FLUKE 87III MULTIMETER REPAIR

[pierrechan68]

I have a fluke III multimeter, as 9V battery no power, i use a power adapter to provide power, unfortunately, i make wrong polarity, + and - are change with wrong polarity. the meter light up with beep sound continously and the reading are show on the screen. but  the reqading are not accuate, which part sould i change to repair, thank you.

[shakalnokturn]

The Fluke worshipers are going to be frowning at you!

Is it an impression or has the small tantalum capacitor near PCB edge on underside turned from yellow to brown?

[strawberry]

fuse
main controller could have separate 3.3V or 5V regulated
If MCU operates normal then some op amps maybe

[pierrechan68]

I FOUND 2 transistor maybe burn. THE CODE is P99 and JC2 on the transistor. what is these true value? thank you.

[floobydust]

NEVER run a handheld multimeter from a bench power supply or wall wart. Why are people so cheap.
Because input com(-) is not DC power(-) and any short or voltage forced between them will blow up the main multimeter IC or diode CR5. Check CR5.

For the Fluke 87III, I'm not sure if I have the specific service manual for it. edit: 87IV,V I think: It has three voltage regulators (power rails) U10 making 5V, U6 making 3.3V and U8 making 2.5V. BUT to get +/-2.5VDC and +0.8VDC, they use a floating common to the +2.5V regulator as analog common. Yes it's confusing.
Running the meter from a 9V battery, check the three rails with another multimeter its (-) to the 9V(-).
Should measure around 9V at TP16 U10 input, 5V at TP17, 3.3V at U6 output or C13, 2.5V at UI8 output or C16.

added block diagram and typo up top? VDD should be +3.0V not -3.0V I think.


I would think the main multimeter IC is blown, it usually gets damaged because it's only a 5V part. Or dual diode CR5, or Q8 have failed but the power would not switch off.

Power Switch
Q3  NPN BJT, MMBT3904, Fluke 742676
Q8  N-ch MOSFET, 2N7002, Fluke 927538
Q10 P-ch JFET, SST175/J175, Fluke 832477. SOT-23 DGS, SMD marking code "S5" or "P05".

CR5 dual diode, BAV99, Fluke 742320

Input Protection clamp
Q13,Q14  NPN BJT, MMBT6429, Fluke 912469, SMD marking "1KM" or "M1L".
Q1,Q2,Q6 NPN BJT, MPS6560 but also given as SMBT1388LT1, hand selected for low IEBO, Fluke 821637

Backlight etc.
Q5,Q11 PNP BJT, MMBT3906, Fluke 742684