Hi Hextejas
I just happen to come across this posting.
The signal on the LA1266 pin 12 is the FM demodulated output. It is the raw signal that contains the audio, the 19khz pilot, 38khz stereo difference signal, and possibly other digital or "muzac" type signals. The input to the stereo demodulator chip, LA3401 pin 2, only wants to see the audio, 19khz pilot, and 38khz difference signal, as these are needed to decode the left and right channels. The other signals need to be filtered out, which is what the network that includes FL5 does (low pass filter). To test and see if there is a signal present at LA1266 pin 12, you need to be familiar with what an audio signal looks like on your scope. A slower sweep speed, with higher gain, will help. If you tune in a strong local FM station, and probe pin 12 you should see audio present (plus some lower level fixed frequency tones). If you see it there, move to LA3401 pin 2 and probe for a similar, but probably lower level signal, since the filter will have some loss. If it is present there, then move on to the output pins, 5 and 8. If you see a signal there, then it must be in the discrete circuitry following. There are a couple of muting transistors (Q402, Q409) that could be pulling down the signal if they are getting switched on inappropriately. See what you find and report back.
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