Working on this rec. for somebody. Uses the awful PA 1001 1002 3001 chips. At first the audio was garbled, no meter, no mute. Replaced bad PA3001 IF/Det. with the Hitachi equivalent. Restored meter, mute, and clean 550 mV sine to the 1001 MPX chip with 30 KHz deviation. Output of the L and R of the 1001 is down in the noise. With an OTA stereo signal, there are huge 3.7V 38 KHz tilted square waves on the L R outputs of the chip. Of course that goes thru a LPF to the audio amp chip. There is recovered stereo sound from the tuner but the audio level is a great deal lower than AM. Even with 1 mV RF from the test gen. But the tuner is super sensetive in the RF stages. I am thinking the MPX chip may be bad too with such tiny recovered audio. But the audio chip has it's own issues. An apparent 2 MHz oscillation that appears at the output at about 50mV Peak at all times except when the inputs were disconnected. Anyone have a Pioneer that they would consider taking a few measurements on with this chip set? BTW I do not have MPX in my RF gen, just FM selectable dev.

Typically that problem was caused by bad coupling or 'lytic caps. Been 40 yrs or so since I worked on one of those.

I'd replace the caps, which btw I don't necessarily suspect. But they are a lot easier to get than the chips, which is most likely the problem. They are notorious, and getting extremely hard to find. You may get lucky and a few caps will fix it. I'm not betting on that

Did extensive cap testing in the problem areas. I sent it back to the owner telling him to take it to one of the pro shops in the region that have or should have MPX generators for one thing.