Mackie CR1604VLZ mixer channel gain problem

[belzrebuth]

Hi all,
My mixer recently developed signal gain loss on 3 channels.
The trim pots (input signal gain pots) of channels 7, 12 and 15 did not work at all.
Signal was passing thru but no gain adjustment whatsoever.

I disassembled the mixer and spilled some isopropyl alcohol with a syringe on the top of the trim pots (where the shaft meets the body of the pot) exercised them well and that seemed to fix the issue.
I then put the mixer back together.(it's a real pain to disassemble btw)

I was happy with it for a couple days until I noticed that the trim pot of channel 15 is dead again.
At first I suspected the trim pot itself but the problem occurs 10-15 minutes or even less after I turn on the mixer.

Initially the trim pot works as expected and then completely stops working.

One time I've heard the signal gradually losing gain but I'm not sure if it does that consistently.
I think the trim pot can also suddenly stop responding.
 
Since it's very space consuming having this mixer opened up on my desk I thought I should post here first before opening it up again..
I've attached schematics of the input section.

[Hornethandler]

I have the same mixer.  They are known for having intermittent connections at the ribbon cable connectors.

I've fixed mine a couple/three times now in the past 15 years or so.  Remove the cover on the bottom of the unit (a cover plate about 10"x4"); it's held on with 4 screws.  You'll then have access to all the ribbon cable connectors (6 or 8 if I remember). Disconnect one at a time and and spray with contact cleaner.  This has always fixed my low signal and intermittent problems.

Good luck and let us know how you make out.