Behringer has terminated its retail sales agreement with The Big National Retail Music Store Chain, which was probably a smart move on their part. Fortunately, service literature from Behringer is fairly easy to obtain these days. So, the Unbrokenstring Crew launches off into another mission sortie, this time to repair this powered mixer.
This unit needs new power FETs in the output circuit.
This unit still has a price tag on it!
Made In China pretty much tells the story these days…
I believe the date code is in the format of MMYY.
The entire power output stage is mounted on this massive aluminum heat sink. This will be fun!
The power devices are held against the heat sink with spring tabs and screws, which can be removed with a Phillips screwdriver.
I cleaned off the conductive grease so that this thing could be handled without making much more mess than I had already made.
The discrete devices in the amplifier all test OK, so new power FETs were ordered. These are a complimentary, matched pair of N-channel and P-channel devices.
Applying heat sink grease is logistically easier with a cotton applicator.
Both faces get a little compound. We want complete, void-free contact between each device and the heat sink.
This is the Big Squish.
The plastic output jacks were busted up pretty badly, so new ones were installed.
The power FETs form a Class A-B push-pull stage. Per the documentation, between five and six millivolts are measured at the test points when the idle current is correctly set. Exercise for the Graduate Student: knowing the voltage developed across a resistor that can be easily identified on the schematic (available for download) what is the bias current?
After testing, I went back into the unit and replaced the bipolar transistors that drove the gates of the power FETs. Now this thing really does a yeoman’s job of battling against the Silence.
Thanks for reading all the way through!
CONTACT – David Latchaw EE