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Time for either a low-ohm meter or brute force - a single fully charged AA NiMH cell & some short thick wires with negative to the ground plane and positive to the track I marked in yellow back on post #36..
Touch the wire very briefly, as quickly as possible, and see if anything smokes or gets warm!
If nothing happens, try slightly longer - but don't let the PCB track itself get hot. It's so small it will not take much time to heat and burn..
1.5V is too low to cause any harm to the electronics byt can often locate - or clear- a PCB short.
It needs to be a NiMH as it may take several amps when you connect the wires & normal dry cells usually cannot provide that.
Yes, ideal if it will turn down to around 1V, or even lower.
Or with that, set it to 1V and use a 10 ohm resistor to limit the current, then use that and a voltmeter to find the lowest resistance point on that marked PCB trace? With a fixed current the fault should be fairly easy to find.
Everything seems to point to the fault being somewhere along that?