bass amp

Ampeg BA115 Bass Amp Falling Apart (Literally)

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Back in August of 2017, one of these combo bass amps came through the Unbrokenstring Shop with cracked solder joints, which were probably a result of brittle lead-free (RoHS) solder on the circuit board.  The owner of this Ampeg saw that post and called up the Unbrokenstring Crew to ask if this amp could be fixed as well.  Here we go!

Disassembly and reassembly of the combo amp is exactly the same as was performed on the earlier post.  Here, we are starting this blog post with just the chassis on the bench.

 

Name, Rank, and Serial Number, please!

 

From above, everything appears to be as it should be.  No wires are hanging loose as was seen in the amp serviced in August, 2017.

 

But once the circuit board was removed from the chassis, this rotary switch came loose from the circuit board!

 

As was seen in the other repair, the metallurgy involved with the soldering process was to blame.  In addition, in my opinion, this switch was not the exact part that matches the footprint on the circuit board.  Note that the pins are bent inward to the center of the switch.

 

These pins are plated in gold.  This is a good thing for the component, but gold, in solution with molten solder (yes, the metals mix) makes the resulting solder joint brittle.  Here, some activated rosin flux is added to the gold plated pins to prepare them for a coat of tin-lead (non-RoHS) solder.

 

Tinning is complete.  In this picture, we can see the intentional bending of the legs to match the holes on the circuit board.

 

This is a high quality part, and works well in this application.  However, the manufacturing engineer at SLM was off his/her game that day.

 

Our new solder joints will probably outlast the amplifier.

 

No parts were required for this repair, only labor.  This unit plays 100% now!

See the previous Ampeg 115 post for reassembly.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Ampeg B2-RE Bass Amp Gets New EQ Potentiometers

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Bass players use SERIOUS hardware, and this Ampeg B2-RE is a serious, heavy, high-end bass amp.  So why in the world does Ampeg use those tiny little plastic slide pots for the equalizer controls?  The Unbrokenstring Crew digs in.

Ampeg is a name that needs no introduction nor elaboration.

 

Three of the plastic handles are broken off.

 

A tour of the rear panel shows the cooling fan, the quarter-inch speaker jacks and Neutrik jacks for output power.

 

The preamp and power amp can be separated using these jacks,  An effects loop is nice, and a balanced output for the poor sot in the control room is a nice touch.  The balanced output has a switchable attenuator.

 

A look inside shows a big green circuit board that handles the power amp duties.  This view is dominated by the toroidal power transformer on the right.  The ribbon cable on the left carries power to and signals from the preamp board.

 

And here is the preamp board.  Interesting, the slice potentiometers for the EQ are located on their own circuit board on the left side of this picture.

 

The preamp board needs to come out to get to the slide pots.  This end of the cable comes loose easily.

 

Screws hold the preamp board in place in the chassis.

 

All the knobs come off, as well as the nuts beneath them.

 

The input jack has its own nut and washer.

 

While we’re here, an inspection of the unit reveals a solder joint on the path to failing.  This is an easy fix.

 

The slide potentiometers used by Ampeg are special for a couple of reasons.  One reason is, they have this nice vinyl flap over the slot that helps keep dust out of the internals of the potentiometer.

 

The potentiometer board is held down with these screws and brackets.  This needs to come apart next.

 

The other thing that makes these slide potentiometers special is that they use three legs to attach to the circuit board.

 

Here is the exact part number that needs to be specified when purchasing replacements.

 

A similar part is available from other sources, but if you look closely, the ‘pin-out’ does not match.  The correct part is in the foreground, and the ‘new’ incompatible part is seen in the background, with the ‘1736’ marking, which is probably a date code of year = 2017 week = 36.

 

I was curious if the slider and handle could be removed from the new parts and substituted into the old pots.

 

The interior view of the slide pot.  The black plastic thingie with the fingers, on the left, is the wiper, and the black strip on the right is the resistive element.  The fingers slide along the resistive element to achieve the variable resistor function.

 

Oh, drat.  The sliders are a different thickness.  We won’t be able to use the new parts to repair the old controls.

 

So, a week later, a batch of the new controls come in.  We are going to change them all out.

 

What do you think, folks?  Should I clean up the front panel and remove all these presets?

 

The new parts are installed.

 

The workmanship appears to be pretty good, if I do say so myself!

