Had a Technics SA-300 with no FM at all. Ran that down to this IC which is the final limiter and detector. Also handles the muting which I think it stupid to put in that chip. But I prodded a bit to make sure some dumbass 100K resistor isn't open somewhere.

Your chances of getting an AN377 are about as good as the lottery. So I used an NTE788 from our wonderful ESI out in Mrntor, Ohio. Now I got FM, which wss not before, but it is severely overmodulated. Nothing in the service manual indicates the coil alignment would affect gain. It is as if this chip was designed fro 25 kkHz deviation rather then 75, seriously. And how it connects to the tank circuit at the end makes me think it might be a slope detector, which ain't the best thing but maybe they fixed that somehow.

If anyone is familiar with this chip, or Technics of this vintage, I want your opinion on this NEW chip being bad. Wouldn't be the first time. What's more, sometimes with ECGs and NTEs and SKs there are problems due to specs. Been through that before. I got the coils tweaked where I have the tuning meter reading pretty right. The waveform at pin six is only wrong in amplitude. It has the correct DC value on it : 5.8 volts.

The old chip was missing some DC voltages. Really, since I got something out of it now I have not checked all the voltages on that IC. But does anyone think a DC voltage would cause this ?

This is like REALLY overmodulated. On rock stations clipping like 30 % of the time. Nice and symmetrical though. I look for failure modes, could that chip have blown something else to cause this ? I could not get the PDF on the AN377 but I found it in HTML and it looks right. I see nothing that could affect the gain of the FM detector like that.


thanks in advance if anything...