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

Fishman Loudbox Mini has Defective Tolex from the Factory

This Fishman Loudbox Mini is about a year old. The owner loaned it out, and when she got it back, the covering was in tatters. Could The Unbrokenstring Crew do something?

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Fishman will give you a Return Authorization to return these little amps back to the factory, at your expense, and another unit would be returned to you. With supply chain issues, no stock at the factory, and not really wanting to be without the amp until a replacement became available, the owner decided to have it recovered locally.

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What was really irritating was, whenever you picked up the unit, the space under your fingernails would fill with this rubbery ‘stuff’ that was shedding from the original covering. This is particularly annoying for finger-pickers with long fingernails.

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To make matters worse, the owner would have to vacuum up little bits of covering two or three times a week in her apartment, and her car and the stage at church was always a mess after she used her amp.

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The first thing we did was remove the feet. The old covering stuck to everything!

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The handle bolts hold down this wooden stiffener on the top of the unit. Off it goes!

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This little hex bit is used to remove the bolts that hold the electronics in the chassis.

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Six bolts hold the chassis in the body of the cabinet. The unusual style of this cabinet will make the job of recovering the unit a little out of the ordinary compared to the usual Tolex job. Along the edges, both sides as well as the edge will need to be covered, with closely-matching seams.

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I’ll take a few pictures to show where the wire harness connects. These are polarized, but it is possible to swap a couple around and cause who-knows-what havoc.

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Power to the board comes from a power transformer inside the cabinet. The secondary voltages come through this cable.

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This is an audio connection from the front panel. We don’t have to remove this, but I snapped a picture just in case it came loose later.

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The front grille is held in place by three screws, almost hidden inside the electronics compartment.

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The grille tilts away from the cabinet, but is still held captive at the bottom somewhere.

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This little tab fits in the slot cut into the cabinet. We need to keep track of this slot lest it be covered with Tolex and forgotten.

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The inside corners, next to the control panel, are covered with these little plastic corner covers. Naturally, the sticky covering is all over these corners. A neat little feature of these corners is, some of the seams of the original covering come together under these corners, and are hidden from sight and protected from damage.

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These corners need to be cleaned up. Heat is not an option, as the plastic will probably deform. Most chemicals will likely attack the plastic.

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A little old-fashioned elbow grease is applied to break the adhesion between the old covering and the plastic. It rolls off slowly, with this electricians’ screwdriver.

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Once the posts are cleaned, this can be set aside, ready for reassembly later.

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While we are cleaning up hardware, every item that touches the old covering requires cleaning. Here we are cleaning under the heads of the machine screws that hold the electronics in the cabinet.

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Little washers under the heads of the screws that held the power transformer are particularly sticky. Here, we are using a sharpened edge of a popsicle stick (excuse me, they are called tongue depressors nowadays) to scrape the goo from the washers.

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Even the edges of the hardware that holds the handle strap to the unit is messy.

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We now turn our attention to the cabinet itself. We will remove the loudspeakers and keep them in a safe place while we work on the cabinet itself.

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As with most Tolex removals, a heat gun is just the thing for speeding the process along. This is a paint stripper, run on the lowest setting.

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Once an edge is heated, the old covering can be pulled away from the cabinet material. Here we go!

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The cabinet is sealed particle board, giving us a good surface to support the new Tolex.

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The old covering is shedding each time we handle the cabinet. What a mess!

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Now, we’re starting to get somewhere! Note that the aluminum tape shield around the electronics has been peeled back, so that the old covering could be completely removed.

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To re-Tolex the small panel under the handle, we are arranging to have all the edges meet at the screw holes, so that the pressure of the handle screws can secure all the edges. No over-lapping of Tolex is allowed anywhere, because of the tight tolerances between the existing cabinet and the pieces that fit into it (the electronics, the grille, and this panel.)

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One side of the cabinet is done. Here, we are opening up the holes before we forget where they were.

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Once the sides are covered, a simple rectangular strip covers the top, back, and bottom. But rather than get contact adhesive on the new Tolex, we will mask off everything, starting with these inside corners.

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I don’t think that it will be a deal-killer if adhesive is accidentally smeared on the speaker baffle, but my obsessive/compulsive disorder forces me to do it anyway.

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The blue tape indicates the start and ends of the last piece of Tolex to be applied. It wraps around the bottom, back, and top of the cabinet.

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The masking tape must conform to the Tolex already installed to prevent any wicking of the adhesive, so this brush pushes the masking tape into the contour of the new Tolex.

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This corner, at the bottom, is the fussy-est edge because of the small geometries involved. The contact adhesive on both the Tolex and the cabinet must be completely sticky and dry to hold this thin edge down. Here, more masking helps control the spread of adhesive on the bottom of the cabinet.

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I took a clear picture of the bottom before the Tolex was applied, because I will need to find those holes again, for the feet as well as the power transformer.

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And this is why we need to mask off the edges. The disposable brush works well, but it’s a little hard to control near the edges. The white stuff is the contact adhesive.

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When the adhesive is dry, it turns clear. A little heat will help speed the process along where I put a little thicker layer of adhesive on the cabinet.

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Here is the top and the right side of the covered cabinet.

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Here is a view of the back and bottom of the recovered cabinet. This is starting to look nice!

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The power transformer is bolted to the bottom. Remember those little washers that we were cleaning up earlier? They go here.

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The clean feet are returned to where they belong. Note the pink eraser; It’s just the thing for cleaning up spots on the new Tolex where adhesive leaked under the masking tape. It rolls right off, without a trace.

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The tweeter is mounted back where it belongs. The wiring is polarized so that we don’t need to worry about + and – connections on either loudspeaker.

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The other loudspeaker is reconnected and installed now.

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Remember the slot that holds the tab on the grille? This is it.

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This is a better view of the tab/slot system.

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The tab goes into the slot, and the grille fits tightly against the new Tolex.

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You have seen this before, from another angle. Three screws hold the grille in place.

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A grounding lead on the electronics is attached to the aluminum shield inside the cabinet, as shown here.

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These big screws hold the electronics in place. Recognize that little hex bit I showed you earlier? Here we are again!

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Two screws hold the handle and the rectangular spacer in place on the top of the cabinet. This project is all done!

Thanks for reading all the way to the bottom!

CONTACT – 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