 

Back on the air, this Ampeg is ready to really rumble through its four hour burn-in!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey MKIII Bass Head Needs Un-Smoked

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This solid state Peavey Bass head is also capable of handling mixer, equalization, and preamp roles for public address, monitor, and other sound reinforcement roles.  But power amp quit!  Can the Unbrokenstring Crew help?

The AutoMix function that Peavey developed has been discussed elsewhere in the blog.  Lots of EQ knobs here!

 

Note the graphical EQ and bi-amp capability.

 

Woah!  An Instrument System!  Ooh.  Aah.

 

On the rear panel is the power switch and speaker connections.

 

Peavey mixes and matches front panels (inputs, preamp, eq) and rear panels (ps, power amp) to build different heads.

 

As is the case with many pieces of electronics, the City of Los Angeles Fire Department approves this unit!

 

Name, rank, and serial number, please.

 

The blue circuit board at the top is for connections to the power transistors.  The I/O connections are to the left and the power supply filter caps are seen here.

 

More blue boards at the top, for power transistors.  Driver transistors are found on the square heat sinks.  Do you see the problem yet?

 

This circuitry is all preamp and tone circuitry.

 

This sucker got HOT!

 

The worst damage was to components that were near the root cause.  They burned because the transistor on the aluminum heat sink suffered an internal short circuit.

 

The heat of the electrical fault was high enough to melt solder, which happens around 650 deg. F.

 

A matched set of driver transistors were installed and the circuit board cleaned.  The destroyed components to the right have been removed and will be replaced.

 

The new parts are mounted just above the circuit board.  We can get flame-proof resistors now, unlike when the unit was built with in the 1970s.

 

More collateral damage was found on one of the blue boards.  This solder trace acted as a fuse at its narrowest point.

 

The circuit board is now cleaned up and the gap is bridged with a bit of 16awg solid copper wire.

 

Some of the power transistors were shorted as well, so all of them are now replaced with a matched set of eight.

 

These parts are still made by ON Semiconductor, the heir apparent to the Motorola semiconductor product line.

 

The electronics are back together.  The filter capacitors are original, but are still in great shape, so they remain in service.

 

And, of course, after all the components and circuit boards are in flames, the fuse finally does its job.  Of course.

 

With a new fuse, the electronics are connected again and initial tests begin!

 

This unit is back on the air!  This unit is almost hifi sound quality, with endless bottom end.  Good Job Peavey!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Mesa Boogie Bass 400 Repair and Refurbishment

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Craig was very proud of his rack-mount tube-powered bass amp.  But there were crackles and pops while playing.  He wanted to have everything looked-at.  Could The Unbrokenstring Crew get it tuned up?
This is a heavy monster.  Yes, that implies 400 watts out.

 

These are the tubes that were still alive.  We’re missing a couple of pairs.

 

Some contact cleaner/lubricant will be applied with a pipe cleaner into each socket pin.

 

The sockets are in good shape, but just need some chemical attention.  You can still get pipe cleaners, but instead of checking the tobacco aisle, you might look at the gun cleaning supplies section of WalMart to find them.

 

The individual pins on each tube are wiped with the contact cleaner/lube before they are inserted into the unit for testing.

 

The push pull class AB1 pairs are oriented from front to back in the chassis.  Once the operating point was established, pairs of tubes were sorted that had similar characteristics.  So we matched six pairs of tubes to within 2% of each other.  Power and output transformers are seen in the foreground.

 

The amp was driven by a sine wave and operated at 60 watts into a dummy load.  Each pair of tubes were verified in sequence.

 

And this is what six pairs look like while putting 400 watts into a dummy load on a messy workbench.

 

Craig gigged with this amp.  When someone asked what he thought of his rig, he said, “It’s brutal.”

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey KB4 Keyboard Amp Disintegrating

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This Peavey keyboard amp works intermittently, but the modular AC outlet has completely come loose from the rear panel. Can the Unbrokenstring Crew fix this?

This unit is pretty cool, with a built-in luggage roller and extendable handle as standard equipment from the factory.

 

The modular AC plug was covered in RTV rubber.  Was this a user ‘fix’ or did it come from the factory this way?

 

Fortunately, all the electrical conductors were insulated.  Otherwise, we would have sparks.

 

User controls are on the top.

 

The ground polarity switch is a throw-back to the days of two wire electrical cords.

 

A simple mixer is integrated into the unit.

 

Name, rank, and serial number, please.

 

The panel layout allows for some space for the handle.  Good Job!

 

We removed the head from the cabinet,

 

Here is our intermittent.  This power resistor had broken free from its solder pad.

 

A circuit board trace had broken.  An Exacto knife clears away some of the solder mask to allow for a repair.

 

This crack was very small, so a good solder jumper is all that is needed here.

 

This unit appears to have been wet.  Do you see the minerals left behind after the water evaporated?

 

Here is another little blob of mineralization.  This may have been from solder flux residue left after the assembly was manufactured.  Some fluxes turn white in the presence of water.

 

This screw was loose inside the grille.  This screw holds the loudspeaker in place.  This is not good.

 

Most of the screws were loose.  While we have this unit on the bench, we should be sure that the loose screw is not a sign of a more sinister problem lurking with this unit.

 

Here is the loudspeaker in this unit.  Nice!

 

My guess is, humidity has softened the baffle upon which the loudspeaker is mounted.  These Tee nuts will be removed and new holes drilled in the baffle in different locations.  The Tee nuts will be reinstalled and we should be Good-To-Go.

 

Some black nail polish will camouflage the new fastening hardware.  You do have black nail polish, don’t you?  Doesn’t everybody?  Hint: This also makes good thread locker.

 

OK, now for the IEC power jack.  This unit has mounting ears, so we won’t rely on friction or glue to keep it in place.

 

As an added bonus, this IEC jack has a built-in noise filter.

 

The hole in the chassis was enlarged to accommodate the new jack.  This hand grinder is adequate for the job.

 

Yep!  Just fits.

 

Now we will bore the steel panel to accommodate the mounting hardware.  This will be SO much better than glue!

 

This doesn’t look too bad, does it?

 

All of the original wiring goes straight onto the new IEC jack.  This is better than factory!  Hot glue was apparently used at the factory to secure the switch and the old IEC jack to the rear panel.  So that answers that question.  Shame on you, Peavey!

 

Here is one last look of the internals before we reassemble the head.  The mixer is at the bottom and the power amp and power supply is at the top.

 

This unit is literally ready to roll!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Behringer Ultrabass BX4500H Bass Head Needs a New Fan

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Billy said that this unit worked very well until it overheated and shut down after about an hour and a half of use.  He also didn’t hear the fan run.  Could something this simple be all that is wrong with this bass head?

The Unbrokenstring Crew operated this unit for an hour and verified that it overheated and shut down, as it should.

 

Here is a tour of the rear panel.  The power switch is in the back of the unit.

 

I really like the Neutrik Speak-On connectors for power at this level.  The 1/4th inch plugs could be operated above their data sheet limits for current and voltage if used at the 450 watt level.

 

We’ve removed the top of the unit.  Removing the finger guard lets us verify that the fan itself is completely locked up.

 

To get to the fan, we need to remove the heat sink assembly, which is held in place with these screws on the bottom.

 

This pic just allows us to keep the wiring straight for reassembly.  This circuit board handles the power output duties.

 

The fan is bolted to the end of the heat sink assembly.  The power amp is also attached to the whole stack.

 

We need these specs in order to specify a replacement.

 

This power cable is specific to the mating connector on the circuit board that supplies the 24v for the fan.  We need to keep this and transfer it to the new fan.

 

There is a chance that we can remove the whole cable assembly from the old fan and move it to the new fan.

 

The new fan has three wires.  The third wire is probably a tachometer output so that the speed of the  fan can be known.

 

The wires on the new fan are soldered to the fan with lap joints.

 

So, we can make new lap joints when moving the old cable to the new fan.  I’ve got a good feeling about this.

 

The new fan is bolted back on the stack and the whole arrangement is put back together.

 

This amp ran for four hours continuously until the neighbors called the cops complaining of a noise ordinance violation.  I believe that the overheating problem is fixed!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Intermittent Ashdown EVO III 500 Bass Head

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This head was soldiering away in the studio when the output signal became distorted.  Can the Unbrokenstring Crew un-distort the output and keep it from happening again?

Stock photo credit: Ashdown Engineering

 

Name, rank and serial number, please.

 

The oscilloscope shows the waveform presented across an eight ohm resistive load.  A sinewave is applied to the input jack.  We should have a sinewave here.  But we don’t.  This gives us something to work on!

 

Oops!  As soon as we touch the chassis, the output waveform changes!

 

Since we’re on a roll, let’s touch it again!  This is what we should have seen all along.  I think we know where to look.

 

You might be surprised to know that the big metal heat sink I was touching has a high voltage on it.  So here I am safely draining the high voltage before I touch it and get shocked.  Like in the previous picture.

