Behringer MX 9000 PFL problem

[dbdoc]

I am working on an MX 9000 24x8x2 mixer on the side for a friend. The unit goes into PFL mode on its own after an hour of operation. I have the schematics from Elektrotanya, which I learned about on this forum. What a lifesaver! Just make sure you spell it correctly or you will end up on a number of other sites.
The main pfl control circuit is just a transistor followed by a dual op amp and is on the master board. This all works as it should. It appears that some of the 5volt supply is leaking thru three of the input channel pfl switches. One section of the 4PDT pfl switch connects 5 volts to the anode of a diode. This diode is followed by a resistor which connects to the pfl control buss, which connects to the base of the main pfl control circuit. The anode of this diode is normally negative, up to -1.5 volts with pfl off. But when enough positive voltage leaks thru the switch, the diode is forward biased and voltage on the base of the transistor rises and turns on the transistor when it reaches around .5 to .6, triggering the pfl mode. I still need to remove one of these switches and confirm voltage isn't getting through external of the switch.
These switches act more like a solid state switch than a mechanical switch. I was able to isolate a switch from the rest of the circuit. When the board is cold, there is .4 volts coming thru the open switch. I reconnected it with a jumper and after the board went into failure, there was 4 volts coming thru the open switch. It even reacted to cold spray. One switch reacts to slightly moving the switch's push button, so I am reasonably sure that the switches are causing the problem.
I was wondering if anyone else had experienced similar problems with pfl circuits or this type of switch. I also don't see any ID on the switches. I plan on letting the owner do most of the legwork locating the switches. He is good at that. But if anyone has any suggestions, I would appreciate it.

[Jazz P Bass]

It sure sounds like the switches are bad.
The GK Backline amps had a similar occurrance with the channel a/b switch.
They got contaminated at the factory (probably flux residue when the boards where cleaned)

[Enzo]

Are you telling us all the PFL lights come on? Or are you telling us the PFL indication comes on in the master area, but none of the individual PFLs are lit?


Whenever I get one of these larger mixers in for maintenance - and not only Behringer, they all use the same switches - one thing I almost always do is grab my combination pliers and pop off the button caps on all the pushswitches - the sub assigns, the mutes, the PFLs, whatever. The I put my Deoxit spray tube aside each switch shaft and squirt a litle cleaner. It dribbles along the switch shaft into the switch. Push the switch in and out a few times. And that cleans it. SOlves most of this type of problem, as well as typical intermittant channels and so on. I VERY RARELY have to actually replace a switch. SWitches are common types though.

Remember, freeze spray not only chills things, it also adds moisture. So if a switch is already leaky - probably from hydroscopic contamination, freeze spray will make it worse. WHich is consisent with your findings.

[dbdoc]

The master PFL lite comes on but none of the others. In this mode you can use PFL by activating any of the PFL switches, which lites the individual PFL LED .
The one switch I hit with cold spray was on the outside of the switch only, although moisture could form inside. I am not so sure this would be a bad thing if it would clean any contaminate as it dries. And it did react in the favorable direction, but didn't solve the problem. I haven't made anything worse yet. I did touch up all the pins with a solder iron and the heat seems to have helped one switch. The deoxit is the next step. I didn't know how any contaminate would mix with contact cleaner either though, but it sounds like you have been there.
These switches have absolutely no flakiness in the closed position as far as passing the 5 volts through. If I can keep from removing these switches I am all for it, since desoldering 12 pins on a double sided board isn't always fun.

[Enzo]

Please try Deoxit before moving on to more drastic measures.

On those switches, one side controls the signal path and the other controls that relay control line. All it takes is one noisy switch to tie up your control bus.

[Dr Kaos]

I recently repaired a Mackie mixer with the PFL light stuck on (no individual lights) where the cause was a liquid spill thru the panel collecting under a PFL switch. Cleaned board and switch all good. I second the motion for Deoxit in the switches.

[dbdoc]

Cleaning the switches with Deoxit, unfortunately didn't solve the problem. I did remove the four problem switches. The board was fine underneath, so no spill problem. We have some replacement switches on order from E-Switch. I have all components I lifted during trouble shooting back in place. The board is operating as it should, with the PFL buss voltage where it should be with all PFL switches in off position, and PFL/solo works as it should. So I think the new switches will solve our problem.
I believe this is the first time I have run across switches that didn't seem at all flakey and checked ok with an ohm meter, but were still causing a problem. The tops of these switches come off fairly easily. I was able to remove one while keeping everything from flying out. It was still too hard to see if anything was wrong with this degree of miniaturization. It could be simple fatigue keeping the metal sliders from having the needed space between open contacts. As things heated up, there was enough contact to leak voltage.

[Enzo]

Cleaning often works, but you have to hope the cleaning agent gets all the way down in the works or it does nothing.

I tend to think not metal fatigue in the switch, but more likely contamination that filtered down into them. Your meter can;t measure leakage very well. On the other hand, you could be totally right.