Okay, I took a closer look at this, and beware that what I'm going to say is coming from an unschooled novice with a demonstrative ability to be a complete dumba**, but the ON button doesn't interrupt power to the strip. The ON button schematically is more like a traditional muting switch in that it simply interrupts the post EQ signal coming off of U7 to the group assign switches as well as any post-fade enabled AUX busses...and the stereo SOLO-in-place buss. That being said there appears to be nothing fancy in there like a muting transistor or some such thing...its a hard-contact switch right in line with the signal path and that's it. The unfortaunate thing here that I can see is that U7 is a 4556 chip...it can output, like, up to 70mA which is relatively huge and Teac put that particular chip there because that one opamp has the potential to have to simultaneously push 14 busses (8 groups, 4 auxes and the stereo SOLO buss)...not that you would, under normal circumstances, be sending one input to all those places at once, but Teac did the smart thing in selecting the 4556 for that application and engineered for the
potential drive needed from U7 if you were to open up all those busses. Better to be able to drive all 14 paths than get all distorty and whatnot.
SO...my point? There's just a lot of drive potential at the ON switch and there's really no way around it. I wonder if a cap could be put there to buffer the signal, but then you'd have a cap there buffering the signal.
Actually, wait...there's already a cap there...look at C35...that's a 22uF/25V cap on the output of pin 1 of U7...you might try cleaning the ON switch really good as I described (
not trying to dribble cleaner in from the top but inject it from the side) and replace C35 with a good quality 105-degree cap and see...if C35 is getting dry I can imagine hitting the ON switch will be more like shooting a hole at a shaken-up bottle of soda rather than slowly opening the screw-on cap...that's an exaggeration but you get the idea...I really like the Nichicon KT series caps...105-degree and designed for audio...easy on the wallet too.
I seem to recall that on the 4-strip module in my M-520 that had been totally recapped/cleaned/gone-through the noise present when switching the strips on and off was
better, but there was still some *clicking* and *ticking* when working the ON switch.
Is there any difference when you try it with the EQ switched off and the aux busses switched pre-fader?
Also, did you try doing it at the ASSIGN switch(es) instead of the ON switch?
I was thinking of a source routed to a single buss during multitracking and not during mastering. That would be difficult to get them just right.