03-03-2014, 04:16 PM
You should be able to replace the lamp with a LED with a few conditions.
The transistor Q404 just pulls the lamp to ground to light it, but it also controls bias for the demod driver Q403. When the lamp is off (mono) the voltage at Q404 collector is pulled up to +23v, removing bias from Q403 and shutting down the demodulator.
So you need a series resistor to limit the LED current and a shunt resistor to insure that the off voltage rises high enough that bias is removed from Q403. The original bulb drew about 36mA, but you really don't need that much current with a LED. I would try a 1.5 K in series with the LED and maybe a 10K across the combination as a pullup.
The transistor Q404 just pulls the lamp to ground to light it, but it also controls bias for the demod driver Q403. When the lamp is off (mono) the voltage at Q404 collector is pulled up to +23v, removing bias from Q403 and shutting down the demodulator.
So you need a series resistor to limit the LED current and a shunt resistor to insure that the off voltage rises high enough that bias is removed from Q403. The original bulb drew about 36mA, but you really don't need that much current with a LED. I would try a 1.5 K in series with the LED and maybe a 10K across the combination as a pullup.
