Solid State Amps - Unbroken String

Peavey Stereo Chorus 212 Head Stuck In Mono

October 16, 2017 by latchaw

The owner of this amp was so anxious to get it fixed, he removed the chassis from this Peavey Stereo Chorus 212 combo and even helped load it! The amp plays, but only one stereo channel is operational. Can the Unbrokenstring Crew help?

A quick look of the front panel shows the inputs and the normal (clean) channel controls.

Did you catch the missing knob? The Unbrokenstring Crew will find a replacement, which is no small thing because these are no longer available new and are only available from spares or salvaged from a non-working unit.

This amp uses a digital signal processing (DSP) module to make all this audio magic.

Interestingly, the DSP module creates two unique audio signals (stereo) from a single mono input.

The ground lift switch is on the front panel, which is a nice touch from many other designs.

This dual OPAMP is non-functional. However, this is not the root cause of the missing stereo signal.

This is the DSP module in this amplifier. We have only one audio stream as an output from this device.

A piece of cardboard serves an an insulator so that the bottom of the DSP assembly can be probed.

Here is where the DSP processor outputs both audio as a stream of ONEs and ZEROs. We are OK here!

This chip processes the two DSP streams in a manner similar to a successive approximation analog-to-digital converter.

The bit streams are split using an analog switch. Here is the output of the switch assigned to the working channel.

And this is the output of the switch on the non-working channel. Time to change the switch!

Desoldering today is performed with a traditional iron, rosin flux, and copper braid. This keeps solder balls to a minimum.

The circuit board is cleared of excess solder and flux.

The new MUX chip is available through the usual sources.

With the tweezers, the new MUX chip is oriented properly on the circuit board pads. See the tweezers?

After soldering, the workmanship is inspected with this hand-held microscope. The high-powered white illuminator is on the left, and the black cylinder next to the big IC chips is the optical microscope itself.

Any remaining solder flux is removed with alcohol. The rag gives the dissolved mess somewhere to go.

Both channels are operating at full power. The knob is coming from an eBay auction. When it gets here this head is ready to return to the eager customer!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Bugera V22 Infinium Combo Amp Repair

September 28, 2017 by latchaw

This beautiful combo amp excels in all categories except one… no audio out. Could the Unbrokenstring Crew help?

Taking a walk-around, the rear panel tells us that this amp was built in Macao, PRC.

Good corporate branding…

The TRIODE/PENTODE switch is advertised as a ‘half-power/full-power’ switch. The LEDs labelled “Tube Life Monitoring” light up when the fuse in the plate circuit open, due to an internal short in the tubes.

Looking down from the end shows a handsome front panel with the Very Cool chicken head knobs.

The top view of the chassis shows us another good instance of strong corporate branding. I do like the protective cage over the vacuum tubes.

Here we get a good look at the bottom of the main circuit board. I am not a big fan of soldering tube sockets straight into a circuit board, but this seems to work here.

Most everything on this side of the circuit board appears to be OK.

I was documenting the location and orientation of these cables, in case we needed to further disassemble the unit.

These solder joints appear to be cold. We have found our intermittent ‘no sound’ cause!

Every other solder joint is removed, fluxed, then resoldered.

Ah, that’s better!

Ironic that just a couple of cold solder joints would bring this wonderful combo amp to its knees. This guy is 100% now!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Noisy Controls in an SWR Red Head Bass Combo

August 18, 2017 by latchaw

This combo amp really turns heads whenever it is played. However, over the decades, the controls have become noisy. Time for the Unbrokenstring Crew to do some remedial magic and get this unit back in top form!

This unit has a 1U rack space under the head unit, which is a nice touch!

Let’s take a look around the inside of the unit. The power transformer and heat sink dominate the center of the unit.

The preamp is hybrid solid state / hollow state.

Looking at the rear of the front panel, we see the input jacks and equalization controls directly mounted and hand-wired to the circuit boards. Most of the work we need to do today is right here.

The configuration switches are seen here. That big black block in the middle is the top of the heat sink.

We see the fan here, which blows the length of the heat sink. The power transformer has a bit of tape and foam on it.

