The main problem with old, unused Trips is the auto aperture system gets stuck in the fully closed position, usually due to lubrication oil on the blades. They usually just need a good clean to get them to work. However, getting to them is a bit complex.
There are some very good instructions here:
http://www.thermojetstove.com/Trip35/
I followed these and successfully got my aperture system working - until I replaced the middle lens element. Then it stayed fully open most of the time!
The Trip's exposure system is quite clever. The aperture is lightly spring loaded in the closed position. There is a lightmeter with a moving needle which you cannot see. As you press the shutter button a plate moves and makes contact with the needle pushing it onto a rear plate. As the first plate is angled, the position of the meter's needle dictates the amount of movement and, therefore, the extent to which the aperture opens. There is a second similar mechanism which decides the shutter speed (1/200 or 1/40).
In my opinion, the genius of the design is in the 'manual' settings for use with flash. These still use the auto aperture and provide protection from over exposure. e.g. If you have a flash fitted and you decide (based on the scale on the flash) that at the distance you are using it, you need f5.6 then that is what you set on the dial. However, if the meter decides that there is enough ambient light to expose properly at f8 then it will not open any further than f8. The flash then becomes a fill light rather than the main light. If there is not enough ambient light then the aperture opens up to your chosen setting.
Steve.