Below is a photo the bottom side of 651S-1. The part of red arrow is audio output coupling capacitor.
I changed new capacitor from old one. However, I didn't have same shape one. It is very slim. The collins 651S-1 original is "axial-lead type". but my capacitor is "radial-lead" type.
therefore, I decided to make I have two 470uF 35V. Top side of the capacitor put together by vinyl-tape.
My very modern design Collins 651S-1 was fixed audio volume.
This set came from junk store of the "Middle East" two years ago. It was very dirty junk when it came to my shack. Recently, It has been getting better and better.