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

DigiTech Whammy Pedal Checkout

This pedal wouldn’t work regardless of how it was connected. He tried the supplied power supply and a pedal power unit in his pedal board, with no luck.  Could the Unbrokenstring Crew straighten out this mess?

We put the pedal and the power unit (wall wart) on the bench and verified that nothing worked.

Here is the clue.  The customer provided a DC 9v output wall wart, a power standard which is commonly used in the world of guitar effects pedals.  That power standard is not compatible with this unit.

Not much else to see around the rest of the pedal.  The MIDI control is a nice touch.

This is the ‘correct’ wall wart.  This one says 9vac output, which is what this pedal needs.  I love eBay!

Something is rattling inside this unit.  Removing these Allen head cap screws will allow us to open the case.

This is what is rattling.

And here is where it goes.  This hardware works well, so we can set the switch aside for now.

The electrical connection between the circuit board ground and the chassis is made via this metal stand-off.  This connection was intermittent until we took the wire brush to the end of the stand-off.

The light spot on this circuit board is called ‘measling.’  This indicates that something got very hot.

This inductor goes between the power supply and the chassis ground of something plugged into the pedal.  So, it failed.  The head of the destruction caused the measling seen above.

Here is another view.  This inductor serves as a simple noise filter when nothing is plugged into the guitar input jack.

The inductor is replaced and the unit goes back together.  Working with the correct wall wart, this pedal is fun!  It does pitch bending and chorus effects surprisingly well, considering its age and the technology used.

Thanks for reading all the way through!

CONTACT – David Latchaw EE

Peavey MaxBass 158 Combo Amp Repair

Replacing broken input jacks and switches are the bread-and-butter of the amp repair business. This Chinese-built practice amp needed a new input jack. The owner was a college student and didn’t have the money or space to upgrade to something bigger. Could the Unbrokenstring Crew make this unit play again?

The cabinet is a simple, sealed-back unit. The electronics chassis is accessible, but the speaker wire is threaded thru a hole best accessed from the hole where the loudspeaker goes. So, here we are taking off the grille.

As soon as the electronics chassis was slid out the front, the printed circuit board assembly came loose.

Most of the mechanical support for the circuit board is provided by the input jack, which is plastic. As a rule, I replace input jacks with all-steel Amphenol jacks. However, that won’t work here. This forces me to replace the input jack with a similar plastic unit in order to reassemble the amp back the way it was.

The old input jack is gone. Good riddance!

A new, identical jack is sourced from a commercial vendor. Now, we have a chance of a more durable assembly because the new jack is Made In USA.

When sourcing an alternative part, the electrical function must be the same. In this case, input jacks are single-pole switched jacks; the tip circuit is grounded until a plug is inserted into the jack. This keeps the amp quiet whenever nothing is attached to the input.

Likewise, the footprint of the alternative part must mate up correctly with the rest of the amp. Here, we see that the new connector pins match the circuit board exactly!

A little solder to seal the deal!

This part of the job is ready to go.

Cleanup of the unit is easier when everything is apart.

Likewise, any electrical problems can be fixed while the unit is apart. We’re checking this guy out to verify that everything works.

We have achieved success. Time to button it up.

With the chassis in place, the leads to the loudspeaker can be pulled back into the speaker box and secured. My hands were a little full, so I didn’t take any pictures while the loudspeaker was out. Sorry.

Our job here is finished.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE