Combo

MusicMan RD50 Combo Amp Repair and Inspection

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AJ played this wonderful MusicMan combo amp, until it quit suddenly. He was aware of the Big Names in the music business that repaired these, but was there anyone local? D’oh!

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As you might expect, this combo has one 12 inch loudspeaker and has a fifty watt Class AB push-pull pair of 6L6 tubes. One vacuum tube serves as a preamp, and the rest of the amp is built with solid state techniques.

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This looks a lot like a Fender amp, doesn’t it? Leo Fender had sold the Fender company to CBS, but wanted to continue making instruments and amplifiers despite a non-compete agreement that he was required to sign as part of the deal with CBS.

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So the MusicMan amps were born. The Mid-shift switch indicates a slightly different tone stack design.

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IIRC, production was shifting away from the Fullerton factory to Anaheim, with offices in La Brea California.

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The rocker switch for the ground select function and a three wire power cord is evidence that the older design of Fender amps was changing to meet modern regulatory requirements.

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Of course, model numbers were entirely new, and serial numbers had little resemblance to the old way of doing things.

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I have no idea what the paper label to the right is for. Any ideas, anyone?

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The chassis is sound, if not a little cosmetically ‘challenged.’ We can blame the humid Gulf Coast environment.

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Another change from the Old Fender was this pilot light, which consists of a neon bulb and a limiting resistor. The package is held in with a push nut.

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Yet another thing we don’t see here is the iconic brass sheet upon which most of Fender’s controls and jacks are traditionally mounted.

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The wire harness dress is excellent as is the workmanship. The black switch is the Tone Shift switch seen earlier on the front panel.

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These front panel controls work smoothly and are noise-free.

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The Bright/Normal switch is found next to the input jack.

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The dual-section 12AX7 lives here. This amp has been re-capped, including cathode bypass capacitors and all electrolytics. I’m not touching any of this!

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The rear panel jacks are Switchcraft, the best you can get.

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This ‘death’ cap is original. The red paint on the solder joints is an interesting way to indicate that they passed QC. This makes it easier to see where past rework/repairs have been done.

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The fuse holder held a too-high value 32v automotive fuse. The correct 250vac 3A part is installed.

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Most of the preamp duties are done with operational amplifiers. Those connectors in a square configuration are for the reverb tank and pedals.

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This unit is very well built and maintained.

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Can you see the problem? The pair of transistors on the pink heat sink form a phase inverter that drives the output tubes. In the 1970s, televisions, ham radio gear, and other consumer electronics were commonly built using ‘hybrid’ techniques e.g. solid state parts with power tubes. Leo Fender knew his TV stuff, and applied that technique in his new line of amps.

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These high powered resistors are part of the phase inverter circuit. They must be matched closely for good performance. Obviously, these are no longer matched.

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The devices on the heat sink are 75 watt 15 ampere 80 volt power transistors. They should be closely matched for best sonic performance. Also, transistors will drive the next (tube) stage with a bigger voltage swing than two sections of a vacuum tube, because they are inherently lower impedance.

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The main board is coming out of the chassis so that we can solder and desolder parts.

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In the late 1070s, circuit board design was performed on computers. Thus, graphical images could be added to the artwork. Also, this circuit board is electrochemically plated tin, which is a fresh new technology not previously seen in Fender products.

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On a lark, we will measure the value of the remaining 6.8 ohm resistor.

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It reads a little high. No problem. The resistors will be replaced with a matched pair.

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The old transistors are coming out for testing.

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The transistor curve tracer shows that this part is good.

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The other part is shorted internally.

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The heat sink is removed and the old resistors are desoldered. Here, we’re cleaning up the circuit board where the resistors go.

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These two parts were sourced from new stock and selected because their value matches better than 0.1%.

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These transistors were sourced from new stock and were matched on the curve tracer. See the new resistors above and to the left?

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The solid state phase splitter drives the tube stage as it should.

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The tubes are in and it’s time to fire it up!

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This amp comes with the official MusicMan pedal, controlling reverb and distortion.

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When a function is selected, the LED comes on. This is nice on a dark stage!

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This unit is ready to go again!

