Yes -- Turn DS ON. Delay to Zero. When you say gain is set to 50%, is that really 50% gain, or is the channel you're using to control it set to 50%?
Which leads to the real question -- how are you controlling gain, what transmitter, what channel, and what switch on the TX? I'll guess since you said "A", you're using some form of Futaba TX.
Further, the 401 must ALWAYS be started up in Heading Hold mode if you want it to work correctly. When you have correctly initialized the 401, the red LED on the body will come on and will stay on as long as you don't move the stick or the airframe. If it comes up blinking after the power-up initialization, you were a bad boy and tried to start it up in Normal/Rate mode. You'll need to cycle power on the RX, but this time make sure the TX is set to select AVCS/HH mode first.
And just to cover all the bases, a digital servo WILL buzz -- not shake -- but will buzz much of the time when it is at neutral or holding a fixed position, especially if you have some friction and/or binding in your linkage and control setup.
What's that thing in the picture? Seems to resemble a plank...
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