Ampeg SVT6Pro Bass Amp Repair

This Ampeg hybrid bass amp became intermittent during an extended gig.  The unit was ready for a million-mile checkup anyway, so it was time for this guy to visit the Unbrokenstring workbench!

Externally, this unit is in gorgeous shape!  Let’s look inside.

Swollen filter capacitors are never a good sign.

I’m cutting the hot glue away from the bodies of the capacitors so I can remove them.

The circuit board assembly needs to come out of the chassis. I printed these labels to mark the cables as I remove them.

I’m documenting where the cables go with pictures.  And you get a nice look around the inside of the amp!

More cables.

Yet more cables!  Note the fuse holder that can accommodate two different fuse sizes.

The various screws holding the chassis together are different lengths.  So, I’m documenting THAT with more pics.

Same view, different place, different screw length.  This stuff is important, you know.

The AC power port gets its own short screws.

Now, the circuit board is out where we can work on it.

These spacers belong between the sheet metal housing and the various rear panel jacks.

St. Louis Music (SLM) built a lot of stuff for major American brands.  What year was this printed circuit board fabricated?

I like to mark the solder joints to be desoldered with a little liquid rosin flux.  It assures that I desolder the correct joint, and a little extra flux helps the solder to ‘play nice’ during rework.

These are the bulged capacitors.

The replacement caps need to fit on the circuit board footprint.

We probably won’t have a height restriction with any new part, but it never hurts to check.

The replacement caps need to have the same lead spacing as the circuit board.

The site is prepared for the new parts.

These are modern, high-temperature, long-life replacement capacitors.

What’s this?  This rectifier has been very hot.  Those holes are probably for cooling.  However, there has been enough thermal stress (due to the unequal coefficients of expansion between the copper, the fiberglass, and the component itself) to crack the solder joints.

Here is a better view of the cracked solder joints.  Wow!

Whoever was applying the hot glue that day probably over-did it.  There is very little ventilation around these components.

I replaced both rectifiers, leaving a bit of space between the component and the circuit board.  The hot glue has been completely removed, to allow some air circulation.  This fixes everything!

The finished unit looks as it did before we started.  Now, it works as well.

Thanks for reading all the way through!

CONTACT – David Latchaw EE

Fender Twin Custom Foot Switch Repair

Matt complained that the reverb function of his amp was erratic when he used the factory foot switch.  Could the Unbrokenstring Crew look it over and find the rat?

Let’s take a tour of the unit.  This one is in excellent shape.  Meet the ‘normal’ channel,’ on the left side.

Effects channel is in the middle.

Everything in this amp works well.  I wonder what the issue is?

Name, rank, and serial number, please!

One thing I really like about this unit is the ON and STANDBY switches sport dust boots.  Good practice to keep these switches trouble-free for years to come!

The foot pedal goes here.  This is a stereo jack, to support two functions.  Is this the problem?

Matt supplied the foot switch.  This looks as if it has never been out of the studio.

Inside the unit, we see cast frame Eminence units.  Very nice!

The tube diagram is as it should be.  I understand that the schematic is the same as the original Blackface Fender, only updated with modern components and largely built upon a printed circuit board.  Nice stuff.

No reverb here!  There is an intermittent within the pedal.

The vibrato section works fine, so we need to investigate the foot switch on the right.

Let’s verify that everything is OK here.  Very clean inside!

Amazingly, the reverb foot switch does not actuate every time it’s pressed.  A new unit is pulled from stock.

Here is the new reverb switch.  Now, the reverb is functional, but there is another source of intermittent operation.

Aha!  At the plug end, the wire insulation has pulled back, allowing the inner conductors to touch the case and each other.  This is a mess!

There are actually three conductors in the factory cable; two are used for switch functionality and the third is the braid, a ‘common’ conductor for both circuits which doubles as a ground shield as well.

Here, I’m carefully pulling one conductor out of the center of the braid while leaving the braid intact.

Here are the three ‘wires’ that we need.

I pulled the braid until it was a solid conductor.  This piece of clear tubing will insulate it from the other wires in case the insulation on the wires pulls away again.

This is the finished termination.  I had to use a big iron on the solder joint to the outer shell, and the insulation is a little worse for wear.  I’ll do better next time.  A tie wrap was added on the exiting cable to help the strain relief do its job.

Everything is back together and works per spec.

Another satisfied customer!

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE

Big Muff Pi Pedal Refurb and Update

Matt owned this pedal FOREVER and needed to put it to work in the studio for a project.  But, the foot switch had disassembled itself years ago, and the audio jacks were worn out.  This pedal needed an update to the 21st century, adding compatibility with the distributed power in a pedal board.  Could the Unbrokenstring Crew make this happen?

The knobs and controls were in great shape.  Beyond the audio jacks, switching, and power, the unit works great.

With the top cover removed, we see the VERY heavy gauge frame that takes the force from the foot switch.  Yes, stomp boxes get stomped on!  The LED mounting scheme is obviously some Soviet military hardware.

The top view of the circuit board show the ‘in’ and ‘out’ jacks.  The originals are plastic and completely shelled out.

The circuit board is more Soviet military goodness.  Clearly, this board was ‘hand taped’ and not laid out with an automated.CAD program.  I’d call this ‘one step beyond hand wiring.’

Note that all the wiring insulation is white.  This harness started out as ribbon cable, and each wire was pulled out of the ribbon as it was wired to place.

A modern epoxy-sealed foot switch is trial-fitted.  This one is triple-pole double-throw on/off to handle both the required functionality of the old switch and to implement ‘true bypass’ when the effect is deselected.

Here is the user-side of the foot switch.  We need to be sure that the mushroom button is installed as high as possible to simulate the height of the old foot switch.

For those of you who know your ‘one-line’ electrical symbols, you know that these two terminals are ‘no connect.’  What?

These ‘no connect’ terminals are used to mount the current-limiting resistor for the LED.

So the current-limiting resistor is moved to its own spot in the wiring harness.  Yes, that is clear heat shrink tubing.

More ‘one-line’ electrical symbol goodness.  The body of the switch is phenolic.  It is badly cracked and ready to shatter.  Had I been able to find the missing switch hardware, this switch could not be returned to service anyway.

And for those of you who understand the Cyrillic alphabet, this pic’s for you.

The input and output jacks will be desoldered.  The gray plastic is quite brittle after all these years.

Now that they have been removed, we can complete the schematic.  Those jacks have switches in them that need to be analyzed in isolation.  The switching function is not necessarily what Western manufacturers utilize in their jacks.

The printed circuit board is temporarily reinstalled so that we can check the fit of the new jacks.

The new jacks are genuine Amphenol units, professional grade, as they say.

The terminals are bent slightly inward to clear the internals of the pedal.

The stereo jack functions as a power switch, disconnecting the ground to the 9v source when the plug is disconnected.  Both jacks need to be oriented in such a way to minimize mechanical interference with the circuit board.

The original jack escutcheons really dress up the jacks!

All new wiring was made with silver wire with a white Teflon insulation, matching the original SovTek wiring harness.

The new switch is installed.  Unlike the original foot switch, this switch is wired as ‘true bypass’ when the effect is off.

The edge of the circuit board was trimmed away to allow clearance to the body of the new Amphenol jacks.  The trace that was cut is for the sleeve terminal, which is duplicated in the array of remaining pads.

This pic shows the orientation of the notch in the circuit board to the body of the new connector.

A new 9v power jack is added between the two in/out jacks.  This jack is compatible with the ‘Boss’ pedal power “standard” with negative in the middle and 9v on the sleeve.

To reassemble the pedal, the top frame goes on first.

The cover goes on next.  This is beginning to look like a pedal again!

Matt would use the 9v power jack in the studio, but the functionality of the pedal with a 9v battery remains unchanged.

Here is a look at the finished unit, updated with steel jacks and the 9v pedal power capability.  This unit works well!

Thanks for reading all the way through!

CONTACT – David Latchaw EE


Behringer Europower Mixer

Behringer has terminated its retail sales agreement with The Big National Retail Music Store Chain, which was probably a smart move on their part.  Fortunately, service literature from Behringer is fairly easy to obtain these days.  So, the Unbrokenstring Crew launches off into another mission sortie, this time to repair this powered mixer.

This unit needs new power FETs in the output circuit.

This unit still has a price tag on it!

Made In China pretty much tells the story these days…

I believe the date code is in the format of MMYY.

The entire power output stage is mounted on this massive aluminum heat sink.  This will be fun!

The power devices are held against the heat sink with spring tabs and screws, which can be removed with a Phillips screwdriver.

I cleaned off the conductive grease so that this thing could be handled without making much more mess than I had already made.

The discrete devices in the amplifier all test OK, so new power FETs were ordered.  These are a complimentary, matched pair of N-channel and P-channel devices.

Applying heat sink grease is logistically easier with a cotton applicator.

