Acoustic B600 Head Acts As If It Is Always Muted

This unit lights up but does not make a sound. Matt said that other techs couldn’t figure out why there was no audio coming out of this almost new bass head. As the last resort, could the Unbrokenstring Crew succeed where no tech has succeeded before?

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The plastic face plate, bezel, and knobs were coated with a synthetic rubber material that turns into a sticky mess with time. Here, I’m using a little elbow grease to see what works to remove this particular flavor of sticky rubber stuff on the bezel, which surrounds the front panel.

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This rubber coating, seen on this mouse, is popular on all sorts of consumer and pro-sumer gear, and the Internet has many articles and YouTube videos regarding how to clean this stuff up. Once the rubber coating begins to degrade, some sort of sticky chemical leeches out over all exposed surfaces. I suspect that the chemical has contaminated the MUTE push button, causing it to be, in an electrical sense, ON all the time, thus muting the unit. Other techs were looking for an electrical problem, not a chemical or material science problem.

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I took lots of pictures while disassembling the unit. There are no schematics available.

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The pictures would be invaluable when reassembling the unit.

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Here is another cable, hidden on the side.

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The central section of the unit is this chassis, which is a power supply and Class D power amplifier. The chassis serves as a heat sink for all the active electronics. A cooling fan on the top circulates air through the unit.

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With the chassis out of the way, the screws securing the front panel PC board in the chassis can be accessed. Of course, the knobs have to come off. Don’t they always?

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The front panel has all the controls, and some blue LED lamps. The MUTE button is labelled in the silk screen (white lettering next to each component.)

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The push buttons covers themselves don’t need to be removed to allow the unit to be disassembled, but they need to be removed to clean the switch. And they, too, are covered with the sticky rubber stuff.

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We are looking at the world’s first spa for Acoustic Amp push button caps. After trying several different chemicals and soaps, straight isopropyl alcohol seems to work the most quickly.

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Now that the switches and controls are cleaned, we can start the process of re-assembling the unit. The machine screw with the red fiber washer fits into a slot in the edge of the circuit board.

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Everything is reassembled in the reverse order of disassembly. The jacks and buttons on the rear panel are cleaned and lubricated. None of these were contaminated with the sticky rubber stuff.

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Power is applied, and all functions are checked out. Unbelievably, the soldering iron remained cold for the entire repair action. Sometimes you don’t need to be an electrician to make things work again!

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Thanks for reading all the way to the end!

David Latchaw EE

281-636-8626

Ampeg BA115HP Combo Rescued from Pawn Shop

This pawn shop find doesn’t work, parts are rattling around inside, and a knob is missing. Watch The Unbrokenstring Crew get this into the practice room!

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St. Louis Music moved their production to China, which IMHO was the beginning of the end of this fine brand. Que sera sera.

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I had always wondered if stuff from a pawn shop with obscured serial numbers was a ‘little warm to the touch,’ if you know what I mean. But who would steal something this big?

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The electronics are out of the cabinet for inspection. A replacement Gain knob is on order from eBay, because someone cranked it up and ripped off the knobs. The ‘Style’ switch, which was broken internally, was put on order as well.

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This amp sports three band EQ and a Master Volume

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You can play your CD through the Line Input RCA jacks on the top, and mute the amp to play through headphones. In the muted mode, a built-in chromatic tuner comes on. Neat!

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The internals of the tuner was one item rattling around inside the unit.

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The translucent lens over the LED array diffuses the individual LEDs that illuminate when each note is played. This was very dirty for some reason.

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The tuner electronics and LEDs are one assembly, which makes me think that this was an option for this model.

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This power cord needs attention. All I need to do is unclamp it, shorten it, and re-clamp.

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The Chinese did not go cheap on the power transformer. This is the power supply section, with the filter capacitors seen on the left.

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The power amp is all bipolar transistors. The large white objects are wire-wound resistors. Some of these resistors are open-circuit, but the transistors are all intact. This is interesting…

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Removing the preamp board, we note that these spacers are required behind the input jacks. The gain control, which needed a knob, is fine.

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Speaking of spacers, this toothed washer is required behind the Style switch (the white and green item in the center of the picture) to properly set it’s geometry. This is the new switch. The original switch was defective.

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The ‘sleeve’ contact on this input jack was open-circuit, which caused hum with nothing plugged into the amp. Do you see anything?

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The contact on the right is not closing when nothing is plugged into the jack.

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The easiest way to fix this is to just replace the jack, although these can be carefully disassembled and bent back in place. These are inexpensive enough to just replace.

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The open resistors were identified and removed. Note the round holes in the circuit board.

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These round holes are for air circulation. I believe that there is a relationship between the plugged holes and the defective resistors.

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Some of the holes were plugged. The material plugging the holes is adhesive that helps to mechanically support the resistors when the amp is moved. Someone at the factory was sloppy with the hot glue gun!

