path to the Chip U4 , but if it was the big fluke chip I wouldn't think the rest of the functions would work fine but really what do I know haha, so any help would be greatly appreciated! I will post pictures and the schematic very soon! thanks in advance!

P.S. the diodes were all shorted weather I left the good fuses in, or took them out if that matters

[Zermalmer]

The Service manual you will find directly at the fluke server

http://assets.fluke.com/manuals/8xiii___sieng0200.pdf

It is for the models 83 III/ 85 III / 87 III

[Cody Turner OKC]

Hi, thanks, I went through that yesterday and it is "Service information" unfortunately , it just goes over the basics like the user manual, change fuses fresh battery and calibration procedure for dc and ac voltage, but as my meter is reading dc and ac voltage and all other functions just fine i dont think i should adjust it, it is only the current readings that are way off

[Zermalmer]

as far as I know the current range is based on the voltage measurement and the used current shunts of the meter.
So, if the voltage is accurate, then you for sure have a problem with the shunts or the signal path to them.
You can't specifically make a justification on the current ranges.

I hope for you, that I am wrong...maybe someone else have a usefull hint.

[floobydust]

The diode + bridge rectifier is a protective clamp for the uA/mA shunt resistors and the DMM current ranges, in case of overloads.
The fuse is supposed to blow before the diodes, but if the diodes are shorted they are effectively resistors in parallel with the shunts and thus cause current readings to be low.

The older 83,85,87 series schematic shows the circuit and Series III is quite similar.

I think from pictures of 87-III the bridge rectifier is DF06S and SMA diode is Vishay S1D. Take pics so you don't put the new ones in backwards.
Careful not to bend the PCB and crack the ferrite bead filter by the fuse. That happens if a gorilla changes the fuse.

[Cody Turner OKC]

The diode + bridge rectifier is a protective clamp for the uA/mA shunt resistors and the DMM current ranges, in case of overloads.
The fuse is supposed to blow before the diodes, but if the diodes are shorted they are effectively resistors in parallel with the shunts and thus cause current readings to be low.

The older 83,85,87 series schematic shows the circuit and Series III is quite similar.

I think from pictures of 87-III the bridge rectifier is DF06S and SMA diode is Vishay S1D. Take pics so you don't put the new ones in backwards.
Careful not to bend the PCB and crack the ferrite bead filter by the fuse. That happens if a gorilla changes the fuse.

when I got the meter, both fuses were blown, so maybe he tried using it still after and messed it up more, the bridge and diode are both shorted together any way i try and shorted to both fuse holders on one side even through the diodes! the bridge is a Dfo6s, I cant see the diode, I will have to look for some of those to try and replace, do you think anything else could have potentially been damaged if he overloaded it bad enough to blow all the diodes and fuses?

[floobydust]

If the diodes shorted, they would have protected the shunt resistors- which should have smoked if otherwise overloaded.

Test the shunt resistors. mA range 0.99R big black resistor near fuse, and the uA range small 110.1R resistor.

Another approach is to remove the shorted bridge rectifier + diode. Then with another meter measure ohms between the COM and uA/mA jacks with the rotary switch in either mA/A or uA. You should see the (total) shunt resistance of 1R or 100R.
The DMM should also now measure current fine- it's just unprotected with the diodes missing so don't run it like this.

[retiredcaps]

So once I got he fuses, I tried to measure current on both the mA and A range and going from 0 to 400mA read 0-19 or so mA on the screen, going from 0-3A (in the A jack) the display read from 0-192mA consistently,

So to clarify, if you measure a known steady current, like 275mA, the 87 III will show any number between 0 and 192mA.  Does this number fluctuate or just stay at one constant number?

[Cody Turner OKC]

It slowly raises as the current increases, the amps range was going from 0 at 0 amps, to like 80mA when I applied 1.5amps, and then with 3A input the meter read 192mA ,consistent up and Down,

For a analogy, say I have a 0 to 10 volt power supply, and I make a divide by 10(or so) voltage divider and I slowly go from 0 to 10 volts the output follows from 0 to 1 volt,

So my 0 to 3A input to the meter follows it showing 0 to 192 mA o. The display give or take