Looking at the inside of the rear panel, we see the power jacks and fuses. Can you see the bridge rectifier?

This big potentiometer dominates the rear panel, setting the line out level.

From this angle, we can see a the filter capacitors. Electrically, this unit is very solid.

Each control is rinsed out with some Blue Shower cleaner. The cleaner is applied from the rear of the potentiometer, rinsing the crud away from the resistive element.

Then the control is dried with some compressed air in a can.

This anti-corrosive cleaner/lubricant works well on the actual resistive element and wiper.

This synthetic lubricant is just the ticket for lubricating the shaft.

After reassembly, the unit is checked out. Note the ease by which the loudspeakers are connected to the head unit.

This unit is now fully functional, with no more control noise. Back together it goes!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Ampeg BA115 Bass Combo Amp Repair

August 2, 2017 by latchaw

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

Kustom KPC15MP Powered Monitor Repair and Refurbishment

July 5, 2017 by latchaw

This powered monitor/PA box was badly abused but could become the basis for a good keyboard amp. Could the Unbrokenstring Crew put it back together and make it gig-worthy?

All of the parts are here, but they are rattling around inside. Electrically, it worked, but the third-world construction techniques rendered the unit worthless for loading in and out of a venue.

For instance, the control panel was literally kicked inside the enclosure. Yes, those are wood brads fired from a nail gun.

The trim plate was easily removed.

To get a good look inside, the main loudspeaker was removed.

There are all kinds of things rattling around inside this unit.

The horn driver is shot. Proceeding with exploratory surgery, the lens is coming out.

This driver is threaded, which implies that if it needs to be replaced, a standard compression driver can be selected.

This driver is probably a Chinese copy of a Motorola unit. The series resistor is all this unit has for a crossover network.

Disassembling the driver reveals this one-inch cone.

Behind the cone is this piezo driver. It has come loose from its mounting and one of the connecting leads is broken.

The piezo element was glued on to the back of the cone. This is some fine Far East engineering!

The resistor in series with the compression driver has miraculously survived the abuse.

I am a big fan of these compression drivers. They are low-cost, covered by a warranty, and compatible with this setup.

In this application, it is an equal or better replacement for the plastic driver that we are removing.

These crimp-terminals are handy, although we may use some insulated ones that are the same size.

This crossover network is compatible with the new compression driver and loudspeaker, plus it is adequately sized for the power levels involved.

The only weakness out-of-the-box is that the large inductor needs additional support, particularly if this crossover network is installed in gear that will be loaded out for gigs. The shiny glue is actually hide glue.

Here is the crossover network in its new home. The main power transformer is on the left and the power amp with the large black heat sink is on the right.

Shifting out attention to the user interface, the control panel is separated from the piece of MDF board.

A quick inspection of the power amp reveals that it is functional. However, temperature cycling and vibration have broken a couple of solder joints. These will be repaired before returning the circuit board to service.

I tried to drive the brads out of the MDF the same direction that they were driven in. However, they were VERY firmly stuck in the wood.

I decided to just trim them flush and get on with the process.

This black marker is adequate to disguise any exposed MDF around the control panel.

This piece of MDF will be glued and screwed in place from the inside of the cabinet. Here we are pre-drilling the MDF to prevent the screws from splitting the thin material.

These screws will be driven from inside the cabinet.

Oh, look here! More stuff rattling around inside the cabinet!

I wonder where this goes?

The control panel is about ready to reinstall.

A little glue is spread around the inside of the main cabinet where the control panel will be fastened.

The control panel goes here.

A socket wrench drives the bit, seating the screws. Not much room here for a powered driver.

This is another view of the control panel mounting scheme and the 100 watt amplifier.

The AC power for the amplifier is filtered by the yellow across-the-line capacitor seen on its own circuit board.

Time to start putting things together for real.

We’re drilling a hole to mount an L-pad attenuator that allows the user to set the level of high frequency audio coming out of the horn. The fixed resistor seen above will be eliminated by this L-pad.

The L-pad level control looks almost factory.

The main loudspeaker seems OK.

Some closed-cell foam is installed around the loudspeaker to seal the cabinet. Peeling the white paper backing reveals an adhesive back that keeps the foam in place.