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Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

Fender Vibro Champ Combo Amp ‘Almost’ Lost to Hurricane Harvey

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Texas Amplification, operated by the late Darryl Shifflett, built some of the finest Fender Blackface clones available. Much of the inventory of Texas Amplification was subjected to the flood waters of Hurricane Harvey. This newly-completed combo amp was high enough to escape immersion, but did not escape the subsequent rain, humidity and condensation. Could the Unbrokenstring Crew make this new unit like-new again for its new owner?

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The nickel plated feet and corner hardware are new, but a light coating of rust from the screws has leached onto the hardware. The Tolex covering appears to be unaffected by the water.

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Here’s a close-up of the rust. Not a big deal, but this triggers my OCDC (like obsessive-compulsive disease with a bit of AC/DC tossed into the mix.)

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The back panels of the amp are held on with the Correct screws, but they are showing signs of iron rust as well.

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This back panel is plywood. It had been wet but had been slowly drying out and was no longer warped. Surprisingly, the Tolex covering was still glued in place.

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This bit of Tolex covering, however, had become unglued.

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The Jensen loudspeaker was high and dry, but we’ll check it for any damage.

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The loudspeaker is more-easily inspected by removing the baffle board.

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With the baffle board out, it’s easy to verify that everything is in good shape.

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More importantly, no apparent water damage had occurred here! The Unbrokenstring Crew is fairly certain that this amplifier was at least partially submerged at the height of the flooding. This loudspeaker and grille cloth appear unaffected!

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Rust Biox is a tool of the museum curator. When old objects are carefully cleaned and restored for display in a museum, such as old weapons or other artifacts, Rust Biox slowly removes iron rust while preserving the un-oxidized material under the rust. This was once sold in the United States as an automotive rust remover, but did not become a ‘hit’ and was removed from the market. The Unbrokenstring Crew, however, is just cool enough to have a source.

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After each item is processed with Rust Biox, a water rinse and hot air dry prepares it for re-use.

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The feet of the unit are nickel-plated steel over a rubber bushing. Here, the bushing is separated from the metal foot for processing.

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These screws hold the feet onto the bottom of the amplifier cabinet.

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The metal feet are restored. Next, the Rust Biox will remove the rust stains from the rubber feet.

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Interestingly, this line may have been the ‘high water mark’ and so this unit could have been partially submerged. Furniture polish will clean and condition the Tolex covering to like-new condition.

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Heat from the hot air pencil softens the Tolex adhesive. The hot Tolex is pressed into place and allowed to cool.

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The hot air pencil has done the trick! This cabinet appears to have never been wet.

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The electronics are brand new, with no signs of water damage or corrosion.

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The Fender Vibro Champ is a single-ended Class A design, a low-parts-count, simple-to-build amplifier with surprising response and tone.

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All magnetics used in Texas Amplification products are procured through Mercury Magnetics. Top-of-the-line!

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The violet jewel in the pilot light tells us that we are ready for business!

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All back together, this amp is running a four-hour-long burn-in to verify that it is 100%. …And dry out anything still wet. This unit was delivered to its new owner, who promptly placed it in his recording studio.

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Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626

Fender Princeton Reverb Amp is Snatched from the Jaws of Hurricane Harvey

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Partially submerged in the flood waters of Hurricane Harvey, this combo amp was rescued when the waters receded.  Could the Unbrokenstring Crew turn this insurance claim into a working unit again?

 At first glance, this unit is in pretty good shape.  Fortunately, the flood waters around this unit were not salty, but fresh rain water.  The grille cloth was not badly stained, and much of the exterior grime was superficial.

 

Not much damage had occurred to the cabinet; some warpage was beginning to appear in the bottom baffle.  The interior was still wet.  This implied that, if the drying-out process could be controlled, no further damage to the cabinet would be sustained.

 

Can you see some rust on the screws?

 

This side has some mold.

 

The bottom Tolex has some mildew beginning to form.  Look at the rust beginning to form on hardware in the foreground.

 

The handle was beginning to rust.  This could be managed.

 

The handle and the Tolex is cleaned and reconditioned with this, which also gives us a clean lemon scent!

 

This is the top of the reverb tank.  Yes, beads of water, still on the exterior of the tank.

 

The previous owner had padded the top of the tank with gray foam, and the bottom with cardboard.  The cardboard was soaking wet.

 

Reverb tanks are inexpensive, so we will just order a new one.

 

The paper cone of the loudspeaker was intact.  This loudspeaker will be replaced by the new owner.

 

Moisture inside the amp chassis has swelled the turret board.

 

Water has reacted with the solder flux, creating a brown crust around all the solder joints.  The components still look pretty good, although they cannot be trusted now.

 

Corrosion on the tube socket contacts testifies to the presence of liquid water here.  Note also that the zinc plating on the once-shiny chassis is turning cloudy.  This tells us that the zinc is doing its job as a corrosion-inhibiting plating, sacrificing itself to protect the steel underneath.

 

The cabinet hardware is washed in Rust Biox to clear away the rust.  This chemical is available in Europe, but of course, The Unbrokenstring Crew is just cool enough to have this material here in the U.S.

 

The nickel plating has very little iron to rust;  This deposit is probably mud.

 

All the hardware is cleaned up.  The Tolex is cleaned and conditioned with the furniture polish.  The cabinet looks good as new!

 

A new tube chart is pasted inside the cabinet where the original one was located.

 

For the electronics, a hand-wired chassis from the estate of Darrell Shifflett of Texas Amplification is pressed into service.  The Unbrokenstring was truly fortunate to buy the remaining inventory of Texas Amplification.  This chassis was part of the inventory.  Look at those shiny new jacks!

 

The knobs are, of course correct.  This is a clone of a Fender Blackface Princeton Reverb, not built in California but rather in Houston, Texas.

 

Darrell was a master of the details.  Even the front panel is Correct for this unit.

 

As a testament to Darrell, let’s just take a look at his workmanship.

 

The wiring and component placement is meticulous.

 

If original components were available, such as the carbon composition resistors, he used them.  Modern flame-proof components are used where an improvement in reliability and safety without sacrificing sonic performance justified the upgrade.

Even the wire is period-correct, fabric-covered was used for the point-to-point wiring, just like the originals.

 

A bias check for EACH output tube is added to the rear panel.  Millivolts measured from red to black correspond to milliamps of plate current.

 

The jacks and controls are name-brand and not the cheap stuff.

 

But just look at that fresh brass sheet used for the ground plane under the controls.  The original brass probably didn’t look this good in Fender units when they were new!

 

The underside of this amp is just a voyage on the Good Ship Eye Candy!

 

The electronic tremolo circuit is duplicated on this turret board.  Not sure why this turret board is warped, but it is electrically 100%.

 

Speaking of turret boards, just look at the meticulous care used to mount each component and route the leads.  Even the bias potentiometer is nicely placed.

 

Comparing this layout against the original Fender drawings is just breath-taking.

 

I’m really jazzed about how the fabric-covered wire is carefully routed around the tube sockets.

 

We needed a new rectifier tube for this amp.

 

Darrell used Mercury Magnetics for all the transformers on this chassis…  the best you can get!

 

With the power on, all the voltages are correct.

 

The new reverb tank arrived today.

 

The bag protecting the reverb tank is dry and ready to be used again.

 

These straps hold the reverb tank bag in place in the bottom of the amplifier.

 

The ON/OFF switch works as it should.  Since the AC cord is a modern three-wire unit, the original ‘GROUND’ switch is wired as a STANDBY/ON switch.

 

This unit is ready to go back to the new owner, who will install the new loudspeaker.  Pretty nice unit for having been under water!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

A Journey To Planet Unobtanium – Yamaha 50-112 Combo Guitar Amp

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A secret weapon of many an acoustic and jazz artist, this mid-seventies line of Yamaha solid state amps were well-regarded among those few who knew about them. This like-new specimen had been suffering from a strange ailment, then went mute. Could the Unbrokenstring Crew revive this unit?

On the exterior, this amp was in very good shape considering that it had been built forty years ago!

 

Starting our tour, the power switch combines the ON/OFF function with the AC polarity reversal switch seen on many tube amps of the same period.

 

The high and low level input jacks are typical for the era.

 

The tone stack includes a ‘bright’ function, a precursor of the ‘presence’ control seen on amplifiers today.

 

The presence of the reverb function demonstrates that this is an early unit.  Many of the later ones did not have a reverb tank at all.  The distortion function is an attempt to add ‘fuzz’ and is nothing like the metal/shred distortion heard today.

