[Mondial]

Well we are still making progress. You now know its not the cap or the transistors!

I have a feeling that part of the problem is the LED conversion. If you connected an incandescent bulb to your MPX board, the lamp probably would not light all the time because it would have only a few volts across it. The LED needs only 2 volts or so to light so its much more sensitive to marginal conditions.

Are Q 404 and Q 406 mounted in sockets so you can unplug them? If so, completely removing them would only leave R414 as a path to ground for the LED current. This resistor may be passing enough current to light the LED but would never be a problem with the original bulb.

Another thing you can try is to connect a 1K directly in parallel with the LED itself. This will reduce its voltage sensitivity so it will take at least 4V across the circuit to light it instead of just 2V, which will make it behave somewhat more like the bulb.