The first thing I personally check with all such units - particularly of a certain age - is the power supply rails. Are they all where they should be? Any dried or shorted caps? Smaller electrolytics tend to dry or leak, DIP tantalums (not sure these have those) then to short, and not always as fireworks.
Also, if using power carbon comp resistors, those can drift like heck and are really not suited for that type of service. A lot of underdesign has happened with power supplies in the relatively early days (= a couple of decades) of solid state.