Fluke 77 IV - Repair

[MichaelPI]

Hello,

I bought a defect Fluke 77 on ebay some weeks ago. Initially, I thought it could not be that difficult to fix it, but after a couple of hours digging around without a schematic of this device, I have to admit, I need some help to get the Fluke 77 working again.

I inserted a new 9 V battery into the battery holder and hoped it will power up, but the LCD did not show any signs of life.
After opening up the case, I recognized, the previous owner must have tried to repair it already. The screw underneath the fuse holder was missing (the one that is connected to the shield). Furthermore, there were finger prints directly on the LCD and the 400 mA fuse was blown. So someone must have tried to bring it back to life - obviously without success.

As I had already read of issues with the LCD's conductive rubber strip, so I cleaned it and put every back together: No change at all.

The overall condition of the PCB and the components is good (no burned components, no burning marks on the PCB). Except one thing: I observed some flux residues (already cleaned with IPA) onto the PCB. One trace and via seem to be corroded (picture attached). I checked the continuity of the trace and it seems to be alright.

I measured the current consumption in different rotary switch positions. I looks like it does not drain any current at all. Furthermore I checked the input voltages of the two LDOs (TPS71533DCKR digital and TPS77050 analog), I could not measure any voltage referenced to battery minus (inputs of both LDOs are connected). I guess, there is something wrong with the connection of the battery plus pole - the via could be the cause of it, but I do not want to do this surgery yet, unless I gained more confidence that this is likely the root cause.

The only components that are currently directly connected to the battery plus are the CD4069 (piezo driver) and the backlight leds. I was supprised, that non of the rotary switch pads seem to be directly connected to the battery plus pole (at least my unit does seem to have any connection). Maybe someone has more information on that.

There is also this mysterious circuit (marked as 'purpose?' on the image) - which could be some sort of switching logic to apply the battery voltage to the MCU and the analog circuitry, if the rotary switch is not in the off position. Vbatt on the images is only checked by continuity testing, except were 'measured' is written.


Many thanks!
MichaelPI

[Andy Watson]

Try this:
https://elektrotanya.com/fluke_70-series_sm.pdf/download.html

Actually, your PCB looks like a much later version - try here for the series II and III service manuals:
https://mods.dk/manual.php?brand=fluke

[MichaelPI]

Thanks - sorry, I did not mention, it is Series IV Fluke. But I will check the Service Manuals of the earlier versions.

[retiredcaps]

There is no service manual for the 77 IV, but check out the following modemhead posts.  They may yield some clues.

http://mrmodemhead.com/blog/fluke-77-iv-copper-corrosion/

http://mrmodemhead.com/blog/fluke-26-iii-corrosion-repair/

http://mrmodemhead.com/blog/fluke-23-iii-dmm-repair/

[floobydust]

Just in case the 77-IV power section is similar to the 87-V, I'll post this snippet. It seems to use a discrete latch Q13/Q14 which I suspect is your mystery section.
I could not read your SMT makring codes, Q13 Pch mosfet is IRLML6302 with Q14 MMBT3904 in the 87-V.

edit: note battery(-) is not circuit common or GND in most digital multimeters. Here it goes to the -2.5V rail and COM is derived from a voltage reg IC.

[mzacharias]

Thanks - sorry, I did not mention, it is Series IV Fluke. But I will check the Service Manuals of the earlier versions.

Email me at:

mark_zacharias@sbcglobal.net

I will reply with JPG's of the 77IV service manual including schematics. It is the US military BN version (Navy, Marines). Same thing.

[MichaelPI]

Thank you all for your kind support!

I managed to get it working again (sorry, bad pictures - the camera seems to need a repair...). I had to create a bypass for the defective via, but it finally works. I will try to fix the via somehow - don't know, if this is going to work. Anyway, I don't really want to leave the wire flapping around in the case.

@floobydust: It is indeed the same circuit. The transistor on the left side of the picture is the PMOS (package marking CBF) from Infineon. The bipolar has the marking M1L7.

@mzacharias: I have got your email - thanks!

Next steps in the next couple of weeks: Checking the multimeter for any hidden issues, ordering parts (fuse, ...), cleaning and calibration against my Prema 5000! I will keep you updated.

Thanks again.

Regards
Michael

[tgthiencl]

Thank Bro . My Fluke 179 worked !

[malwnakos]

I have  Fluke 177 and after crash my lcd and one trasistor broken. When you power on backlit flashes. Transistor is lost to find the same. I found from another fluke 177 one photo and transistor   says on top 1RN. Could you please anyone verify if it is correct and which type- p/n is this part?  Thanks in advance

[shakalnokturn]

MMBT5089

[Paceguy]

I have a 77 IV that I picked up earlier this year. Problem is everything appears to work EXCEPT the backlight and piezo beeper. I purchased a replacement piezo beeper since the one in mine appeared to be damaged. I looked at the corroded trace blog by mrmodemhead posted here by retiredcaps and found copper corrosion on a trace exactly in the same location on the bottom side of the board as mentioned in the blog. I made the repair and now the backlight works! The beeper still doesn't beep. Made some measurements, no voltage reaching the beeper contacts. Does anyone one know which components drive the piezo beeper? Thanks in advance.

[malwnakos]

MMBT5089

The following is acceptable equivalent?  It wasn't available  MMBT5089 at this moment, should I order it or use BC847?? Also does anyone one know what drives this transistor or which part of multimeter? Thanks in advance.

[shakalnokturn]

malwnakos:

If you look you may find that the 87V service manual is close enough to be helpful, it seems quite similar.
It's downloadable here:
https://xdevs.com/doc/Fluke/87V/Fluke_87V_Full_service_manual_with_schematic_and_electrical_parts_list_Service_Manual-87VANFullService.pdf

That should at least tell you that among other things the MMBT5089 are used for backlight control.

Edit: Should also be of use to Paceguy. (CD4069 a quite common failure I believe...)

[malwnakos]

Thank you  for your kind support!

[MichaelPI]

The CD4069 drives the piezo buzzer. The ground line of the CD4069 is switched by a bipolar transistor MMBT5089. The transistor malwnakos has shown on the picture is actually the one for the beeper. The light is switched by another transistor (also MMBT5089) in the the digital section of the circuit board (see initial post - marked it as NFET, but it is actually a bipolar transistor).

In guess almost any high current gain npn transistor should do the job. The transistors are controlled by the microcontroller in the orange section in the initial post via 100k base resistors, hence the replacement transistor should have high beta.

Regards