This is for the 91, and it's very straight forward.
The 94 is different. First, looking at the 94, the connector will be on the left side at the bottom of the picture, rather than at the top. The access hole for the transistor-to-heat sink screw will be at the top center. It would be more appropriate to flip the thing 180 degrees and look at it with the access hole at the bottom (sorry, I don't have the digital camera with me to take a pic). You have to stick the screwdriver in at about a 45 degree angle to contact the screw (look into the hole with a flashlight to see the screw - also, look at the unit from the side to see how the 45 degree angle will put you perpendicular to the heat sink surface, as the screw is). Use a #2 phillips screwdriver, use firm pressure and work deliberately - the screw appears to be retained with some sort of thread lock compound. It's tight, and you don't want to strip out the screw head.