I would use the smallest gauge wire you have on hand. Wire-wrap wire is perfect.
I would solder one end of the wire to the signal source and just touch the other end to the lead going into the input capacitor shown in the schematic. Make sure you are connecting to the capacitor lead that is connected to the IC1 pin.

I can't see your setup. Are you able to access the area around IC1 with the power on? If not, then you will have to disconnect the AC power and solder both ends, one test at a time.

 

IC1 is mounted on a board underneath another board, and the board on top is connected on all sides by jumper ribbons. They also covered many of the input jumper pins on the green side of the board with some kind of white non-conductive isolating substance (probably because they put another board on top of it). So this is not going to be the easiest set of tests in the world. But I'm determined, so I'm going to get there. I'll report back soon.

 

Ok, test results!

I confirmed that the crossfader signals at IC5 and IC6 are between 0 and +4.2 V on opposite sides of the crossfader. The voltage changes smoothly as I slide the crossfader. Signals at IC5 are the opposite of IC6... when the pins at IC5 read +4.2 V, the ones at IC6 read 0, and vice versa.

I took a 26 awg wire and attached it to the "CH2R" pin on CN123 on the DSP assy (luckily those pins weren't covered with the white foam-looking stuff). I turned down the channel fader on channel strip 2 and plugged some audio into the right input. Then I attached it to the input capacitors which were wired into IC5 and IC6 pins 8 and 9. And something I neglected to realize is that this would be MUCH LOUDER THAN ANTICIPATED.

Anyway, indeed, the signal when hard-wired into IC5 and IC6 would get nice and loud on one side of the crossfader (i believe the right side for IC5 and the left side for IC6), and quiet on the other side (although not entirely silent... just really quiet).

I believe this leaves us at least in a place where we know IC5 and IC6 aren't grossly malfunctioning. I don't have an oscilloscope (although some friends might have one, so I'll ask around) so I can't test the control signals coming into IC1. This means it's either IC1 malfunctioning, or the microprocessor sending it garbage signals... except bad control signals wouldn't explain the mixer channel bleed that I'm seeing, I don't think. So it's probably IC1.

I'm not sure if I should expect that the signal when hard-wired into IC5 or IC6 would get totally silent vs. just really quiet when putting the crossfader at the quiet side. At least the behavior was the same (except crossfader inverted) with both IC5 and IC6, so if one's malfunctioning, they both are

 

That sounds good.
I would go ahead and order a replacement for IC1.
Have you already unsoldered IC1? If you need assistance I can guide you with some ways to do so.

 

Just ordered a replacement for IC1. Actually ordered a few in case I smoke them trying to solder them... found some for $2 a piece. Also ordered some better hookup wire and some shrink wrap tubing to clean up the mess I made on the input PCB.

I haven't unsoldered IC1 yet but would appreciate the help, I've never done any SMD soldering at all.

 

You will need finer soldering tools when working with SMD components. I routinely assemble SMD boards by hand. However, my eyesight is not as good as it used to be and must resort to using a microscope. If your eyesight is good, a magnifying visor should do.

I use a Weller W-TCP station with a PTS7 0.015" soldering tip.
The solder is Kester 66/44 0.015" diameter. I prefer to use leaded solder. A flux dispenser pen is useful to have if you plan on doing a lot of SMD boards by hand.

If you know the component is bad and don't care about saving it, you can clip the legs of the IC and unsolder each leg one at a time.
If you are attempting to save the IC, it is possible to slide a fine razor blade or Exacto blade under the leg and lift the leg away from the pad while unsoldering. You do this one leg at a time on one side of the IC. After that it is easier to get at the other side. It also depends on how much space you have away from surrounding components.

After you have removed the bad IC, clean the PCB pads with solder wick. Do not dwell too long on any pad otherwise you are in danger of lifting the pad. You don't have to remove all of the solder. It would be best to leave a slight amount of solder behind so that you can easily mount the new IC.

Always double check the orientation of the new IC. I bet everyone has soldered a chip the wrong way at least once in their lifetime.

Depending on the pitch spacing of the IC leads, it is possible to hand solder the IC with 0.015" solder and tip. It does take some practice. The worst thing is when you create a solder bridge between two pins. Then you have to use a combination of solder wick and/or an Exacto blade to break the surface tension on the solder bridge.

 

Sad turn to this story.

There were two things that I didn't predict when trying to remove the old IC.

One was that the pads on the board were incredibly easy to lift. I tried clipping the leads, but maybe my cutters were too big for this task... I didn't have anything smaller (and I don't even know whatever a smaller cutting tool would be named, so I don't know what to search for!). The force of clipping the leads was pulling the pads right off of the board! Afterwards I tried using an exacto knife to lift the pins while heating them, but after about a second of heat applied with my soldering iron, the pads would lift.

The second thing I didn't expect was that the IC was glued down to the board, and even after the leads were all desoldered, I had a really hard time getting the IC off. I ended up having to pry it off. I wonder whether years of overheating or something is what caused this IC to stick down to the board, or whether it was some coating or something?

Anyway, I now have a board free of IC1, but also free of about half the pads that IC1 requires. So I'm considering running jumper wires for those connections, but it is not going to be an easy or pleasant task. (I'm also missing a small-gauge stripper and small enough wire to do this, so back to the store...)

 

We did it! After replacing IC1, everything is back to normal and the mixer works!

I am so surprised that I was able to solder jumper wires to individual pins on the IC... it all looked impossibly tiny. And as so many of these things go, if I did it a second time, I'm sure I'd do a much better job with all of this.

Thank you immensely for your help!! I very obviously could not have done it without you. I would PM you messages of extreme gratitude but it looks like I can't do that on this forum. (Perhaps I haven't been here long enough to send messages?)