 

A few attempts had been made over the years to clean the controls.  Unfortunately, the lube spread onto the front panel around the controls.  Yuck!

 

The open cabinet is clean and functional.

 

We have the usual name-rank-serial number information here.

 

We have two unmarked jacks.  What in the world?  But we see foot switch jacks which are not out of the ordinary.

 

Both the AC power into the unit and the DC power to the final amplifier block are externally fused.

 

And, we have a QC sticker!

 

Removing one of the rear baffles reveals the solid state amp and gives us access to the chassis.

 

Obviously the original loudspeaker, the response graph demonstrates the heritage of this unit to the high fidelity world that Yamaha dominated in the 1970s.

 

This ground lead connected the chassis of the amplifier to the frame of the loudspeaker.

 

With the chassis out of the cabinet, we see a reverb tank in the foreground, a power transformer to the right, big capacitors in the center, and a mono-block amplifier to the left rear.

 

This strain relief for the AC power cord is really over-the-top!

 

The black, finned heat sink is the foundation upon which the power amplifier is built.

 

This large electrolytic filters the DC power for the amplifier, which is nominally 80vdc.

 

This electrolytic capacitor is in series between the amplifier output and the loudspeaker.  This amplifier’s circuit topography shifts the DC operating point of the amplifier to one-half of the DC power supply voltage, effectively forming a class AB amplifier using a single power supply.  This capacitor passes the audio current to the loudspeaker while protecting the loudspeaker from any DC current.

 

Underneath the chassis we find this fused, low-voltage power supply which supplies floating DC voltages for the circuitry.

 

Remember those two unmarked jacks on the rear panel?  Someone added them so that a quarter inch cable can be connected to another quarter inch cable.  Yes, this is a home-made 1/4″ mono to 1/4″ mono jack adapter/coupler.

 

Here is the bottom side of the two large electrolytic capacitors we saw up top.

 

More fuses and bypass capacitors are visible here, in vinyl tubing, to shroud the terminals from touching something they shouldn’t.

 

The ON/OFF/ON switch is seen to the right and the Power ON indicator lamp, with limiting resistor, are seen here.

 

Look at the thick steel shield that keeps any signals running around the inside of the amplifier away from the input jacks!

 

While we’re here, let’s service the unit.  Jacks are cleaned with De-Ox-It.

 

This circuit board handles all the signals surrounding the front-panel potentiometers.

 

These controls will be properly cleaned and re-lubricated.  And we can clean that nasty front panel while we’re here!

 

After removing two large bolts, the power amp assembly lifts off.

 

The six pin connector handles power in, signal in, and amplified signal out duties.

 

Inside this assembly, we see all the components for a transistor-based solid state power amplifier.

 

A pair of these transistors handle the power amp duties.  The screen separates everything from the collector of the transistors, which are at +80vdc potential.

 

This screw under the little bump in the sheet metal holds a temperature-compensating diode array in close contact with the heat sink.  This diode array provides temperature compensation for the transistorized amplifier.

 

Note that this module is stamped 50W/8 ohms.  The Japanese think of everything!

 

With the cover removed, we can see the inner details.

 

These low-level driver transistors are pure unobtanium, which means that if they are bad, there is no modern direct replacement.

 

Fortunately, all of those low-level driver transistors appear to be OK.  The curve tracer indicates that this is a PNP device.

 

This is another bit of pure unobtanium.  Three silicon diodes with special forward voltage characteristics over temperature are housed in this component.

 

This diode array appears to be functional for now.  These are HIGHLY SOUGHT AFTER by techs who rebuild those 1970’s era Kenwood and Pioneer stereo receivers.

 

Every component will be checked, including the power transistors.  Replacements are available for these, if we need them.

 

Almost every component will be removed from the circuit board and verified against the schematic and the markings on the device.

 

This capacitor was more than 30% low in capacitance, and will be replaced.  (No, the leads are not touching.)

 

With the power amp assembly back together, we can perform some initial setup of voltages and levels.

 

One of those two big power transistors with the copper-colored tabs is intermittent.  Can we find a matched complimentary pair to replace them both?

 

Yes, after some research, an adequate replacement was ordered.  Whew!

 

Here they are, those black boxy devices in the center of the picture.  I marked the collector pin locations with a C and the base pin locations with a B on the circuit board so I could get the new parts in the right place.

 

OK, now we’re cooking.  The center yellow trace is a signal called C.VOLT on the schematic, and represents the voltage value of the midpoint of the DC power supply.

 

Over a few hours, the value of C.VOLT changed, creating bad distortion.  Look closely at the green capacitor at the top of the picture.  Can you see something ‘wet’ on the circuit board under it?

 

That ‘wet’ looking stuff is similar to contact cement.  The Japanese used this stuff extensively in the 1970s to secure electronic components so that they did not come loose from the circuit boards when shipped to the United States and elsewhere.  Over time, this ‘stuff’ becomes conductive, which will upset circuit operation.  Many a Japanese-built bit of electronics, including televisions and Ham radios, were taken out of operation by this stuff.

 

The site where the green capacitor goes has been cleaned.  This needs to be repeated for any ‘stuff’ remaining in this unit.

 

Here is the C.VOLT test point.  The crusty brown stuff is solder flux, which will also be removed.

I uploaded a video of the working amplifier to YouTube, which then blocked the video and hit me with a take-down notice about a minute after I uploaded it.  The audio content of the video was copyrighted, and I was caught.  Don’t you just love YouTube/Google/Facebook?

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

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

Peavey MX Combo Amp Rescued from a Bad Amp Tech

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This combo amp had lived a hard life and had finally quit.  Grandpa wanted his grandson to get the amp fixed so that they could jam together again.  Could The Unbrokenstring Crew bring this unit back to life?
 We begin with a quick tour of the rear panel.  The ground switch is a tip of the hat to the Old Days of two wire AC.

 

 The foot switch plugs in where the REMOTE SWITCH jack is coming loose.  This gets fixed.

 

 I’m surprised that this hadn’t ripped loose.  The whole connector wil be replaced.

 

 

 Name, Rank, and Serial Number, please!

 

 What have we here?  We found grandpa’s stash.

 

Let’s get this line cord wired correctly.  Do you know what’s wrong?

 

 The black wire goes under the brass screw. “Black on brass will save you ass.”  You’re welcome.

 

 The reverb tank connects to the main circuit board with this connector.

 

 Even after all this time, the high voltage capacitors are still charged.  Woah!  This is my discharge wand at work.

 

 Our first mystery… where does this nut go?

 

 This is a fuse.  No, you think that it is a piece of 16AWG wire, but it is a fuse.  Or, it is where a fuse goes.

 

 And here, someone was tired of the fuses falling out of the holders, or what was left of the holders.

 

 The heat from the flow of current has wreaked havoc on this solder joint.

 

 This probably smelled bad when it was hot.

 

 Now that the introductions are out of the way, we need to start replacing this nonsense.

 

 These are commercial fuse holders.  These will replace all of the preceding nonsense.

 

 The plan will be to install these new fuse holders at a spot in the circuit where they will be functional, yet out of the way.

 

 The new fuse holders are held down with a screw.  This hole is where the screw goes.  Here goes!

 

 Another hole is drilled for another fuse holder.

 

 This hole is in the center of a trace.  We won’t miss that copper.  Much.

 

 Insulating nylon nuts and bolts are used to keep the new fuse holders in place.

 

The traces in the burned circuit boards are replaced with this Teflon-covered wire.

 

 Everything is now stuffed back into place.  Not too shabby, if I do say myself.

 

Turning our attention to the rear panel, your sharp eyes may recognize this connector as a MIDI female panel connector.

 

 To keep the connector hardware in one place, some of this Thread Locker is all we need.

 

 We have the original foot switch.  It needs a new cable, with a connector to match what we just installed in the amp.

 

 This MIDI cable will be repurposed to replace the cable on the footswitch assembly.

 

 We don’t need this connector.  Instead, this end of the cable will be wired to the switches themselves.

 

 The new cable is soldered directly to the switches  Note the strain relief installed to the right of the picture..

 

 This pedal is ready for action once again!

 

 The amp is reassembled and is ready to go!

 

 The four hour burn-in test is underway.  I think we have rescued another vintage Peavey amp!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Crate Vintage Club 30 Recap

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Robin had another of these wonderful Crate combo amps for servicing.  The Unbrokenstring Crew knew just what to do!

