22-10-2018, 12:14 PM
First thing to do when you get a confusing voltage reading: use an oscilloscope 
It's just about possible that there is an AC waveform at that point - perhaps hum or similar - and if the AC waveform is not symetrical either side of 0V, then there will be a DC offset that could easily be negative, depending on the waveshape.
With a x10 probe the 'scope will present a 10M resistance to that point, so shouldn't affect the signal too much. The capacitance will be around 10-20pF, give or take, so shouldn't affect an audio signal like hum significantly (if it's HF oscillation, that might be different). If nothing else, checking this might give you more information about where to go next...
It's just about possible that there is an AC waveform at that point - perhaps hum or similar - and if the AC waveform is not symetrical either side of 0V, then there will be a DC offset that could easily be negative, depending on the waveshape.
With a x10 probe the 'scope will present a 10M resistance to that point, so shouldn't affect the signal too much. The capacitance will be around 10-20pF, give or take, so shouldn't affect an audio signal like hum significantly (if it's HF oscillation, that might be different). If nothing else, checking this might give you more information about where to go next...
