Check the value of R15, R14, R9 (by color code).Check the direction of C12, C11, C6.Try to follow the signal in TP1, TP2, L1 top pin.The DMM must be set to AC volts. The voltage you read must vary with the position of pot P1.
Check the solder joints on transformer T1.Check the solder joints on connectors CN3a and CN3b.Check if you can measure the input signal between the 2 side pins of pot P1.
Part number: H2RA-20-UAhttps://www.digikey.com/en/products/detail/adam-tech/PH2RA-20-UA/9830622?s=N4IgTCBcDaIAoAkwCUCCBaMAGdBVVIAugL5A