active loudspeaker - Unbroken String

Two active loudspeakers have the same problem – the high-frequency driver has quit. Does the Unbrokenstring Crew have what it takes to get into these loudspeakers and do the repair?

The enclosures of these units are molded from a very durable plastic material.

So are we dancing?

The rear panel has a crossover frequency control, equalization, overall volume, and audio inputs.

The bottom of the real panel has an input power IEC jack and an on/off switch.

The ‘Suggested EQ Settings’ suggests to me that this is a little more consumer than pro.

Perhaps we can gain access to what’s wrong by removing the loudspeaker. These nice Allen-head machine screws hold the loudspeaker frame to the case.

With the loudspeaker out of the way, we realize that we cannot really get any tools inside to replace the high frequency driver. We will need to split the case apart, which is no big deal but at least thirty screws are used! Let’s get the electric screw driver warmed up and get to work.

The first thing to remove is the handles. This flat-head screw comes out with the aid of a magnet.

Inside the handle, a nut falls out when the screw is removed. I’ve retrieved the nut with a magnet.

The nut is held in place in this molded socket. This may be interesting to reassemble.

This one foot long screw driver bit will allow us to reach all of the screws.

Some of the screws are long.

Some of the screws are shorter. We make note where they all go.

Here is the foot-long screw driver bit at work.

There are screws holding the case halves together underneath the plate of the amplifier. Off it comes!

More screws come out. Glad I got this long bit!

The plate that the amplifier is mounted on is gasketed in place with this L-shaped plastic strip on two sides.

At last, we are in! The two sections of the enclosure come apart.

And here is the high frequency driver that needs replacement.

To completely separate the two halves of the case, this cable to the pilot light can be removed.

These drivers are held in place with four screws.

Interestingly, the voice coil inside the driver is intact, yet the unit did not work. This tells us that the voice coil had separated from the diaphragm.

The exact replacement, like the original unit, is made in China.

The new driver is installed with four screws.

All of those screws go back in where they came from.

And there are a lot of screws!

I used blue tape to keep the nuts captive while the handles were reinstalled.

The handle screws are easily tightened as the nuts are held captive in the plastic socket.

Now that the handles are tightened down, the blue tape keeping the nuts in place can be removed.

A little originality is necessary to finish the job.

At last, these units are ready for testing and return to service!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

This powered monitor/PA box was badly abused but could become the basis for a good keyboard amp. Could the Unbrokenstring Crew put it back together and make it gig-worthy?

All of the parts are here, but they are rattling around inside. Electrically, it worked, but the third-world construction techniques rendered the unit worthless for loading in and out of a venue.

For instance, the control panel was literally kicked inside the enclosure. Yes, those are wood brads fired from a nail gun.

The trim plate was easily removed.

To get a good look inside, the main loudspeaker was removed.

There are all kinds of things rattling around inside this unit.

The horn driver is shot. Proceeding with exploratory surgery, the lens is coming out.

This driver is threaded, which implies that if it needs to be replaced, a standard compression driver can be selected.

This driver is probably a Chinese copy of a Motorola unit. The series resistor is all this unit has for a crossover network.

Disassembling the driver reveals this one-inch cone.

Behind the cone is this piezo driver. It has come loose from its mounting and one of the connecting leads is broken.

The piezo element was glued on to the back of the cone. This is some fine Far East engineering!

The resistor in series with the compression driver has miraculously survived the abuse.

I am a big fan of these compression drivers. They are low-cost, covered by a warranty, and compatible with this setup.

In this application, it is an equal or better replacement for the plastic driver that we are removing.

These crimp-terminals are handy, although we may use some insulated ones that are the same size.

This crossover network is compatible with the new compression driver and loudspeaker, plus it is adequately sized for the power levels involved.

The only weakness out-of-the-box is that the large inductor needs additional support, particularly if this crossover network is installed in gear that will be loaded out for gigs. The shiny glue is actually hide glue.

Here is the crossover network in its new home. The main power transformer is on the left and the power amp with the large black heat sink is on the right.

Shifting out attention to the user interface, the control panel is separated from the piece of MDF board.

A quick inspection of the power amp reveals that it is functional. However, temperature cycling and vibration have broken a couple of solder joints. These will be repaired before returning the circuit board to service.

I tried to drive the brads out of the MDF the same direction that they were driven in. However, they were VERY firmly stuck in the wood.

I decided to just trim them flush and get on with the process.

This black marker is adequate to disguise any exposed MDF around the control panel.

This piece of MDF will be glued and screwed in place from the inside of the cabinet. Here we are pre-drilling the MDF to prevent the screws from splitting the thin material.

These screws will be driven from inside the cabinet.

Oh, look here! More stuff rattling around inside the cabinet!

I wonder where this goes?

The control panel is about ready to reinstall.

A little glue is spread around the inside of the main cabinet where the control panel will be fastened.

The control panel goes here.

A socket wrench drives the bit, seating the screws. Not much room here for a powered driver.

This is another view of the control panel mounting scheme and the 100 watt amplifier.

The AC power for the amplifier is filtered by the yellow across-the-line capacitor seen on its own circuit board.

Time to start putting things together for real.

We’re drilling a hole to mount an L-pad attenuator that allows the user to set the level of high frequency audio coming out of the horn. The fixed resistor seen above will be eliminated by this L-pad.

The L-pad level control looks almost factory.

The main loudspeaker seems OK.

Some closed-cell foam is installed around the loudspeaker to seal the cabinet. Peeling the white paper backing reveals an adhesive back that keeps the foam in place.