speaker

Mackie Thump Active Loudspeakers Need Tweeters

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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

KRK Rokit8 Powered Studio Monitor Repair

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Steve used these monitors in his studio constantly. Unexpectedly, they broke. Can the Unbrokenstring Crew bring them back to life?

These monitors have no grille so the raw loudspeakers were given a little protection for the trip to the shop.

 

These are entirely self-contained, with power supply, amplifier, input attenuation, and high frequency controls built in.

 

Name, rank, and serial number, please.

 

The high frequency control rolls off audio energy going to the tweeter, thus acting as a simple tone control.

 

The balanced input line is a nice touch!

 

The fuse is located in a drawer built into the IEC power jack.  This fuse is open.  What caused this?

 

Let’s take a look inside.  I like all the acoustic attenuation batting inside this unit.

 

The swollen electrolytic capacitor is highly suspect.  Let’s test it.

 

This Chinese capacitor meter shows that all is well.

 

This kit ESR meter shows a very reasonable number for internal losses for a capacitor this size.  What gives?

 

The old 1950s technology Heathkit capacitor checker tests this unit at its working voltage.  Only at the full working circuit voltage does this capacitor fail.  Old Stuff Rules!

 

A couple of matched electrolytics are installed.

 

These monitors live to rock it another day!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey 1810 Bass Enclosure Crossover Doesn’t (Crossover)

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Most of what you see in this post works well. However, a crossover network inside the speaker cabinet doesn’t work at all. Could the Unbrokenstring Crew sort out a solution that would put this cabinet back to work?
Here is some file old hardware from the 1980s.  This cabinet has an eighteen inch subwoofer underneath two ten inch loudspeakers.

 

Behind this panel is found a crossover network that routes the input signal to the proper destinations.  The crossover automatically routes the really low frequency stuff to the 18 inch loudspeaker and the rest is routed to the ten inch loudspeakers.  Or, you can specify which signal goes to which driver using the BI-AMP jacks.

 

The sheet metal screws hold this circuit board in place.  An inductor plugs into the pins in the center of the picture.

 

The input jacks are seen in this view.  The capacitor is part of the crossover network, and has been replaced.  As this is in a high powered audio network, the electrolytic capacitor is a non-polarized variety.

 

We can examine the circuit side of the printed circuit board to figure out the schematic for this assembly.

 

This inductor is cooked.  Unfortunately, this part is no longer available.

 

An examination of the inductor may give us some clues that we could possibly use to fix it and use it again.

 

This component can take the place of three different inductors, thus the three wires.  Any two wires yield a different inductance.  Unfortunately, the insulation is thoroughly cooked.  No salvaging this guy.

 

The functions of each ‘net’ on this circuit board is labelled with a felt tip marker.

 

For this particular model of crossover, the inductor wires we need to use are indicated by this inductor symbol.

 

This is an inexpensive crossover kit, with similar specifications.  We can harvest this inductor for use in the crossover.

 

Here are a few details, listed on the end of the box.

 

This inductor is has a laminated bar core.

 

We can remove this inductor from the circuit board and use it in the Peavey circuit.

 

The back side of the donor network is covered with some self-adhesive foam rubber.

 

The foam sticks really well!  But now we have access to the solder joints that need to be unsoldered.

 

The mechanical mounting scheme is VERY robust for this heavy part.  We can use all of this in the Peavey circuit.

 

These white plastic caps are handy to hold the mounting nut and to insulate the exposed iron core.

 

We will use the original circuit board as a template for finding and marking places to drill new mounting holes in the Peavey circuit board.  We can search around for a practical mounting location.

 

The mounting holes are marked.  Away We Go!

 

One end of the inductor is electrically wired here.

 

The other end of the inductor is wired here.

 

We will make lock the threads of the mounting screws with this stuff.

 

The thread locking compound is thin enough to seep into the threaded fasteners and lock them.

 

Now, we can reassemble the crossover network assembly.

 

The original inductor was mounted in the foreground.  This doesn’t look to tacky, does it?

 

One last look before it disappears into the enclosure.

 

The panel is ready for reassembly into the enclosure.

 

No.  Wait.  We need a new gasket between the I/O panel and the cabinet enclosure.  This self-adhesive foam strip material is just the ticket for this application.  Note the mitered corners.  Because I’m OCD like that.

 

Holes for the mounting screws are cleared with the Exacto knife.

 

OK, NOW is one last look at this assembly.  The wire pairs go to the loudspeakers in the cabinet.

 

This system is pretty awesome.  Everything tests out at full power.  Life Is Good!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

JBL SoundFactor SF12M Loudspeaker Repair

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Sal is a busy DJ who had to use his backup system after one of his JBL speaker cabinets quit. Can the Unbrokenstring Crew help get him back on the air?

This loudspeaker has been a reliable performer but had lost its high-end, then quit entirely.

So we found this inductor inside the cabinet, probably part of the crossover.  This is the ‘quit entirely’ issue.

The grille and loudspeakers need to come out to get access to everything.

This horn driver is Made In The USA!

This pic documents the wiring polarity.  One yellow wire is solid and the other yellow wire has a black stripe.

All we are measuring across the horn terminals is the crossover impedance.  This is the ‘lost its high end” problem.

This speaker is also Made In USA!

This is a pic to document the loudspeaker wiring.  One of the green wires has a stripe.

This loudspeaker seems to be OK.  No dragging or other issues were noted.

The loudspeaker wiring was bundled up to make it more compact…

so that we could remove the crossover network and repair the missing inductor issue.  This is a simple soldering repair and has been covered elsewhere in the blog.

The date of manufacture of this assembly is stamped inside the unit on the bottom of the cabinet.

Some of these horn drivers are repairable by the end user.  Let’s see what’s involved with a repair.

The threaded horn adapter comes off when four screws are removed.

The voice coil can be removed from the magnet as shown.  This voice coil assembly is widely available from many Internet vendors, Amazon, and eBay.  This is a repair that you can do yourself if you are handy with hand tools.

I searched for this JBL assembly number and found the whole assembly on sale for about the same price as the voice coil.  And, it comes with a warranty!

This horn driver has a higher power rating than the original OEM part.  These are made in Brazil now.

Oh, and did I mention that it came with a warranty?

This dust cap keeps junk out of the interior diaphragm of the horn driver.

The new driver even looks cool.  Too bad it’s hidden inside the loudspeaker cabinet.

The assembly fits in the loudspeaker without modification and is compatible with the existing wiring harness.  The thread pattern is compatible with the horn lens.  It just screws on by hand without tools.  What’s not to like?

 

Another happy customer!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626