Behringer MX3282A help

[thehave]

I have a behringer mx3282 that I use on a weekly basis. Recently it has had a problem that has progressed to the point that I can not use the board.

With none of the switches set to solo, the solo led on the main display is flashing and the main output can not be seen. It acts as if one of the solo buttons are engaged, but none are.

Any help would be greatly appreciated.

[Enzo]

But it works in all other ways?

This will be excruciating, sorry.

But if it is a hung switch, at least the switches are through-hole, so you can get at the leads from the underside.

Isolate the problem.

The master board will function on its own. There are some short ribbon cables connecting the master board to the last channel input board next to it. Part the ribbon that carries the solo button line. Chances are it will be the one at about the same level across the board as the switches. Does the solos stick on when the master is alone? That wopuld localize to that board.

If the master board alone relaxes and kills the solo thing, then we connect teh channel boards up one at a time to determine which one had the problem.

However we decide which board has the beef, the next step would be go down the row with an ohm meter and check each switch. Just because it clicks down;t mean it isn;t shorted.

If there are no hung switches, and it is on the master, we can talk about the control circuit.

[thehave]

Enzo,

Thanks for your reply.

Yes the board works in all other ways.

I have isolated the problem to the main board and I have checked the impedance of all the switches. There are none that are shorted. Without the schematic it is nearly impossible to find the problem. If you have some other things for me to try that would be great.

[thehave]

Without having a schematic, I think the problem may lie in the PFL/SOLO enable. This comes from looking at the block diagram.

Does anyone know where this would be on the main board?

[Enzo]

SOmewhere on the master board - there are no layout drawings - is circuit with 4580 IC7 and TIP29C T8. I believe that to be the flash driver. Got the blink rate signal on the TIP29? Probably do.

Then there is a 339 comparator IC25. That takes the solo control bus. Pick some SOLO button, and toggle it while watching pin 1 of the 339. Does it follow the switch? That feeds through some small stuff to the other side of the 339. Pin 2 of that side droves a line called CVswitch.

I believe it is this area you have something stuck

[thehave]

Enzo,

I appreciate you taking the time on this. It is driving me crazy.

The blink rate is on TIP29. As for IC25, pin 1 does not follow any solo switch. I checked pins 1 through 7 and this is what I found:

Pin 1 ~ 16V
Pin 2 ~ -16V
Pin 3 ~ 16V
Pin 4 ~ 16V
Pin 5 ~ 0.8V
Pin 6 ~ 4V then pulsed when a solo switch is pressed.
Pin 7 ~ 4.2V

While blindly probing things and seeing this on IC25, I replaced it to no avail. Hopefully you may have another idea.

[Enzo]

This won't be easy, don't expect it to be.

That blink on C of TIP29 is a line called FLASHBUS and it is a bus to all SOLO buttons.

The input to the 339 at pin 6 is a bus called SOLO_CTL. it returns from all the switches. However each switch has an isolating diode in series, oriented anode toward SOLO_CTL. SO I visualize it as the TIP29 pulsing on to ground, pulling SOLO_CTL low through the diode at any switch that is on. Also at each switch, the local LED is connected to the switch and the FLASHBUS drives it as well.

4.2v at pin7 of 339 is dead on, set by voltage divider as a reference for pin 6 to compare to. Pin 6 is pulled up via a 20k R287 run to +5. it is against that that SOLO_CTL pulls down.

In some cases there may be multiple switches grouped together. In those cases, there is a little transistor taking the place of the switch between FLASHBUS and SOLO_CTL. The push switches then control its base. The subs do this using T14, and I see T13 used by a bunch of AUXs. And T9 for some AUX returns.

Those are NPN type 546. I don;t offhand know if that means 2SC546 or what. But a shorted one could cause this. They are small, so...

I could be wrong, but your pin 6 on the 339 looks low, and something on that SOLO_CTL bus would have to be doing it unless R287 is open.

[thehave]

Enzo,

Thanks again for your help with this.

You are right about pin 6 being too low. I removed T14 and low and behold the voltage increased and all worked (except for the solo on the 8 buss being that the transistor is gone). Removing T14 at least got me through a rehearsal sinse I did not need the solo for the 8 buss.

I replaced T14, swaped D29 with another of the same type on the board, pulled the LED for the 8 buss solo and checked the resistor tied to the base of T14 with one leg lifted. The resistor measured approximately 10K. None of the other things corrected the problem. The voltage at T14C is 3.8V with the transistor in. I'm not sure what is droping the voltage.

This is going to take some time to fix. Especially because I use the board every weekend for rehearsal so I have to keep putting it back together and taking it apart.

At least I can use it for practice without the 8 buss solo function so thanks for getting me at least to that point. I really do appreciate it.

[Enzo]

OK, so T14 is the gateway to the solution. With T14 gone ALL the rest works, right?

You replaced T14 and the diode, no help. Joining those at the collector of T14 is the LED LD46, which helps pull up that collector. The LED is powered through R525 from +5. I don;t think so, but an open there could cause grief.

More likely in my mind - always suspicious - is something pulling up the base of T14. I could be wrong. You have the eight sub switches, SW44-51. All are in parallel from +5 to a common gathering at R615, the base resistor for T14. the switches are multisection, so just one part is all it takes leaving the other sections working. So not ALL contacts on each switch connect to the +5, onlly the section relatd to the solo.

Find R615, the end away from the base of T14. What voltage is present there. If +5, there you go, someone is leaking or shorted. Ground the base of T14, does that stop the syptom, allowing the rest of the system to work?

T14 is causing the symtpom, so there is not much circuitry behind it. Find out what is turning it on.