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

Ampeg BA115 Bass Amp Falling Apart (Literally)

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

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 1810 Bass Enclosure Crossover Doesn’t (Crossover)

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

Ampeg BA115 Bass Combo Amp Repair

Mysteriously, this modern Ampeg bass combo amp quit working.  Could the Unbrokenstring Crew make it right?
The unit appears to be completely dead, with power present at the fuse holder.  Let’s go inside to take a look around.

 

More goodness from St. Louis Music.

 

This picture documents the wiring polarity on the main driver coil.  This needs to be right when reassembling the unit!

 

And the one on the right is the ” + ” terminal.

 

The screws that hold the chassis in the chassis come in from the sides of the enclosure.  The captive nuts in the chassis have sharp corners on them, which snag the Tolex covering that wraps inside the enclosure.  If you ever venture here, beware!  TIP – slip a thin scraper or putty knife between the chassis and the cabinet to keep this from happening.

 

The circuit board is separated from the front panel of the chassis.  Note the white nylon spacers on the jacks to the right and the inside-toothed lockwasher on the rotary encoder to the left.  These come off now and are stored with the knobs lest they fall off and get lost on their own.

 

Do you see what I see?  This white wire carries the AC power neutral to the main circuit board.  The flag terminal came off the main circuit board.

 

The solder used in this unit is ‘lead-free’ and compliant with RoHS, the directive to remove harmful substances from the supply chain.  This kind of solder is brittle, so solder joints made with lead-free solder often fail from cracking stress.

 

This is the other side of the circuit board.  The solder fillets are OK but the mechanical joint failed entirely.

 

Rosin activated flux was added and the old solder removed.

 

Here is another failed joint undergoing rework.  This one is at the DC common point of the amplifier.

While we’re at it, let’s look for other failed joints.  Can you see these?  These are still electrically OK but will fail soon.

 

Let’s put this guy back together.  The shiny metal plate next to the circuit board is the heat sink for the power semiconductors in this unit.

 

These pics were made earlier to document the location of the flying wires attached to the circuit board.

 

These wires carry DC power to an off-board circuit.

 

That shiny metal heat sink gets a new coat of silicone heat sink compound, to minimize thermal resistance to the chassis.

 

Power is applied.  Look, we have an indicator light now!

 

Before final assembly, let’s take one last look around.  See those components stapled to the rear of the cabinet?  That is the crossover network for the tweeter.  Yes, I said stapled.

 

This is the pair of wires that go between the chassis and the crossover network.  When the chassis is installed, this hole will be resealed with RTV to control the moving air behind the main loudspeaker.

 

All back together except for the grille, which you saw.  This unit works very well for a solid state unit.  It is loud, and light-weight.  The future holds many more years of service.for this unit.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Ampeg SVT-200T Bass Head Update to IEC AC Power Entry

This absolutely mint-condition solid state bass amp head came through the shop for a quick once-over.  More significantly, the owner wanted one of those new-fangled IEC AC power jacks installed in place of the existing power cord.

01SVT the patientThese units are highly regarded by many bass players.  Some consider them to be an equal to the tube-based models in the realm of versatility and sound.  And they are sure a lot lighter to carry around!

02SVT line outsThe rear panel is straight forward, with everything you need for a decent bass amp setup.  More goodness from St. Louis Music Electronics!

03SVT amp1Inside, we have the interface to the front panel and some power transistors.  The unit with two flag terminals is a thermal switch that opens when the heat sink gets too hot.

04SVT amp2Here we have more power transistors and the driver transistor pairs.

05SVT amp3Power supply capacitors and the choke are found on this end of the board.

06SVT amp4The power transformer is bolted here, and the input protection circuit board is mounted on the side of the chassis.

07SVT fp1Input jacks and controls are found in this view, facing the back of the front panel.

08SVT fp2This is the rest of the front panel.  Note the big solid state rectifier for the power supply in the center foreground.

09SVT technologyAmerican-made Texas Instruments semiconductors are widely used throughout this unit.  Here is a preamp chip.

10SVT cordThe original line cord is removed with a snip.  The wire remaining inside the chassis will be soldered directly to the IEC connector.  The big chunk of insulation will be removed.  The AC wiring will then be dressed in the same manner as the rest of the under-chassis wiring.

11SVT pwr inThis blue IEC connector will be installed in the rear panel where the strain relief bushing goes.

12SVT hole dimsA rectangular hole will be cut to mount the IEC connector.  The pencil lines show up pretty well in this view.

13SVT magnetA magnet is positioned to keep the metal chaff from the sheet metal nibbler away from the electronics.

14SVT holeThis is the rough-cut hole.  More filing will gun-smith this into a rectangular shape.  Note the handle on the magnet visible through the rectangular hole.

15SVT trial fitHere, we are trial-fitting the IEC connector.

16SVT screwholeThe location of the hold-down screws is marked with a center punch.

17SVT drillNow the holes for the screws are drilled, as you might have guessed.

18SVT trilobed scrThe machine screws are torqued, mounting the IEC connector in its new home.

19SVT magnetsThe magnets have really done their job.  None of these filings will be left loose inside the chassis!

20SVT wire prepThe cut ends of the power cord inside the chassis are stripped and tinned.

21SVT shrinkAs a nice touch, some heat shrink tubing of the correct color is slipped onto each wire.  Pure cosmetics!

22SVT solderThese are tacked into place for now.  Before the final joint is made, I’ll verify that the wires go to the right place!

23SVT shrinkWiring orientation is confirmed as correct!  The soldering was completed and the heat shrink tubing is shrunk into place.

24SVT line out testHere, I’m checking the functionality of the low-level signals to feed a bi-amp setup.  Note the Marshall Stack!

25SVT final testThis unit is ready for the 21st century.  The modular cord makes setup and transportation more convenient.

 

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

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
281-636-8626

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
281-636-8626