I picked up a DX-200 at the AWA Spring Meet and am in the process of enjoying it. No matter what length of long-wire antenna I use, the antenna trim control doesn't peak through-out its travel. It will have maximum signal strength at the far end of its travel, so it does have a compensating effect.
I would just like to have the control 'peak' the antenna within its travel so I know that I have maximum signal.

The circuit uses the Varicap (FC53M) as a capacitance and varies the reverse-bias voltage on it in order to change the trim. The adjustable Main Tuning cap is in parallel to the Varicap, but has no padder or access to plates to tweak the value. In order to get the trimmer control into range, I think I have to replace a component.

I read briefly about how to test a Varicap, but I am not equipped to accurately measure it. Do these drift capacitance value with age (40 years) ? Should I just try to replace it?
There is a series C103 (220pF), should I try a 200pF or 250pF to get the Trimmer in range?
The FC53M was a common component in its day and they are still around. Should I just replace it?