Robin’s 50 watt combo was serviced earlier, in this blog post: https://www.unbrokenstring.com/crate-vintage-club-50-amp-repair/

 

This is a 30 watt unit, with very similar construction.  Look at all that St. Louis Music goodness!

 

The loudspeaker is 100% and made in U.S.A.

 

This pic just documents where the wires go.

 

These wires go to the reverb tank.  They are marked with a magic marker so that they can be correctly re-installed.

 

The tubes need to come out as we are removing the printed circuit board from the chassis.

 

These numbers correspond to the tube numbers on the schematic.

 

While we’re here, we’ll make a check of the condition of each of the tubes.

 

Restoring the washer stack is essential to keep the strains on the printed circuit board to a minimum when reassembled.

 

So we are now able to remove the main circuit board.

 

 

As we did with the 50 watt unit, we are taking this opportunity to clean up the front panel.

 

Sure enough, the capacitors have reached End Of Life.

 

This circuit board is now recapped!  Pretty!

 

After reassembly, this quick check shows us that we can drive 30 watts continuously into eight ohms with no problem!

 

This unit has an easy bias setting arrangement.  Here, we’re using the 4 1/2 digit Fluke meter to measure current.  The AC power is supplied by the Variac on the shelf.

 

Some of the cabinet screws were cross-threaded at one time in the past.  So, we can chase them with a die.

 

The captive nuts are cleaned up with a matching tap.  We’re good for Final Assembly!

 

Another fine combo amp is ready for the next million miles!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey Bandit Combo Amp Refurb

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David found this fine old Peavey combo amp in a pawn shop for almost next to nothing. Could the Unbrokenstring Crew resurrect it?
This saga begins on a cool Saturday morning in the Guitar Center Pasadena parking lot.  If you look closely, you can see right through this unit.  Literally.  What are we getting ourselves into this time?

 

Electrically, this unit is mute.  No sound comes from it.  But, as the story is told, this amp languished in the barn for years so we have to expect the worst.

 

Name, rank, and serial number, please.

 

This part number goes to a Peavey Spider loudspeaker.  This is gonna be fun, as you shall soon see!

 

The grille is off.  This poor thing is a mess.

 

If a rat gets hungry enough, it will eat anything.

 

Perhaps it was not tasty enough, as the rats left plenty of bits of uneaten loudspeaker cone behind.

 

The brunt of the rat nasty-ness was taken by the reverb tank cover.  We need to clean this up right away!

 

The spring reverb tank is in perfect condition.

 

The tank cover did its job and kept this reverb tank in a pristine condition.

 

Pledge furniture polish contains solvents and emulsifiers that are excellent for cleaning Tolex.

 

The nice lemon-y scent is just the thing to counteract the barn smell.

 

The bottom of the unit is a mess, though.

 

The amplifier electronics are on the bench.  The unit is upside down.  The reverb tank connector is to the left.

 

The bottom cover comes off.  We need to go over this unit with a close eye for damage due to humidity and dust.

 

Perhaps moisture (rat urine? mouse-ture?) has seeped under the solder mask and attacked the copper on the circuit board.  Everything gets a bath.

 

The top side of the electronics is pretty filthy, as we might expect.

 

The circuit board is free of the chassis.  Everything is being scrubbed with water, alcohol, and compressed air.  The switches, jacks, and controls are cleaned, flushed, and lubricated.

 

Tracing signals, this guy is bad.  This IC is replaced from stock.

 

I want to clean up around the cases of the transistors.  The heat sink is at zero volts while the transistor case is at +70 volts or so.  The dirt may bridge across the insulator.  Not on my watch!

 

These power transistors are just fine.  They are also super-rare.  Luckily, I won’t have to replace these.

 

But they sure do clean up good!

 

The transistor on the left is already installed.

 

We use Wakefield heat sink grease on power devices at the Unbrokenstring Shop.  Old Skool.

 

The original mica insulator is reused.

 

This is actually quite messy, but is a thing of beauty when I do it.

 

Both sides of the washer are coated with compound.

 

The whole stack is installed as shown.

 

While we are here, some of the solder joints could use some attention.

 

This amp is now delivering 75 watts into 8 ohms.  This chassis is Good To Go!

 

Back in the case it goes.

