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