I've seen a few YouTube videos where the presenter, who appears to be experienced in board repair of Apple (and presumably other) circuit boards recommends to use your multimeter in "diode mode" to make measurements from a suspected problematic part of a circuit, and compare to a known good board. The boards are not powered during the measurement.
Importance of DIODE MODE measurements for troubleshooting - Louis Rossmann
In both cases they recommend putting the red lead to ground and the black lead to the test point (around 6:40 in Jessa's video, and 2:45 in Louis' video).
The advantages are apparently that diode mode measures somewhat faster than simply measuring resistance. In my testing resistance mode took around a second to measure while the multimeter auto-ranges, but diode mode seemed virtually instantaneous.
They both recommend finding a problem connector or IC, and then (with the board unpowered) take a measurement of each pin, and write that down, and then compare to a known good board in the same place. Any readings which are substantially different point to a possible problem.
My questions are:
Why reverse the polarity? You are injecting a negative voltage into parts of the circuit which presumably are designed for positive voltage.
Won't injecting negative voltages damage the underlying circuit? I measured three multimeters I have here in diode mode and found that they used:
- 7.3V at 1.0 mA
- 3.3V at 1.4 mA
- 2.5V at 0.91 mA
I would have thought that injecting -7V into a logic board would cause assorted problems, but both of these presenters are swearing by that technique as a way of quickly debugging. Maybe their meters only inject 3.3V but even so.
From a comment:
how many volts/mA does the ohm feature use?
Same meters in the same order:
- 2.77V at 1.0 mA
- 0.48V at 0.65 mA
- 0.53V at 0.31 mA
A possible explanation?
Thinking about the questions above, particularly the one about "why use negative voltages?" I have come up with one possible explanation:
If you connect (say) positive 3.3V to a board then parts of it will try to power up. For example, if you connect to Vcc of a chip, the chip will try to power up, or if you connect it to a data line it will parasitically power up.
The resulting readings won't prove a lot. Effectively your multimeter has become a rather under-powered power supply.
However by injecting a negative voltage then the main chips will reject that (through their protection diodes) and thus won't power up. What will be left is the "path to ground" via various resistors and voltage dividers. This would more readily pick up missing or bad resistors, broken traces, bad connections, etc.
Does this sound plausible?