FM Stereo one channel not working correctly

mannyo1221

New Member
Hi all

I know this is my first post on audiokarma, but thought I'll get stuck right in.

I am currently restoring a 1976 Toshiba music centre, model SM-3100. When I got this system it had issues with everything.

The turntable and tape deck are now working, so I am turning my attention to the tuner and amp, which seems more of a problem.

Tuner Observations.

LW= Working perfectly
MW= Working perfectly
FM Mono = Working perfectly

FM Stereo mode is where the problems are. When I press the FM Stereo selector button, I lose all audio from the right channel. There is something there if I turn the balance control all the way over, its just very, very quiet.

I've got the schematic for the tuner, and managed to get the pinout layout for the FM decoder IC, but I struggle with reading circuit diagrams and my only diagnostic tool at this point is a digital volt meter so I am not sure where to start.

is it ok on here, to upload the tuner schematic and the pin out for the decoder IC, perhaps another member may be able to advise me where to look.

The amplifier is working ok on all other inputs, and also the aux in on the back, it has minor difference in volume between left and right channels, but that is for another section.

The link below is to my youtube video of the actual system, you can see the controls on the front panel for the source selection.

 
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Have you cleaned the switches with deoxit. Sounds like a dirty contact issue. Do you get a stereo light lit when in stereo?
 
Thanks for the reply.

Yes the Stereo light comes on and the function switches cleaned as they were pretty bad. I've also done the volume, balance, bass and treble controls.

When the tuner is in Stereo mode and you are listening to a mono station, audio is still through a single channel only. When using the tuner in Mono mode, I think both the merged left and right channel can be heard.

From what I can work out, there are two IC's for FM in this unit, one works for FM mono and when you select the FM-ST function, it switches over to a different IC. Mono looks like it's running through an AN217 and Stereo is running through an AN271
 
Looks like you did the easy stuff, but it's worth deoxiting the selector again, any tape switches, mono/stereo switch, etc. You mention the switches were pretty bad.
If only one channel still comes through, then my thought is that you would need to trace the audio path as it exits the MPX chip. Since it seems you are getting a signal there, see what happens downstream. If the IC is sending along a signal on both channels then you will have to follow it and see where it disappears.
I am no expert, but that is what I would try. You will need a schematic to figure out what pin does what.
 
I've got the tuner schematic and the pin layout of the AN271. I did try and upload, but cannot get the upload a file function working.

The system is very easy to work on luckily. You can get access to everything by removing the turntable or cassette deck, depending on where you need to get to.
 
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