Something which has bugged me in the past. When does the startup sound happen in the movies? I seem to recall that in the elevator Ray tells egon to switch him on, so the pack goes on with the full start up sound when egon pushes some never used again switch on the pack. However, in many other scenes the start up sound seems to happen when the wand is switched on?
is there now an established consensus as to when the pack is supposed to start up?
Good question. I dont have the answer but Alex Newborn has a great thread on the never used again switch and he also ingeniously incorporated that into his pack. (Mental note, add that to current pack build).
Short and simple: There is no switch in the pack. Ramis’ improvised a switch somewhere around the power cell area, but the on switch is, indeed, on the gun. The scene in the elevator was intended for comedic effect, since no one had yet tested the packs and Peter and Egon back themselves into the corner as it’s turned on.
There are two kick on sounds. One when the whole pack is turned on, and a shorter one when the vent switch is turned on. So the answer is both. Think of it as turning on the pack in a standby mode then kicking it into full power when preparing to fire.
thanks for the answers everyone! 
The same sound effect used in the elevator scene is used when Egon exits the lift and switches something on his thrower, and then again when Ray says “I’m gonna have to hold him myself” and his vent lights come on at the same time. Heard again at the end when they do the ‘heat em up’ preparation for blasting Gozer.
So the ‘switch me on’ switch on the pack and at least one switch on the gun can independently activate the power. (Why didn’t Ray draw his own wand and turn on his power in the elevator scene, then? Shrug.)
Yep, we discussed the possible placement rather exhaustively in this thread: Main Power-Switch on the Pack - Proton Packs - GBFans.com
In the end, I decided to add a rotary switch to the Legris elbow on the underside of the ion arm.
It’s wired into the same terminals on the GBFans soundboard as the ‘vent lights’ switch on my thrower. So either one will activate the PowerUp sound. My pack will recreate ‘switch me on’ as well as ‘I’m gonna have to hold him myself’.
When I went to a Dan Aykroyd signing in Jackson MS two weeks ago, I got to meet several GB’s from MS, GA, and other nearby states. The ‘switch me on’ switch was the first thing they asked me to demonstrate when I arrived.
Alex
Wait a minute… Did I miss a thread about you talking about this/ photos?
Sorry for the hijack.
Yep, that’s the thread and video. I knew you would join the discussion shortly.
Whoops! I thought I had mentioned it, but you’re right! I didn’t post anything here about it. Situation has now been corrected.
Another cool thing was that one of the people who wanted to see my ‘switch me on’ function in action was Sephiroth, who had been following my pack build. I in turn was excited to see his belt gizmo with lighting Nixies, which I knew about from his father’s thread on here. We both have Dads who are retired electricians!
Alex
you know i never got the comedic effect there, I always assumed Egon was turning round to get Venkman to do the same in a confined space…but I think your idea is correct 
Oh yeah, Alex’s “switch me on” feature was awesome to function in person. My dad and I have been working on my pack and trap builds for a while now and I truly appreciate how he has dedicated so much time and talent. I’ll add some pics once I feel they are ready to show off.