Electronics > Repair
Fluke 8808A not measure
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eduaqa:
I bought this multimeter as part, and I'm trying to fix
Initially when powering on, it was beeping, beeping and display was come on and off fast
Doing a initial investigation I notice that the 7805 marked was warming ... I removed it and test the rail that it supply voltage and was short circuited
I discovered to IC (latches) marked on the picture in short ... I replace the 2 IC and the rail now is having around 24ohms resistance, which still seems having some type of short circuit, because is cause a consumption of 370mAH, what seems too much, because when compared with the others rail which read very high resistance

When I turn it on now, the display works, the push button on the front change as should, however none of the measures works
I noticed that the 7805 is warming, as it should because it's providing 370mAH, also I notice that doesn't matter what function I choose I don't hear relays "clicking" and relays are also warming ... also measuring the voltage on the relays S and R coil that have around 4V, on both ... seems strange, I understood that should not have voltage on both at the same time, and maybe even not continuous

Any help are welcome
Harry_22:
Hi!

Did I understand correctly that the software part of the multimeter works well now and all modes switch correctly. But you don't see any measurements and the relays don't click.
It feels like the measuring part itself is not controlled and seems to be missing.

Could you attach a photo in good resolution so that the names of the chips can be seen?
eduaqa:
I'm adding some more pictures which has text of notes on them.
I replace the MOSFET that activate the relays coils but just realize that probably I used wrong MOSFET, because 1 side of the coil goes direct to the +5 rail, the other side of the coil goes to the source of the mosfet (yes strange because normally goes to the drain, I think is call high side). This assuming that the device that I noted is a MOSFET, than the drain goes to 0v ... in this scenario assuming it's a N channel (what I think I assumed wrong) the MOSFET will acts as a diode and relay will always have at least around 5v on the coil
Look the picture where I noted as "what device is this?". I assumed a nchannel MOSFET but seems that is not, because of the explanation above
eduaqa:
Some pictures ... and more, it's no allowing me at onc3
eduaqa:
More pictures with notes
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