Hello friends. All good? I hope so.
I'm here working on a repair, on a Fander 212R amplifier that is keeping me up at night!
He had the famous problem in the Mute circuit, which was corrected after replacing the Q13. I didn't think this modification with a resistor between the Q7 collector and the cathode of the D18 diode was correct. This just "deceives" the negative voltage that should go to the differential pair, anyway...
What happens here is strange: There is a 120Hz noise as soon as the mute circuit stops working (when plugging in the guitar). And, interestingly, the noise only enters if the 1K2 (resistor that goes to the AC signal), at the input of the effects switch circuit. If you remove the R119, the noise disappears immediately, but of course, the effects switches or the footswitch don't work.
I've already replaced all the diodes in that sector, as well as the capacitors in parallel to the resistors in each switch.
Detail: Even opening the circuit, lifting the diodes that go to 2N transistors from the board, leaving only R119 and the switches, the noise persists.
If you connect the guitar directly to the "AMP IN" port, there will be no noise either.
Thank you for any exchange of ideas or information here.
I am grateful for the help and whenever I can help, it will be a pleasure too.
A big hug and greetings to everyone!
I'm here working on a repair, on a Fander 212R amplifier that is keeping me up at night!
He had the famous problem in the Mute circuit, which was corrected after replacing the Q13. I didn't think this modification with a resistor between the Q7 collector and the cathode of the D18 diode was correct. This just "deceives" the negative voltage that should go to the differential pair, anyway...
What happens here is strange: There is a 120Hz noise as soon as the mute circuit stops working (when plugging in the guitar). And, interestingly, the noise only enters if the 1K2 (resistor that goes to the AC signal), at the input of the effects switch circuit. If you remove the R119, the noise disappears immediately, but of course, the effects switches or the footswitch don't work.
I've already replaced all the diodes in that sector, as well as the capacitors in parallel to the resistors in each switch.
Detail: Even opening the circuit, lifting the diodes that go to 2N transistors from the board, leaving only R119 and the switches, the noise persists.
If you connect the guitar directly to the "AMP IN" port, there will be no noise either.
Thank you for any exchange of ideas or information here.
I am grateful for the help and whenever I can help, it will be a pleasure too.
A big hug and greetings to everyone!
