87v, no current reading?

[RogerThat]

hi all,

Bought a Fluke 87v recently advertised as "As new". It is like new on the outside and inside but the current range is dead.
I've checked everything in the path and everything checks out all the way to U1, I'm suspecting this one is broken.

When measuring current the indication jumps when there is a transient on the input, at steady input it slowly goes down to 0.04mA(or something like that, completely de-attached from reality).

My thinking is that it have been overloaded on the amps input and that U1 is broken to the point that there is only capacitive connection between the input pin and the internal circuitry, hence the jumps when the input changes.

Anyone had this problem or have further ideas?

[jwet]

Don't get upset if this is too simple, did you check both fuses?

[RogerThat]

Not at all, sometimes one gets blind for the obvious, but they are fine.

[BILLPOD]

Goof Morning RogerThat,  If the seller described it as 'NEW', and some functions don't work, then you should be able to get your money back, or at least a healthy portion refunded.

[RogerThat]

Good morning(or evening here)  I tried to get some refund but he only accepts a full return if I wish. The price was good(and with some nice accessories) so if it's just a 10€ IC to repair it I will keep it.

[floobydust]

Don't stay too strong on your theory U1 is damaged. How does it perform on the basics like DCV or Ohms?
For current-function problems, check the usual fuse, shunt resistance seen, bridge rectifier CR2/CR1, jack detect, ferrite-beads etc. U1 is not easily killed reading current.

[shakalnokturn]

It reacts to transients but will it display anything tangible to low frequency ACA input?

[Shock]

Check there are no socket insert faults on the other sockets pins and short the two contacts on the two sockets behind the pcb to emulate shorting two inserted probes. Check correct fuses are installed and with them removed check resistances back to the socket. Replace battery as well with new, clean the pcb switch traces and check the contacts are clean and not broken. Believe it or not this has uncovered several problems before.

If you are attempting repair though like this keep it. It's not cool to work on stuff you are sending back. In a reverse situation noone wants someones failed repair attempt.

[Manul]

Look at the schematic diagram and measure between TP12 and TP9. Do you get a reasonable and proportional voltage based on current range and current? There is 1M series resistor going to U1, so it is highly unlikely that current overload damaged U1. It would need a lightning strike.

[bdunham7]

Are you sure it is set to DC current? 

[RogerThat]

Are you sure it is set to DC current? 

No 

I just assumed it defaulted to DC like my other. All good. At least now I got a good insight how it works.

[floobydust]

Are you sure it is set to DC current? 

Good point and it brings up a new question - what happens when measuring AC current with a DC component? A few threads talked about multimeters malfunctioning when measuring AC voltage with DC present.

With the 87V there is no coupling cap for the current function, I expect drama.

[RogerThat]

I think I have tested that  . In AC mode I believe U1 just routes the signal out to U12 which in turn muxes it to the RMS/AVG circuit via the coupling cap C6...it's there and the reason I just saw readings during transients. Output from the AD737 goes back to the U1 DC amp and on to the ADC.


Regarding DC voltage offset, the meter is floating so any DC offset would be invisible to it.

[jwet]

I wonder if there is a hairline crack in the board somewhere- this would look sort of capacitive.  Someone jamming in a banana jack?  Can you see continuity through the current circuits from the outside (with another meter)

[tooki]

I wonder if there is a hairline crack in the board somewhere- this would look sort of capacitive.  Someone jamming in a banana jack?  Can you see continuity through the current circuits from the outside (with another meter)

It pays to read the whole thread before responding, since the problem has already been identified.