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Ron, looking at the schematic, it appears that PL8 is part of the circuit. When a stereo signal is received, pin 6 of IC 30 is pulled to ground, lighting PL8. Transistor TR307 is an inverter, so when PL8 is on PL9 is off and vice-versa.
It looks like PL8 is the actual stereo light and PL9 is only on with a mono signal. The base drive for TR 307 is supplied through PL8 and if it is open, TR307 cannot turn on and light PL9.
Try connecting a 470 ohm resistor across the open bulb PL8 to supply base drive to TR307 and see if PL9 now lights in mono.
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RE: Realistic STA-225 - by Mondial - 12-25-2016, 11:49 AM
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