I just bought a DC 3 wired voltmeter and it was working perfectly. Here is it image:
I used the yellow wire for measuring the voltages. But for some reason, I desoldered the wires and soldered pin headers. Now it gives unknown reading like 0.XY (eg 0.37) whenever I just connect the positive and negative to battery. It was not happening before.
Can I be helped to troubleshoot what I did wrong? Have I damage the circuit?
Here is the underneath image of the one I soldered: