Behringer Europower Mixer

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

Dropped Ampeg SVTIII Pro Bass Amp Quick Fix

This Ampeg hybrid amp was dropped.  And of course, it quit working.  And, of course, the guy who borrowed it was the guy who dropped it.  Can the Unbrokenstring Crew get it going again before the friendship gets damaged?

The exterior of the unit is in great shape, because it was in a hard sleeve that absorbed most of the drop.

The handle is now useless.

The rack ears (those aluminum angles) broke away from the sleeve.

The rack ears are pop riveted into the sleeve.  The guy who dropped it said that he would fix all that… just repair the electronics.  Got it!

A quick tour of the front panel, from left to right.  The midrange frequency selector has five discrete positions, which moves the center frequency of the midrange tone control up and down in frequency.

The graphic equalizer is switched in and out by the button below the mute button.  It took me a few seconds to figure that one out.

And here we are at the right end of the front panel.

This panel handles the internal switching and foot switch duties.  The fan and filter are to the left of this picture.

The name, rank and serial number dog tag is here.  Interestingly, this unit has binding posts as an option for speaker connections.  More goodness from St. Louis Music!

The toroidal power transformer dominates the interior view.  The emitter bias resistors rate their own heat sink, in the path of the cooling fan air.

All of the tubes in the preamp were brand name except this one.  Which tested fine and works well in this circuit.

The real issue with this amplifier is that the jolt of the drop unseated this cable connector.  This is easy enough to fix.

This amp is operating at 450 watts into 4 ohms.  I think we fixed it!  And the owner will be none the wiser!

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE

Acoustic B20 Bass Combo Amp Repair

Acoustic is an in-house brand name, sold by a national music store chain.  No service information is available, as these are built in China and are intended to be disposable, not service-able.  Could the Unbrokenstring Crew fix this one?

This combo bass amp is dead.  We will tear it down to see how it works, and perhaps fix it if we find something wrong.

Name, rank, and serial number, please!

The grille covering the loudspeaker is held in place with Velcro hook and loop fasteners.  Off it comes!

Removing the loudspeaker is a good way to gain access to the interior of a combo amp.  This pic documents where the wires go.

The loudspeaker leads are secured with hot glue.  This also keeps the cabinet sealed, acoustically.

This picture documents where the wiring goes.  The red and yellow wiring is AC from the power transformer.  The black and white wires go to the loudspeaker.

The knobs and other hardware has been removed, allowing us to remove the circuit board.

This guy gets very hot.  This is the power amplifier device for this amplifier.

This device is a TDA2050, commonly found in small, active loudspeakers and in other consumer applications.  This circuit appears to be a copy of the circuit appearing in the Application Note section of the data sheet.  Who needs a schematic?

To put the least stress on the circuit board, the bad amplifier device is unbolted from the heat sink.  Then, the leads of the device are individually cut at an angle  The device is then removed and the holes in the circuit board are cleared of solder (and of the old legs.)

The old device is gone.  We will add a little white heat sink grease when we install the new device.

Here is the new device, with some of the mounting hardware.  This heat sink compound is the same stuff used for years for power transistors and processor chips in personal computers.  A small tube like this will last a long time.

This doesn’t have to be very neat.  But coverage needs to be complete.

The legs of the new device are threaded into the holes in the printed circuit board.

The insulating mica washer is slipped behind the device, and the device is bolted in place.

A rectifier had become very hot.  This is the forward resistance reading…

And this is the reverse resistance reading.  One ea. new rectifier is now installed in place of this one.

These new capacitors take the place of two radial-lead components.  These axial leads fit well into the other footprints.

These capacitor leads and the power amplifier device leads are now soldered and trimmed.

The heat sink is bolted back down to the chassis.  These fasteners will have some thread-locker applied to keep them from coming off.  But, I cannot find the second hex nut.  I had this threaded spacer, left from another project, that has the same thread pattern, so I’ll use it here.  Hey, the additional metal will just add thermal mass to the heat sink.

This variety of thread-locker is thin enough to weep into the threads of “pre-assembled” hardware.

It’s time to reinstall all the front panel hardware, which also mechanically supports the front of the main circuit board.

The ground lead goes here.  The unit is really noisy if this is left off.  Don’t ask me how I know this.

The wires for the loudspeaker are fished through this grommet in the bottom of the chassis.

After looking over everything, I decided that the new capacitors would be more mechanically stable if they were glued to each other and to the circuit board.

While I’m thinking about it, I’ll put those knobs back on.

The loudspeaker wires are now fed through the hole in the cabinet as the electrical chassis is slid back into place.

The chassis is secured with four of these machine screws.

When we started this project, the speaker wires were sealed in place where they entered the cabinet.

So, we’ll seal them in place, and seal the cabinet, with this clear RTV.

This looks a whole lot like how we started.  This unit works very nicely, now!

Oops.  I found that missing hex nut.  It was stuck in the socket.  Perhaps I can use it on the next project!

Thanks for reading all the way through!

CONTACT – David Latchaw EE