Smoked capacitor

[musicman88240]

Hello. I got this Behringer (please don't laugh too much) PMP5000 powered mixer for a spare. The intention was to put it in our rehearsal space as an extra mixer. I bought it cheap...and now I know why. I plugged it in and it smoked and smelled like burning cap.

So I opened the thing up yesterday and it was, in fact, a smoked cap. There are four 4700uf, 100V electrolytics on a board between the power supply and the power amp. One is smoked for sure and I will probably replace the one beside it too just to be safe. This board provides 120v to the power amp, so I'm pretty sure that it's hot. Last time I plugged it in was about a week ago.

So here's my question: 1. Is there a bleeder resistor in this thing to keep me from getting zapped while working on it? And 2., I read that sometimes there are some SMT resistors that blow at the same time as these caps. How would I check those to make sure they are OK? I have zero experience with SMT, but I have a low-wattage iron to use.

I could always send this mixer off to get it repaired, bit where's the fun in that? For such a simple fix it seems a shame to pay big bucks to get it repaired.

And yes...I'm going to buy a Presonus.

 

[Heliman]

You always want to discharge big caps before working on unit. I use a old screwdriver to short out the caps.

 

[musicman88240]

Yeah, that's a given. I built a rig awhile back with a 2K 2W resistor. I think that will discharge the caps nicely, but I'm concerned about getting the board off of the standoffs without getting zapped.

 

[cjdelphi]

Screwdriver's quicker

 

[musicman88240]

I've used a screwdriver on projects with lower voltage, wasn't sure if it would work with this. High voltage scares me. Still, I'm concerned about getting the board away from the standoffs without getting zapped.

There are eight caps in all, one is popped, and the rest are fine. Do you think it's still hot a week later?

 

[cjdelphi]

If the caps were in the 10000uf+ range i'd bleed it, if you're unsure use a multimeter

 

[musicman88240]

I'm at 4700. Better safe than sorry, unless there is already a bleeder resistor in there someplace. I can't find one on the schematic and nobody seems to know. The company is mum because they don't want me fixing it.

 

[73's de Edd]

.


Sir musicman88240. . . . . . .

No power supply bleeder functions are being noted on schematic.

No confirmation of surface mount use in that area but they prolifically like to use a 47 ohm in series with a 1 nf cap as being a snubber pair, across their power rectifier diodes.
Your problem is probably a blown diode, that feeds your mentioned capacitor.
That snubber pair probably wasn't damaged.

bleeder ?
Time to pull out a double clipped test lead and cut in half and then join to a 1K 10 watt wirewound resistor . . .then you are ready . . . ready . . .ready for the future.

73's de Edd


.

 

[musicman88240]

.


Sir musicman88240. . . . . . .

No power supply bleeder functions are being noted on schematic.

No confirmation of surface mount use in that area but they prolifically like to use a 47 ohm in series with a 1 nf cap as being a snubber pair, across their power rectifier diodes.
Your problem is probably a blown diode, that feeds your mentioned capacitor.
That snubber pair probably wasn't damaged.

bleeder ?
Time to pull out a double clipped test lead and cut in half and then join to a 1K 10 watt wirewound resistor . . .then you are ready . . . ready . . .ready for the future.

73's de Edd


.

Thanks, Edd, and 73s.

Jeff/musicman/KD8DQK

 

[elebish]

You always want to discharge big caps before working on unit. I use a old screwdriver to short out the caps.

Do NOT use a screwdriver to discharge caps, Use a 5 watt 1k resistor. If a cap is blown there is the possibility that a diode in the PS is shorted.