I think IC10 / PIN7 12V is not correct.
- R38 120k is defective / faulty solder joint
- C22 is defective (poor insulation) or installed incorrectly
- IC10 is faulty (or installed incorrectly?)
The voltage at IC10 / PIN5 is somewhat low, but unlikely to be the cause of the 12V in the PINs.
"While doing BPF adjustment, the volume of the tone goes up suddenly from near zero to almost 100% in a very small range around the middle." - that is correct, good sign