You can use a 2N4403, or a ZTX 750, or ZTX 751. Replace them as pairs if you do, so the amp is balanced.
You should replace both of the 5uF using 4.7uF caps of the same brand, so they are matched. You can disconnect the fail safe diodes if you think they are causing a problem.
If you look closely at the one-shot (reference pulse generator) you will see how the transformer primary works as a summing junction. On the secondary side, you get the difference in pulse width, and polarity, to drive either direction.
Make sure you don't have an end failsafe servo, which would be throttle.
I would also replace all the electrolytic caps, especially the big one in the failsafe circuit, as when one goes, they all are suspect. Many times, I have fixed radios with weird problems by just replacing the electrolytics, it's amazing.
We used to take apart the Digimite servos in shoe box to capture the balls. Use a dab of silicon grease to keep them in place when assembling.
Last edited by jaymen; 05-24-2016 at 01:50 PM.