Fluke 87 III 4 1/2 button causes reset

[fls]

Hi,

I have a Fluke 87 III with a strange problem.
When I press the backlight button I hear the short bleep, but no backlight. I actually do not care much about it, but I tell you this because of my 'real' problem.
If i hold the 'backlight' button for more then 1 second to enable the 4 1/2 mode, the meter reset.

The backlight led is actually fine, I dismantled the meter and removed the flex cable with the led to see if it shorted out or something. But the led is fine.

Anyone have any suggestions?

[mzacharias]

The III model does reset to enable the 4 1/2 digit mode. The V model enables the high resolution without a full reset. I double-checked on my III to be sure.

[fls]

Oh my! you are right!
That's the problem having multiple fluke multimeters from different series...!

Ok, then one thing stands, Why is my backlight not lighting up?
I tested the led (and flexcable)...

[retiredcaps]

Ok, then one thing stands, Why is my backlight not lighting up?

The schematics in service manual for the Fluke 80 series I might be close enough to help you troubleshoot the Fluke 80 series III backlight circuit?

[mos6502]

Does your meter have a green backlight or a white one?

[fls]

Thanks for the replies!

(My backlight is white.)

I think it is Q7. Q5 is not getting any signal when pressing the button and I followed the trace to Q7. Q7 is getting signal from R53, but does not change the output, hence Q5 is doing nothing.
One thing that I really do not understand. How on earth can Q7 blow? It is just switching Q5. I really doubt that there is any reasonable amount of energy flowing through. It is a MMBT3904 that can handle 250mW!!!

Anyway, I'm going to replace the transistor and see if that fixed it...

[fls]

For-people-who-want-to-know: After replacing the Q7 transistor, everything is fine!
My best guess that is was physically damaged for some reason. While desoldering the transistor broke in half...

[retiredcaps]

For-people-who-want-to-know: After replacing the Q7 transistor, everything is fine!

Thanks for following up and reporting your findings.

[Resonant]

I had 2 Fluke 87-III both with non working backlights, after tracing the signal through R53 which was OK. Checking Q7 with another multimeter as if it were 2 diodes in both cases Q7 base-collector were faulty. Changing Q7 has given a complete repair. Looks like it could be a more common problem than you would expect.