03-13-2014, 08:36 PM
Ok, it seems that possibly you are not getting a strong enough stereo signal to turn on Q404, because when it did turn on the voltage at the collector dropped to .5 V as it should to turn on the LED.
The two diodes in Z403 rectify the 19 kHz stereo pilot signal and the resulting DC is fed to the base of Q404 turning it on and thereby lighting the LED. If there is not enough signal the Q404 will not turn on fully. The control voltage is also modified by the voltage from the stereo trigger control, which may need adjustment also to make the transistor switch properly.
Keep in mind that the LED is much more sensitive than the incandescent bulb, which would not light at all until Q404 turned on completely. So maybe the low level illumination you are getting does not actually indicate a proper stereo signal because it would never occur with the bulb.
The two diodes in Z403 rectify the 19 kHz stereo pilot signal and the resulting DC is fed to the base of Q404 turning it on and thereby lighting the LED. If there is not enough signal the Q404 will not turn on fully. The control voltage is also modified by the voltage from the stereo trigger control, which may need adjustment also to make the transistor switch properly.
Keep in mind that the LED is much more sensitive than the incandescent bulb, which would not light at all until Q404 turned on completely. So maybe the low level illumination you are getting does not actually indicate a proper stereo signal because it would never occur with the bulb.
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