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The new replacement resistors are modern high-temperature resistors, spaced above the circuit board for additional heat dissipation. All of the holes in the circuit board are clear now.

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This part of the circuit board is all cleaned up and ready to reassemble.

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The raw corners around the mute button can be stained to match the black anodize aluminum. A permanent Sharpie pen works well for this task. A little blending with a finger, before the marker fluid dries, makes the repair invisible.

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The front view of the finished unit is pretty unremarkable, since the controls are on the top and back. The control in the lower center is a pad for the tweeter. This unit is now 100% and ready to get back to the practice room!

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Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

Cool Sunn Practice Combo Amp Blows a Fuse

The fuse blew in this Sunn Stinger 20, but the new fuse blew as well. Then, it quit working for good. Cool Under Fire wanted this combo amp back in action. Could The Unbrokenstring Crew sort it all out?

    

Our patient still has that cool Sunn vibe after all these years, even if it doesn’t work.  That name badge is recognized by all.

    

Getting up close and personal to the front panel, we see an input jack and three tone controls.

 

Independent Gain and Volume controls show that this unit means business!  The mute button and headphone jack give this amp a family-friendly advantage over other inexpensive practice amps.

  

As is found on many guitar amps, the cabinet is sealed with a closed back.

 

Oh no!  The sticker says “DO NOT OPEN.”  What are we going to do?

 

We open it, of course.  How long has it been since you’ve seen a loudspeaker with a square magnet?

 

Obviously this is a four-ohm loudspeaker.

 

The steel chassis has circuit boards for the preamp functions, just behind the controls, and power supply and audio power amplifier at the rear of the unit.  Nothing appears out of order here.  No, wait!  Look here!

 

This wire has come un-crimped from the terminal, seen in the background.

 

We can just open this terminal up a bit, re-insert the wire, and solder it in place.  But the question remains:  Could this have been the reason that the unit blew fuses before it finally quit permanently?  I don’t think so.

 

While we’re waiting on a copy of the Sunn Service Bulletins for this amp to come via email, let’s take a minute to clean this unit up.

 

The controls hold the front edge of the circuit board in place, and a couple of screws hold the back edge steady.

 

The headphone jack is entirely isolated from the chassis of the unit.  Even though the chassis is wired to the green wire safety ground in the AC cord, taking measures such as this makes the UL Certification easier.

 

The input jack is shielded from interference with this metal bracket.  This kind of additional shielding is almost never done on inexpensive amps…  this Sunn is definitely a Cut Above!

 

The switches and controls are easily cleaned now that they are easily accessible, as shown here.  The Unbrokenstring Crew NEVER forces cleaning fluid around the shaft of the potentiometers as a cleaning procedure, because dirt and old lubricant is forced inside the control.  It cannot end well.  Sorry, StewMac.

 

With all the hardware out of the way, it is a trivial matter to clean up the face plate.  Gibson Guitar Pump Polish is pressed into service for this step.

 

Reassembly also involves tightening the woodwork.  Over time, wood shrinks (even in humid South Texas) so most amplifier cabinets will develop buzzes and rattles as they age.

 

Maybe if we work quickly, the “Do Not Remove” police will not catch up to us and put us in jail.

 

The Service Literature arrived!  It states that the next-higher ampacity of fuse should be used in this unit;  There was an error at the factory wherein units in this serial number range had an inadequate fuse installed!  This little amp has run for hours with no issues!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey 260 Booster Amp Needs a Boost

A local church has a rack of Peavey gear that drives their public address system.  This unit had failed.  A phone call to The Unbrokenstring Crew was all that was needed to get this unit onto the operating table.
Unlike a traditional guitar amp or stand-alone amplifier, this unit takes high level signals and buffers it to the loudspeakers.  They are often used as a means to fill in or expand the coverage of an existing system.

 

And the inputs are daisy-chain-able.

This is, of course, the volume knob.

 

Here is a tour of the rear panel.  The power switch is ‘center-off’ and, on this unit, does not switch the chassis ground to connect to one side or the other of the AC power line.

 

No tour of a rear panel is complete without a high-rez pic of the power cord.

 

And the name/rank/serial number part of our tour.

 

The AC line fuse was popped.

 

This is, correctly, a five amp slow blow unit.

 

Removing the front and rear panels permit access to all the electronics.

 

The chassis is attached to the rear panel of the unit.  Surprisingly, the power transformer is NOT fastened to the wooden cabinet, but is also attached to the rear panel.

 

These filter capacitors are good and will not be replaced.

 

This unit has a shorted rectifier.  New parts were secured, and are shown here.  Although only one rectifier is bad, the other three are the same age and have experienced the same abuse, so they will all be replaced.

 

The new rectifiers are installed.

 

Funny thing is, the new fuse looks a whole lot like the old fuse, except it is not all exploded and burned and stuff.

 

The output from my radio is amplified to a larger signal using my Marshall Stack, to drive the input of the amp at the proper level.

 

This was the point when the neighbors called the cops to complain of the noise.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626