I have an SX-525 that I've just finished recapping. When I went to replace the burned stereo indicator bulb I found that the there is a constant ~10v across the bulb whether I'm tuned to a station or not. I also found a constant .8v at the base of Q12, the npn transistor that drives the indicator bulb. The schematic shows .8v too; is that supposed to be when the tuner is tuned in to a station?
Looking at the schematic I see that there is only a .47uF capacitor between the base of Q12 and ground. How is this supposed to work? How does the Q12 base ever get close enough to ground to shut off Q12 and turn off the indicator light? Does the capacitor get slowly discharged by the base current? Some SX-626s (with the AWE-011 tuner) have a resistor in parallel with the capacitor which I presume would pull the base to ground when the stereo indicator light driving transistor should be off.
Looking for a reason for the indicator to be stuck on I checked Q12 for shorts (in circuit) but didn't find any. I checked the base of Q11 (which pulls up the base of Q12) and found that it's voltage swings between 2.2v and 2.9v as the tuner moves between stations. The swing seems appropriate, but should the range be lower so that Q11 can shut off completely?
Another clue might be that the FM muting switch seems to have no effect. I don't think it ever engages judging from the hiss I get between stations. I checked for continuity in the switch and it seems to be behaving. After studying the schematic some more I can't really sort out how the muting or indicator light should work. Turning muting on seems to add an extra capacitor to pin 29 of the tuner, and force the signal from Q9 through an extra transistor/transformer stage. I imagine that should raise the threshold of the signal the tuner will respond to, but I don't really see how. Can anyone elaborate on the theory or suggest what might be wrong?
Thanks,
/Mark
Looking at the schematic I see that there is only a .47uF capacitor between the base of Q12 and ground. How is this supposed to work? How does the Q12 base ever get close enough to ground to shut off Q12 and turn off the indicator light? Does the capacitor get slowly discharged by the base current? Some SX-626s (with the AWE-011 tuner) have a resistor in parallel with the capacitor which I presume would pull the base to ground when the stereo indicator light driving transistor should be off.
Looking for a reason for the indicator to be stuck on I checked Q12 for shorts (in circuit) but didn't find any. I checked the base of Q11 (which pulls up the base of Q12) and found that it's voltage swings between 2.2v and 2.9v as the tuner moves between stations. The swing seems appropriate, but should the range be lower so that Q11 can shut off completely?
Another clue might be that the FM muting switch seems to have no effect. I don't think it ever engages judging from the hiss I get between stations. I checked for continuity in the switch and it seems to be behaving. After studying the schematic some more I can't really sort out how the muting or indicator light should work. Turning muting on seems to add an extra capacitor to pin 29 of the tuner, and force the signal from Q9 through an extra transistor/transformer stage. I imagine that should raise the threshold of the signal the tuner will respond to, but I don't really see how. Can anyone elaborate on the theory or suggest what might be wrong?
Thanks,
/Mark