 

I wish the heat sink was as well-supported as the rest of the circuit boards.

 

This whole assembly is attached to the circuit board on the bottom.  Out it comes!

 

Can you see the problem?  Me neither.

 

Close examination reveals cracked solder joints.

 

The correct repair for a cracked solder joint is to remove everything and replace the joint with fresh tin/lead solder.

 

This is a good solder joint, if I do say so myself.

 

Further examination reveals more cracked solder joints.  Guess what we’re going to do to these?

 

This little yellow grabber tool is handy to install screws in tight recesses.

 

Time to crank it up!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

 

Peavey Mark III Bass Head – Busted Controls

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These sturdy old bass heads turn up in pawn shops and Craigslist from time to time. They soldier on for years, sometimes making music for decades. This unit came in for some minor repairs and a Million Mile Checkup. Let’s get to work!
This unit has LEDs for the power indicator and a clipping indicator, or something called “Compression.”

 

From the school of ‘crank it up and rip the knobs off’ we have a knob that has been ripped off.

 

Most of these Peavey heads usually consist of a preamp, mixer, and/or EQ assembly behind the front panel, and a rear panel that holds an amplifier and power supply.  The transformer is bolted to the case in the middle.

 

We need to get to the circuit board, so all the knobs and nuts come off.

 

This unit was built in the era of the Plastic Potentiometer Shafts.  Grrr…

 

A few screws keep the panel and circuit board flat.

 

More screws go into the magnetic parts holder.  We are almost there.

 

Here, at last, is the circuit board.

 

Potentiometers that stand off the circuit board like this are sometimes called ‘spider’ pots.

 

These other controls are fine.  The values 10K and 50K refer the resistance, and the letter ‘B’ implies that the taper is linear.  An ‘A’ letter implies an audio taper control.

 

The Alps company made these in Brasil.

 

Here is the new replacement part, with a metal shaft!  If this were my unit, I’d replace them all with metal shafts.

 

The plastic shaft broke off inside this knob.  A few minutes with an Exacto knife is all it took to reclaim the knob and cut my thumb..

 

The new control is soldered into the circuit board.  The factory workmanship on this assembly is pretty good.

 

Back together it goes.

 

With the knobs in place, you can’t tell that anyone has been here.

 

One last look as we reassemble the unit…

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

Fender Rumble 350 Bass Combo Repair and Modification

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I can’t help but think that this is not a real Fender, but “Fender Bass Amplification” is the nameplate on this unit, not the cursive Fender logo seen for decades.  This unit is very versatile, works better than most bass combos, but just screams “Designed and Manufactured In China.”

A lot of ‘motor noise’ came from one of these loudspeakers in this Fender Rumble 350 bass combo amp.  The owner said, while we were at it, could we wire this unit so that it could be used as an external cab, driven by another unit?  The UnbrokenString Crew said, ‘Sure!  Why not?’

 

If you listen closely, you can hear the warped voice coil rubbing against the magnet gap inside the bad loudspeaker.

Access to the inside of the cabinet is accomplished by removing the loudspeakers.  We are using an electric screwdriver to drill a pilot hole for a switching Neutrik connector that will allow this unit to be used as an external cabinet.

This Forstner bit is just the right size to clear the body of the connector.

Using the pilot hole, we can cleanly cut through the Tolex and into the wood cabinet.  The scratches in the Tolex were not part of this project.

Yes, this looks like a hole to me.

We will replace both Chinese loudspeakers with a matching pair of 200 watt ea. bass guitar loudspeakers.  The original loudspeakers were rated for 75 watts each, which is strange considering that they were tied to a 350 watt amplifier.  The bass loudspeakers have a different hole pattern, so we are drilling new locations for the Tee nuts.

These Tee nuts have little barbs that help keep them in place.

I am using this clamp to squeeze the Tee nuts into the drilled holes in the baffle.

 

Now we can begin wiring this unit up.  The black and white pair connect the two loudspeakers in parallel.

The red and black wires come from the power amp in this unit.  This Neutrik connector will disconnect the power amp when an external amp is connected to the cabinet at this port.  The soldering is done outside the unit.  BTW this is a Neutrik NL4MD-V-S.  The mating connector, an NL4FX, was supplied to the customer for his own wiring.

That looks pretty nice, in spite of the marks in the Tolex.

The new loudspeakers are in place and this unit is ready to test!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626