388 Aux Buss Not Working

It is time to get in with the scope and find out where it is stopping.
Many mixers made use of these long buss bars as in the M2524 I worked on recently. These should be taken out and resoldered to higher standards. Also look for broken solder joints at the connectors. The last time I worked on one of those was in the late 80's I think.
 
Okay. Understood.

So since you’re getting signal at the AUX OUT jack when you have a source connected to the AUX BUSS IN jack, that tells me it’s not a problem with the AUX buss summing and output amp section on the MONITOR PCB. The first place I would look is to inspect the solder joints and connections where the M BUSS PCB (the long strip of PCB that spans across all of the mixer boards) connects to the MONITOR PCB.

That was my last prime suspect suggested to the OP; the solder joints where the BUSS PCB connects to the MONITOR PCB...suggested to check the joints on both boards.
 
Okay, finally got the base plate off and removed the m.buss pcb.

All the pins that connect to the aux buss (pin 2 on P901 to P911) are fine. There's no sign of any damage to either the pins or to the light green strip on the back that connects them.

However... Pin 7 on P909 (ground) had sunk (i.e. it was halfway out the other side of the board) and had to be hammered gently back into place. It, and Pin 8 on the same... I don't what they're called... were both askew so I used pliers to straighten them. As a result the green strip that connects all the Pin 7s (I don't know what it's called) is broken at P909 and I have no idea what to do about that. Or if it even affects the aux buss.
 
Can you post some pics?

I’d like to see what’s damaged in order to offer relevant advice. You handy with a soldering iron?
 
Hopefully you can see the damage in this webcam photo (it's the best I've got)
 

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Yeah I think I can kind of see...

So, on the M BUSS PCB (the long connector board across the bottom of the mixer channel cards), both pins 7 & 8 of P909 are audio grounds. They are redundant, but it is important that both are viable. The “green strip” is called a trace. It is not actually green, that’s just a protective coating over the trace, and you can gently scrape that coating off to reveal the bare copper trace. I asked if you were handy with a soldering iron. You’ll need to be to repair it. What you need to do for a broken trace is remove the green varnish coating from the trace on either side of the break, then tin the bare copper trace with solder, and then take a short piece of bare tinned stranded or solid core wire and place that across the break like a bridge, and then solder that all in. And any pins that have been dislodged need to be straightened and put back into proper place and then resoldered.

Because you have pins that clearly have been abused/traumatized that tells me somebody at some point in your 388’s history was not appropriately careful when removing/reinstalling the M BUSS PCB. These thin narrow boards made from phenolic resin (rather than more durable glass fiber) that have lots of relatively high insertion/extraction force connectors MUST be handled VERY carefully when removing/reinstalling. They are fragile. Hammer mechanic approaches do not work here. Somebody went all hammer mechanic on your M BUSS PCB. So, if it was me, knowing this, I would execute the repairs to what I know is damaged and then furthermore I would resolder every single solder joint on the board. Furthermore, as I suggested earlier, for your AUX buss issue I would also inspect the solder joints on the MONITOR PCB. And don’t assume if it looks good to the naked eye it’s okay. You often can’t see cold solder joint or cracked joints without good light and a magnifying glass, and even then it can be missed, so that’s why, if I’m having trouble like you are having and my tracing the issue leads to potential solder joint issues I just reflow the applicable joints even if it only ends up being preventative.
 
There'll be no pictures. It's too small and I can't hold it steady. It's one of these except it's the Tascam version. You've seen the Tascam version, you know what it looks like

s-l1000.jpg

The whole thing has completely detached from the board.
 
I wasn’t asking for a pic because I need to know what the pot looks like. I was asking because you typically provide minimal detail in your posts, which is true of the post about the knob coming off. This requires whomever is helping you to play a game of 20 questions...you make it difficult to help you.

If you don’t want to put up a pic, or provide some greater detail, don’t expect help. Do you want help? I assume so...you keep posting.

So here come the follow up questions to try and figure out what your situation is in order to try and help you.

“Completely detached from the board” is the pot broken off and the board is intact, or is the pot intact and the board is broken? I’m trying to determine if you need a new pot, or the pot can be reused and the board needs repaired/modified to reconnect the pot? You said the knob fell off, but that doesn’t coincide with the whole pot detaching from the board. Fill in the blanks.

It looks like you’ve solved the mystery of why you have no output from the AUX buss.
 
The entire pot fell on (the legs are all broken). The board is undamaged. I don't know why I said "knob". I probably forgot the part is called a pot(entiometer) and then forgot to change it later when I remembered.

So I need to replace it but I can't identify the part from the maintenance section of the manual. I assume it's the same as the other volume pots on the mixer.

I just had another look at the relevant blow up for the mixer. The parts list refers to knobs but never to pots. Maybe that's why I called it a knob?
 
If you go to the parts lists for each PCB, that’s where you’ll find it. They list them under the resistors on the parts list as well as on the schematic. The AUX master level pot is a 10K “A” taper pot. It’s R101 on the schematic and parts list. This is the same part used for the AUX and EFF pots on the input channels, and for the MONITOR mixer GAIN pots, as well as the EFF RTN 1 & 2 GAIN pots.
 
Well I’ve run into this before...it’s easy to find 10Kohm A taper PCB-mount pots with that pin spacing and and everything, but what I’ve had a hard time finding is the correct shaft type/length. It’s easy to find the spline-type shafts, but I’ve not been able to find the ‘D’ type shaft with the rest of the specs lining up. I just looked at Mouser and can’t find the right part. I believe I have some here from M-300 console parts though so send me a PM if you want to go that route.
 
PM sent. I bought a multimeter and solder sucker from Studiospares yesterday, so let's see if I can actually remove the part that's still attached to the board and the new pot on, Assuming you're remembering correctly, of course.
 
This type seems to be closest

Here’s the trick...that pot is correct except the shaft is too long...20mm instead of the 15mm type installed on the M-300 consoles (and many other period Teac/Tascam devices). Everything else is correct...correct resistance, correct taper, correct orientation, pin spacing, etc. Will it mount up correctly? I believe so, but the knob will stick up 5mm than the others on the console control surface.

But this is closer still

And that one is entirely correct *except* it’s a B taper (linear) pot instead of an A taper (logarithmic or “audio”) pot to match the rest of the pots on the console. This may not be a big deal. In my opinion the audio taper is not as critical unless you are executing fades with the control. You are not as likely to be doing that with your AUX master control.
 
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