 

These jacks are on the rear of the unit.

 

The knobs press on.

 

This guy goes in the trash.  Wait?  Aren’t we going to recone it?

 

No.  We don’t have to recone.  Peavey Spiders are available without the magnet.  No tricky alignment issues here!

 

When the magnet was removed from the old loudspeaker, the magnetic gap was cleaned and then sealed shut with tape.

 

Once the old magnet is aligned, three screws hold it in place.  We’re done!

 

This is the new loudspeaker, seen from the front.

 

A rear view shows us the cleaned cabinet and reverb tank cover.

 

The loudspeaker looks new, because it is new.

 

The grille was cleaned up and is seen here, reinstalled.

 

The refurbed chassis slides in from the rear.

 

These washers go underneath the trim plates thru which the chassis screws extend.  Don’t leave these out!

 

These big trim pieces are metal, coated black.

 

And these, boys and girls, are how the chassis is suspended inside the cabinet.

 

Installing this trim piece is all that is left to do.

 

David is really tickled with his like-new Peavey Bandit!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

 

Peavey Artist Combo Amp Refurb

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This Peavey Artist combo amp was WAY too distorted to suit even the most extreme metal head. Could the Unbrokenstring Crew look into this and put this unit back into service?
First, a tour.  You can have two channels, or a mix of the two ‘Automix’ inputs.  This was a ‘thing’ back in the day.

 

The other controls are straight-forward.

 

The standby switch is in the front, whereas the AC power on is in the back.  Actually, I like this because if both power switches are in the back, half the time I switch the wrong one.

 

The AC line duties are all squared away on this side.  The City Of Los Angeles has their own version of UL.  That’s the yellow and red sticker.

 

The right hand side of the rear chassis has the ins and outs for this amp.

 

These are all Peavey-branded tubes.  They are all in good shape and will stay in this amp for now.

 

So we put a clean sine wave in, and this is what we get out.  The positive power supply is weak.

 

An overall gut shot shows power on the left, preamp on the right, and power amp on the bottom.

 

These capacitors have begun to swell and push the seals outwards.

 

We have signs of overheating.  These resistors handle power distribution and are somehow related to our problems.

 

The other power supply has a cooked resistor as well.

 

Here I am just documenting all the plugs and wires so I can get them back in the same place.

 

These capacitors are also bulging and will be replaced.

 

Time to remove the power supply board and work it over.

 

This circuit board holds the tube sockets.  We have an intermittent short to ground under this assembly.

 

At first I thought that the short was under the tip terminals of these jacks, but that was not the case.

 

I am going to pull this assembly out and look it over as well.  The blue, red, and brown wires are high voltage.

 

The blue capacitor in the upper right is the ‘death cap.’  If it shorts, 115vac is connected to the chassis.  Not good if you ever touch the amplifier.  Fatal if you touch the amplifier with one hand and grab a microphone with the other hand.

 

This circuit board is supported by the tube sockets.  All four sockets will be unsoldered.

 

Out this guy comes.

 

Here is our short circuit.  These are component leads from parts installed on top of the circuit board and soldered from the top.  I guess if the excess length is out of sight, then it is out of mind.

 

Here are some of the parts on the top side.  I don’t think these were replaced in the field, but rather it came from the factory with the untrimmed leads.  Sloppy.

 

However, wires that are too long are easier to deal with than wires that are too short.

 

I am cleaning up the bits of crap in the bottom of the chassis, using some sticky tape as a way to capture the crap.

 

I have installed new bleeder resistors and new capacitors on this assembly.  That big blue resistor is a high voltage dropping resistor.  This part is fine and will not be replaced.  However, those are not made anymore, and I have some of the last remaining stock of the OEM resistor.  You’re welcome.

 

Everything gets trimmed and cleaned up before reassembly.

 

The power supply board has new caps everywhere.

 

The Ty-Wrap was my idea.  These big parts need some mechanical support, but I’m not big on lots of hot glue.

 

The original power resistors were way out of spec, so these new parts are higher wattage to take the abuse.

 

These caps on the preamp board were replaced.

 

As was this guy.

 

These were the overheated resistors that we saw earlier.

These new resistors are actually more robust than the parts they replaced.  And they are flame-proof.

 

We are back on the air!

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