Every Track On My 388 Monitors, Except 2

  • Thread starter Thread starter Mark7
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So there’s not an IC that controls routing of any of the input channels to the main buss per se. There’s an IC that controls the source of the main buss, between either the L-R buss or the stereo sum of the monitor mixer. That’s U104 on the BUSS B PCB. However, because it controls global sourcing, like signal passing from all input channels to the L-R to the main fader, if there is a problem with just one input not passing signal to the L-R buss (like input 2 for instance) it’s not likely to be U104 at fault. You would have the same problem with inputs 1, 3, 4, 5, 6, 7 & 8 as well. If you’re having trouble with just one input passing to the L-R buss, but you have good signal at that channel’s direct output jack, it’s usually a bad connection or solder joint between the subject input channel and the M BUSS PCB or the M BUSS PCB and the BUSS B PCB…or both.
 
Am I going to need to send the board away to have the U109 chip replaced or is it a simple pull out/drop in job?

U104 is fine. Just checking

Yes, I/O channel 2 is passing signal to the L-R buss.
 
If you determine U109 needs replaced, and you can’t do it yourself, then yes I guess you would need to send it to a tech.

None of the components in the 388 are socketed, so removal requires desoldering.
 
Update 2. I'm getting sound from the monitor submix for channel 2 with the monitor gain between 12 and 5, but it's considerably lower than when I switch channel 2 to the L-R buss (I can hear the TV through the mic in stereo mode but not in direct mode).
 
Does monitor mixer channel 2 behave differently than the other monitor mixer channels?
 
Well, there's no discernible sound coming through the headphones until the volume control is past 12 o'clock, so that's different to the other seven channels. What else might you be thinking of?
 
I've pulled the board out. Are there any tests I can do with a basic multimeter?
 
So this is a familiar place we’ve been before where what’s needed is to know/understand schematics, hook up your signal generator and using a level meter (yes you can use your DMM), start chasing tone until you find the fault. It’s helpful to compare with a known good working channel. And it is difficult at best to give guidance to somebody who does not understand schematics step-by-step guidance over the internet. It, depending on the fault, device and other circumstances, can be done, but it is generally very, very time consuming. The 388 is a relatively complex device because of the interrelation of the various systems and sections, and the schematics leave some things to be desired in terms of clarity/accuracy. If I have a chance I’ll see if I can offer a next step for you, but you’ll need a tone generator, and your DMM, and I have no way of knowing if you’ll be able to do what I’m advising because it all depends on if you can actually access the node of the circuit I’m referencing while the device is powered. I’m hesitant to engage because of a lack of time for this and the potential for damage and communication challenges. My overarching point is your ask is big. But I’ll see, when I have a chance, if there’s something logical I can advise. And to be clear trying to chase this kind of thing down is best done with the board installed so it is connected and powered. If you have a bench supply you can power it out of the device, but the BUSS A and BUSS B PCBs make the monitor mixer system, so you’d have to have them both out and powered.
 
Okay, I've put the board back in, replaced the M buss board and hooked the A buss board back up. The only way I have to generate test tones is software and the only way I have to transmit the test tones from my laptop to the 388 is a cheap Behringer UCA202 interface.
 
Where do I go from here? Does the manual have anything on this?
 
Track/PGM 2 monitoring is back. Would anyone like to surmise why and how?
 
No not really. There’s no way for me or anybody to know why it’s working now, but the fact you unplugged stuff and plugged stuff back in and now it’s working supports my theory you exercised an oxidized connection or you have a bad solder joint or something if the like that is now making contact.
 
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