Behringer only hums in church

[drewl]

So I repaired three B212D powered speakers.
I was told they went bad after a lightning storm.
All three had input resistors to first preamp stage open/fried.

Customer says they all hum, so I retest them, check all grounds etc. But they still have hum but only in their church.
A fourth unit works fine with their set up.


I'm going to check the input filtering etc, but the power supplies were fine when I repaired them.
Strange.

[52 Bill]

Ground problem in the AC wiring, ground loop in the audio lines. Have they tested the AC since the lightning strike?

[g1]

I was thinking maybe a blown ground trace somewhere, seeing as it was lightning related.

[eschertron]

IMHO most churches have terrible isolation on their lighting dimmer packs. Maybe its not the speakers, but the lighting system that's now damaged?

Take the 4th (good) speaker around to see if it hums when plugged into circuits that supply the other three. That might suggest it's the AC (as 52 Bill suggests).

[drewl]

Supposedly these guys are electricians and know what they are doing.

Also the same set up works fine with other units.

The AC ground and jack grounds all are good. No obvious burn marks or blown components in the AC input filtering, but I'll have to pull the input caps and surge protectors and test them.

[TimmyP1955]

Make sure that both legs of the balanced input are good.

[The Dude]

Maybe Jesus just doesn't like Behringer?

[Leo_Gnardo]

Maybe Jesus just doesn't like Behringer?

Nah, Behringer just forgot the words. All it can do is hum along.

[Randall]

This is why I always carry one of those yellow AC plug testers with the LEDs that test the wall outlets. Quick and easy to tell if the wall power is OK or not.

[drewl]

Make sure that both legs of the balanced input are good.

The AC going into the unit, or XLR signal in?

[g1]

XLR

[g1]

Can you post the schematic and tell us which exact resistors you replaced?

[Jazz P Bass]

Here is the schematic.

[drewl]

They all had one or both of the 1.5k and 12k resistors open.

I hooked all three together, tested them alone or daisy chained and they were operating fine, no noise or hum there, at home or the shop or anywhere else.

I'm sure I'm going to have to go to the church and check it out.

For all I know their "good" units may have an open ground preventing hum.
We'll see.

[Jazz P Bass]

That first opamp had to take some kinda hit!

If you lift either C2 or R8 & the hum stops, you are onto something.