Both faces get a little compound.  We want complete, void-free contact between each device and the heat sink.

This is the Big Squish.

The plastic output jacks were busted up pretty badly, so new ones were installed.

The power FETs form a Class A-B push-pull stage.  Per the documentation, between five and six millivolts are measured at the test points when the idle current is correctly set.  Exercise for the Graduate Student: knowing the voltage developed across a resistor that can be easily identified on the schematic (available for download) what is the bias current?

After testing, I went back into the unit and replaced the bipolar transistors that drove the gates of the power FETs.  Now this thing really does a yeoman’s job of battling against the Silence.

Thanks for reading all the way through!

CONTACT – David Latchaw EE

Dropped Ampeg SVTIII Pro Bass Amp Quick Fix

This Ampeg hybrid amp was dropped.  And of course, it quit working.  And, of course, the guy who borrowed it was the guy who dropped it.  Can the Unbrokenstring Crew get it going again before the friendship gets damaged?

The exterior of the unit is in great shape, because it was in a hard sleeve that absorbed most of the drop.

The handle is now useless.

The rack ears (those aluminum angles) broke away from the sleeve.

The rack ears are pop riveted into the sleeve.  The guy who dropped it said that he would fix all that… just repair the electronics.  Got it!

A quick tour of the front panel, from left to right.  The midrange frequency selector has five discrete positions, which moves the center frequency of the midrange tone control up and down in frequency.

The graphic equalizer is switched in and out by the button below the mute button.  It took me a few seconds to figure that one out.

And here we are at the right end of the front panel.

This panel handles the internal switching and foot switch duties.  The fan and filter are to the left of this picture.

The name, rank and serial number dog tag is here.  Interestingly, this unit has binding posts as an option for speaker connections.  More goodness from St. Louis Music!

The toroidal power transformer dominates the interior view.  The emitter bias resistors rate their own heat sink, in the path of the cooling fan air.

All of the tubes in the preamp were brand name except this one.  Which tested fine and works well in this circuit.

The real issue with this amplifier is that the jolt of the drop unseated this cable connector.  This is easy enough to fix.

This amp is operating at 450 watts into 4 ohms.  I think we fixed it!  And the owner will be none the wiser!

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE

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

Ibanez TubeScreamer Pedal Repair

Steve said that his Keeley-modded Tube Screamer pedal had quit.  Could the Unbrokenstring Crew bring it back to life?

This pedal is in great shape cosmetically!

Robert Keeley made a few circuit changes that are well-documented on the Web.  This pedal has those changes.

The battery box has its own lid.

The bottom of the pedal is shielded with the metal screen seen in the center.

A die cut sheet of black insulative material protects the circuit board from the metal shield.

Let’s get it all out of the enclosure so that we can work on it.

We’ve applied battery power and connected up to my Marshall Stack.  The original problem is still with us.

An oscilloscope probe verifies that the dual operational amplifier is defective.  Part of the Keeley mod specifies a Texas Instruments 4558 dual op amp.  This IC is socketed, so replacing it is trivial.

Here is an example of the softly clipped audio that the Tube Screamer delivers to the amp input.

Reversing the disassembly procedure is all we need to do to get this cool little pedal back together again.  We’re done.

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE

JBL SoundFactor SF12M Loudspeaker Repair

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

Luna Gypsy Spalted Acoustic Guitar Needs Fretwork

Sophia of “Pretty In Punk” purchased this guitar new, but it was almost unplayable. Could the Unbrokenstring Crew get this beautiful instrument in shape?

Nearly every fret was higher or lower than the one next to it.  To mask the problem, the action was very high.

We straightened the neck itself so that it was absolutely flat, then used the Absolutely Flat sander across the frets.

Here, you can see the large amount of material removed from one fret, but not the others.

More material removed from the high frets.

Oh, look, here is another high fret.

Here, three frets in a row were high.

Getting close to the sound board, we’re running out of high frets to sand.

After sanding, we marked the top edge of the frets so we don’t take any more material from them.

This nifty fret file works only on the sides of the fret to round them over.  This file was reviewed in an earlier blog post.

Now, we’re getting somewhere.  This fret board is flat and the frets are even.

Sophia prefers these strings.

If you look closely, some of the over-wound strings at the end almost crested the top of the saddle.  If they get too close, I have some washers that slip over the string and sit on the ball end.  The string would then be passed thru the hole in the bridge from the inside of the guitar.  However, when tuned to standard pitch, we had no trouble here.  Missed It By That Much!


I think she’s happy with the results!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE