I fired up one of my 6632B stored for almost 10 years and smelled burned electronics, despite everything is functioning. I tested the unit immediately when I bought them a decade ago and it was working fine, so it’s an example where electronics can deteriorate by storing (even in temperature controlled, dry environment).

Since I see smoke, I turned everything off immediately and investigated. Turns out one of the tantalum capacitors in the processor/controller board gave in: