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