B2031A active studio monitor repair/diagnosis

[matrx10503]

Can anyone give me some guidance.
Behringer Truth B2031A
Here is the issue. 1 of my studio monitors blew and now won't turn on. The other one in the set is still fine and working.
The monitor was not in use when it happened. I came back to my studio one day and the cone on the 6" woofer was blown out and almost shriveled up (indicative of heat). The woofer is seized up and does move freely. I do not know the status of the upper tweeter speaker.
I plug the monitor in and no red light, no green light when the power switch is clicked.
I have now removed the amp from the back of the enclosure. Nothing smells burnt (electric burn/blown smell) and I cannot see any defects on the board itself.

My question is, how do I go about diagnosing, with a multi-meter, if I need to just replace the amp and possibly woofer to get it working again.
I can supply pictures to familiar everyone with the amp and wiring setup.

[Khron]

Re: B2031A active studio monitor repair/diagnosis

Well, to begin with, some photos of the "almost shriveled up" woofer would help, as well as some well-lit and well-focused photos of the top & bottom of the circuit boards inside.

[matrx10503]

Re: B2031A active studio monitor repair/diagnosis

Sorry, I need to separate the amp from its heatsink/cage to get pics of the bottom circuits.
Upon further review, It does seem like one of the two large caps is bulging upwards a little.

[budm]

Re: B2031A active studio monitor repair/diagnosis

What are the P/N of those IC amps attached to the heat sink? It more likely that the output shorted out and dumped large DCV to the voice coil, I do not see any speaker protection relay.

[stj]

Re: B2031A active studio monitor repair/diagnosis

visible in the second pcb foto
LM3886T

why is it that the "better" the brand name, the more glue & flux you find in them?!!

[stj]

Re: B2031A active studio monitor repair/diagnosis

datasheet
https://cdn.badcaps-static.com/pdfs/...d0fd7f1e56.pdf

[Khron]

Re: B2031A active studio monitor repair/diagnosis

Press down on that black PVC top to see if the metal can itself underneith is bulged, or if just the top cover's sitting tall of the top of the can.

Upon further review, It does seem like one of the two large caps is bulging upwards a little.

[budm]

Re: B2031A active studio monitor repair/diagnosis

Check the resistance between the output pin and -VCC pin, output pin and +VCC pin to see if the output devices shorted out (or have very low resistance) to the VCC.

[Khron]

Re: B2031A active studio monitor repair/diagnosis

For what it's worth, i seem to remember reading about LM3886TF's (the isolated-package version) failing in Event ASP series monitors on several occasions, possibly due to overheating ie. insufficient cooling. Well worth investigating in this case as well.

Also, with the speakers disconnected, checking for DC on the speaker output connectors would be another step.

[matrx10503]

Re: B2031A active studio monitor repair/diagnosis

Sorry everyone, I did not get the reply notifications.
Here are the pictures of the bottom of the board:

@Khron
I'm definitely thinking its a capacitor issue and all the other components like the 3 IC's are probably fine.
One of the 6800uF/50V snap-in capacitors is bulging upward and not to mention when I shake the whole board, I can hear that specific cap. in the inside of it moving around.

[Khron]

Re: B2031A active studio monitor repair/diagnosis

Well, it definitely can't hurt to get rid of s**t-brand caps

[matrx10503]

Re: B2031A active studio monitor repair/diagnosis

Check the resistance between the output pin and -VCC pin, output pin and +VCC pin to see if the output devices shorted out (or have very low resistance) to the VCC.

I see -VCC and +VCC. I don't know what the output pin is.

[Khron]

Re: B2031A active studio monitor repair/diagnosis

Google is your friend

http://lmgtfy.com/?q=lm3886

The first result should answer your question.

[budm]

Re: B2031A active studio monitor repair/diagnosis

I see -VCC and +VCC. I don't know what the output pin is.

https://cdn.badcaps-static.com/pdfs/...d0fd7f1e56.pdf

Did you look at the spec sheet?
The spec sheet page 2 clearly shows Pin 3 is the output pin.
I hope this is not going to be another long thread.

[Khron]

Re: B2031A active studio monitor repair/diagnosis

One other way to find the output pin is to remember that the output of the amplifier(s) goes straight to the speakers / speaker connectors...