Okay...that will be important to know.
I'm targeting U109 right now.
U109 is an
LC4966 quad bilateral switch...a 14-pin chip on the BUSS A PCB. You've got 4 of them on there, each does the monitor switching for each pair of channels (1-2, 3-4, 5-6 and 7-8), so it may be a problem with U109 itself OR the switching that tells U109
when to switch.
This is probably going to be tough and I don't know
how tough because I don't have a 388 opened up or accessible in that way right now, but you need to see if you can measure tone voltage at the channel 1 input of U109. That's pin 1 of U109. If you have the manual you can find the picture of the PCB toward the back of the manual where all the PCB layouts are to locate U109, and then once you find pin 1, if you can get your meter probe in there to see if signal is getting to that point.
BE CAREFUL. I cannot be held responsible at this point or any point if you do something to hose your gear. Not wanting to pass judgement on your skills but this is getting deeper and you
CAN MOST CERTAINLY cause damage to
yourself or
your gear if you are not careful. Proceed at your own risk because you're going to have to do this with the unit powered and tone going into a channel and routed to PGM group 1 as before, but the space will be cramped at best to get your probe in there.
So, if comfortable, check pin 1 of U109 for tone. Then check pin 2. That's the output of that section of U109.
Then set your meter to DC volts and see if you have any voltage at pin 13 of U109. Pin 13 is where the control signal comes from to tell the switch for "channel 1" of U109 to close and let signal through to the monitor buss, tape track and master meters.
What I'm hoping is that you have voltage at pins 1 and 13 (i.e. signal is GETTING to U109 and control voltage is there TELLING U109 to close), but nothing at pin 2 (i.e nothing comes out the other side because U109 is hosed and that switch isn't closing). If this is the case, this would also mean that you can't hear anything playing back from track 1 either in the monitor section or at the tape out jack.
I'm
hoping this is the case because it would just mean that you need to replace U109...unless it got hosed because something else ain't right...but let's hold that thought and check these things out first.