Fluke 87 III - current measurement and backlight problem

[cairn7]

Hi all,
I bought a Fluke 87III just to find out that it doesn't work properly,

Multimeter's in good condition, mV, V (AC/DC), continuity, diode test, buzzer, resistance - no problem at all with these ones.

The problem is with:
1). Current measurement doesn't work properly (same issue for the AC and DC range).
For example, I tested the lamp and other two Fluke 87V units showed proper value at 47mA, this 87III unit showed 1,05 mA (stable 1,05mA but incorrect value),

What I've checked:
- When this unit arrived, both fuses were blown and both were not the original ones, I replaced them with the new ones.
- I checked the fusible resistor, seems to be okay (1k).
- Replaced the battery.
- Cleaned the rotary switch.
- Visually checked whole PCB from the both sides, everything seems to be fine,

2) Backlight:
This function doesn't work at all.
I tested the diode under the display and it's also okay,
Any suggestions would be much appreciated, I can take some photos of the PCB if needed.

[cairn7]

I did some more tests this morning,

mA/A issue:
Tested the shunt resistance, seems to be fine (for A and uA),
Tested the bridge rectifier with the diode - both components are shorted, I need to order replacements (DF06S + VISHAY S1D), hopefully only these two components needs to be replaced.

Backlight issue:
MMBT3904 transistor is damaged

UPDATE:
When I was looking for a backlight issue - I've checked three transistors + few resistors (around the backlight connector) and compared resistance values from good and bad 87III unit, now backlight doesn't work on good 87III unit , how could I damage any of these components only with a simple resistance test? I tested all of the components with a Fluke 87V, battery was disconnected.

[m3vuv]

how come your english grammar is so bad,are you a native?,or an import??

[cairn7]

None of the above "mate", English is not my first language, not even second... hope you can speak 3 languages perfectly. Cheers.

[james_s]

Why is that even important? Not everyone is a native English speaker, so what, I could understand him just fine. It is not remotely helpful to chime in just to complain about a person's grammar.

Anyway, unfortunately I do not know what's wrong with the meter or I'd be happy to assist. Does Fluke offer a schematic for the 87? I've never had a problem with mine so I've never had a reason to look.

[srmahaffy]

Hi Cairn,
I wanted to share a couple ideas on your backlight problem of your 87iii. I purchased an 87iii meter a few months ago and it had a backlight problem. It turned out the root cause was the little 2 pin ribbon cable wasn’t pushed down all the way to the J2 connector. Before I discovered that, I did the following steps to troubleshoot:

With the battery disconnected:
- did resistance check of Q5 and Q7.
- continuity check from Q5 emitter to J2-1 mating ribbon cable
- tested the LED by probing across the two pins of J2 using the diode mode of another meter, to make sure the LED lights up. ( you said yours is good )

With the battery connected:
- looked at voltages of Q5 and Q7 voltage levels. from my notes. VSS = -3.2V, VBT- at Q5 collector = 5.5V. VBT+ at J2-2 = 3.5. I verified/ checked to see if voltage state changes at J2-1 and  Q5 emitter when shorting across backlight push button pads. With J2-2 (VBT+) being 3.5, J2-1 needs to switch to a low voltage to make the LED turn on.

I hope your ribbon cable was just not pushed on all the way like mine, and also with your other meter!
 I attached part of the schematic.

[srmahaffy]

Here’s a pic of voltage level information from the 83, 85, 87 service manual.

[james_s]

That's a strange arrangement, why are they using a PNP transistor as a low side switch? I don't think I've ever seen it done like that before.

[bdunham7]

That's a strange arrangement, why are they using a PNP transistor as a low side switch? I don't think I've ever seen it done like that before.

VBT- is 3-4 volts lower than VSS, if that explains it.  I don't know if it does, I didn't try to work it out...