I've just recently acquired a vintage sony STR-6045 and i've been having trouble recieving a lot of FM stations in stereo. The stereo indicator lights up but theres no stereo seperation in a lot of channels, however ill receive these stations in stereo perfectly fine on my other receiver. I need help diagnosing the issue here

 

Antenna? Have you checked the sensitivity specs? Maybe not as good.

 

I dont think its the antenna because ill get full signal on the signal meter and im using a long antenna going out my window

 

You're down to fixing the radio. There are several ways it can be stuck in mono mode so a schematic is the next step. Got one?

Since the problem is not on all stations, I still suspect the antenna. Can you try a TV antenna? The FM band is between channels 6 and 7 on the TV dial so a TV antenna should cover the frequencies, be directional, and have gain.

 

Nope. I got it used off ebay and it didnt come with a manual. Im pretty sure the problems in the tuner part of the circuit, because I get perfect stereo seperation on the phono and auxilary inputs. I need to clean all the controls on it anyway, so ill try cleaning the tuning capacitor, but i dont think thatll do much

 

You never know until you get to digging around in it. You might find a crystalized solder joint and be back in busines for 10 cents worth of solder.

There are circuits that decide whether or not to activate the stereo mode, and they are not in the tuner. I'll post a drawing of a chip. Notice that it has a switched input that tells it whether to go to stereo mode. That switch input is derived from something else. I forgot which part, but there are ways to tell if stereo is being broadcast and if the signal strength is high enough. I'll also try to attach a block diagram of an FM receiver so you can see where this fits into the big picture. (It's in the demodulator section.)

 

FWIW, you could buy the service manual here or here for $15.

It does exist as a pdf, but I couldn't find it free.

 

I think it might be the antenna now because I just found out that im using a 75 ohm antenna, but its supposed to be 300 ohm