Slow down, first make sure the tuner and MPX section has proper power.
It's weirdly powered by the pilot lamp power (D3, C313, R309) for around +9V for +B2 in the MPX. How are the other pilot lamps, they affect that voltage if burnt out or LED it will go high, and there is a 7.5VAC/6.3VAC transformer tap to see what you have.
The other important rail is B+ 14V regulated from QC9. Many 16V caps on that 14V rail too, they might be a bit cranky.
Tuned to a stereo FM station, see if QC7 gets any base voltage at all. The stereo-mono (STEREO DEFEAT) switch seems to short that out to force mono mode.
The actual current drain of the stereo lamp (30mA) sets the hysteresis-amount of the stereo/mono switch Qc8/Qc9 make. It can be hard to get a proper lamp.
With a burnt out stereo lamp, Rc9 27k (by discriminator) front end might get stuck in stereo but not sure.