The fourth generation of the classic Fluke 77 shares the same basic design and circuit board as the rest of the 170-series (175, 177, 179). Apparently a common problem with these circuit boards is the appearance of a white powdery residue that collects in certain areas. A couple have passed my workbench, and I’ve heard reports of more.
The source of this residue is unknown to me. Chances are, it is probably environmental, perhaps humidity. What is clear however, is that the residue can be accompanied by the corrosion of parts of the copper PCB traces. [Added later: best theory is water-soluble flux residue has reacted with humidity to create a corrosive salt.]
This 77-IV was advertised as working, but missing its backlight and continuity beeper functions. This description was accurate. It has been well-used and is very grimy. Other than the overall dirty appearance, there is no real evidence for what sort of environmental conditions this meter has been used in. Salty air? High humidity? Who knows…
Inspection of the PCB reveals various locations on the board with concentrations of the mystery residue. The area around the piezo beeper contacts shows a particularly heavy concentration of the stuff. There are other deposits, including some around the backlight LEDs. None of these areas are near the battery compartment, therefore battery leakage is not suspected.
The usual procedure for PCB contamination is a good scrubbing with a toothbrush and 91% isopropyl alcohol (IPA). After this procedure, the PCB is allowed to dry thoroughly. This gets rid of the white residue. Examining the entire PCB closely with a magnifying lens reveals only one area of permanent damage. One of the copper traces connected to the backlight LED circuit has the appearance of being completely eroded away, breaking the connection. This damage is repaired by scraping some of the solder mask away, applying flux, and soldering a tiny piece of 30AWG tinned wire over the break.
After the repair, all functions including the backlight and continuity beeper work. A thorough clean-up and lens polishing also improves the appearance.
Epilog: The next unit I found with this copper corrosion problem was a 177, with the exact same PCB revision as this 77-IV. Unfortunately I did not photo-document the repair procedure, but it was quite similar to one described here. One of the pads for the backlight LEDs was eroded away causing the backlight not to work. The meter was non-functional with some digital-domain fault that caused it to always display “- – – -“. The trouble turned out to be a corroded via that was open from the top layer to the bottom, due to the plated-through hole being eroded away. A wire patch soldered in brought the 177 back to life.