12-27-2016, 09:35 AM
OK Ron, I think I see what the problem is.The lamp driver circuit is very poorly designed.
TR705 which lights the green lamp is never completely turning off. With the incandescent lamp, a bit of leakage through TR705 makes no difference because it takes 250 mA to light up, but the LED is much more sensitive and still lights with a low current.
TR703 is what controls TR705. TR703's collector switches between 1.6 V and 15.5 V, and supplies base current to TR705 through R711. The problem is with 1.6 V at TR703's collector, there is still enough voltage at TR705's base to turn it on, leaving the green led lit. The solution would be to decrease the resistance of R715 to less than 1K, which shunts the base of TR705. This will form a voltage divider with R711, so that even with 1.6 V at the collector of TR703 there will be less than 0.7 V at the base of TR705, insuring it turns off completely.
Before you do anything else, connect a 1K resistor across the existing 2.7K R715 and see what happens.
TR705 which lights the green lamp is never completely turning off. With the incandescent lamp, a bit of leakage through TR705 makes no difference because it takes 250 mA to light up, but the LED is much more sensitive and still lights with a low current.
TR703 is what controls TR705. TR703's collector switches between 1.6 V and 15.5 V, and supplies base current to TR705 through R711. The problem is with 1.6 V at TR703's collector, there is still enough voltage at TR705's base to turn it on, leaving the green led lit. The solution would be to decrease the resistance of R715 to less than 1K, which shunts the base of TR705. This will form a voltage divider with R711, so that even with 1.6 V at the collector of TR703 there will be less than 0.7 V at the base of TR705, insuring it turns off completely.
Before you do anything else, connect a 1K resistor across the existing 2.7K R715 and see what happens.
