04-07-2014, 10:32 AM
I took a closer look at the stereo beacon circuit on the MPX board, and there are only a couple of possibilities for the cause of the problem. One is the transistors themselves, Q404 and Q406. Since you already replaced them, they can be eliminated as a cause.
The second possibility is cap C419. Any leakage here will apply current from the +24 V supply to the base of Q406, causing it to conduct regardless of any detected stereo signal. This is a 1 uF tantalum, and although they have very low leakage, the circuit applies 24Vdc to a 25 Vdc rated cap so it will be suspect. Fisher should have used a higher rated cap in this location, especially since low leakage is critical.
You can try lifting one end of C419 and see if the stereo light remains on at all times. If it goes out then the cap is leaky.
If it is bad, I would replace it with a 35 V or 50 V tantalum, or even better a mylar. A regular aluminum electrolytic will have too much leakage to work properly in this application.
The second possibility is cap C419. Any leakage here will apply current from the +24 V supply to the base of Q406, causing it to conduct regardless of any detected stereo signal. This is a 1 uF tantalum, and although they have very low leakage, the circuit applies 24Vdc to a 25 Vdc rated cap so it will be suspect. Fisher should have used a higher rated cap in this location, especially since low leakage is critical.
You can try lifting one end of C419 and see if the stereo light remains on at all times. If it goes out then the cap is leaky.
If it is bad, I would replace it with a 35 V or 50 V tantalum, or even better a mylar. A regular aluminum electrolytic will have too much leakage to work properly in this application.
