I believe the RA-103D is like RA-103 in not having AGC. On the 103, the contrast control varies the gain on the 1st and 2nd video IFs. As I understand it, the contrast acts as a manual gain control, like the "picture" control on an RCA 630TS.
Re your main question, the instability and drifting of the gain doesn't sound typical for this model. My RA-103 is one of the most stable early TVs in this house. The whole set seems rather over-engineered, apart from the Focus pot, which is a notorious weak spot.
Phil Nelson
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