Crubio, Depending on how much time your stock SR MPCs have seen in the field, due to age, a diode or cap may have failed, in which case my modification would fix that automatically, and would absolutely make the MPC sound night and day better!
However, another possibility could be that, again due to age, you have a thermal cutoff that opened. This part is like a small fuse located just below the surface of the MPCs transformer primary windings. It is in series with the incoming AC to protect the transformer windings themselves from opening. This part replacement may also get your stock SR MPC functioning again. However, I will only replace the thermal cutoff, if needed, with my modification. All the best.