It sounds like you may have fixed it and want to explain it to us?? Is this correct?
Yes, that's correct. For those who might be interested, here’s a description about how I restored FM stereo function with the P300 unit (and making it easily accessible for all soldering purposes):
1. Snipping off all connecting wires from the pins, marking the wires by white adhesive tapes with the nos. of the respective pins written thereon.
2. Removing all residual wire coils from the pins
3. Tipping all pins with accessory soldering shoes (squeezing the shoes somewhat with a little gripper to get a solid contact with the pins)
4. Placing the disconnected P300 outside the 2275 gear, elongating all connecting wires with longer insulated copper wires and soldering them onto the accessory shoes. The P300 circuit could then be easily handled for all subsequent soldering and inspection purposes.
5. Inspecting the muting component didn’t reveal any defect, nor could the FM stereo signal be restored by changing the muting window
6. Replacing all electrolyte capacitors – except for the bipolar ones: couldn’t find any improvement
7. Replacing the IC HA1156R with a new IC, now placed on top of an IC socket: no improvement
8. Replacing all the transistors 2Sc828 with KSC1815Y, as described by Vehto before. With one of the transistors replaced--unfortunately didn’t make a note which one—FM stereo came back, both with the indicator lamp and acoustically.
Lacking most of the needed measuring equipment, I cannot explicitly state whether the stereo function recovery was due to the replacement of (one of) the transistors alone, or whether it was caused by a synergy with the prior replacement of other components.
Next I will replace the C306/C307/C309 bipolar capacitors by miniature MKS film capacitors, and thereafter put the p300 circuit back inside the 2275. With proper shortening of the wires leading to the accessory shoes, and all soldered joints insulated by heat shrink tubes. Hope everything continues functioning then, too...
Thank you so much for all your valuable advice
Joe / Joachim