repair

Shure 55S Microphone Rescued from the Dumpster

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Dr. Shoen’s girlfriend at the time found this microphone in a dumpster behind a church. What did he really find, and could it be more than a theatrical prop onstage? The Unbrokenstring Crew goes to work!

 

Shure Brothers built this iconic microphone at the factory in Evanston, Illinois in the years between 1951 and 1961. The art deco design is recognized around the world as “The Elvis Microphone.”

 

The 55S is a smaller version of the Model 55 Shure Brothers microphone first produced in 1939.

 

Unidyne is a term coined to reflect that a single (unitary) microphone diaphragm is employed. The moving coil technique employed to convert sound pressure into electricity makes this a dynamic microphone; Thus we have the word “Unidyne.”

 

Units with the ON/OFF switch were produced after 1961

 

The silk wind screen is badly deteriorated. The microphone makes a loud ‘clunk’ noise when it is moved. We need to look inside.

 

Four screws allow the halves of the microphone body to be separated.

 

Inside the microphone, we see the element at the top and a multiple-impedance transformer on the bottom.

 

Two screws hold a bracket that retains a foam vibration dampener in place, which has long since deteriorated and crumbled away. This is the source of our ‘thunk.’

 

Two more foam vibration dampeners hold the bottom of the element. They are also deteriorated. More ‘thunk.’

 

The microphone element lifts out easily once the top bracket is removed.

 

Some of the foam isolation dampeners remain on the bottom two microphone element supports. These are end-of-life and no longer available from Shure.

 

It is easy to see places where the silk wind screen is missing.

 

The matching transformer is mounted along with a couple of boxes that retain the now-deteriorated foam vibration dampeners.

 

Two screws hold these parts in place.

 

We see the back side of the impedance selector switch in the background, and some set screws in the foreground. What do these do?

 

The bottom set screw is supposed to hold this spring sheath around the green and orange wires in place.

 

The top set screw holds the impedance selector switch in place.

 

We need to take everything out.

 

The spring sheath runs through the base of the microphone and protects the wires as the microphone is flexed at the joint.

 

Inside the top of the microphone case we find this sticker, which records the patent numbers employed in the design of this microphone.

 

The pivot between the microphone and base needs to come apart for cleaning and adjustment.

 

This screw can be adjusted to set the stiffness of the microphone head relative to the base.

 

Graphite washers ride between the moving parts for lubrication.

 

A dent in the body of the microphone needs to be removed. Yes, I’m using my luthier’s hammer to pound out the dent.

 

Can you see where the dent was?

 

The old silk wind screen was glued inside the case of the microphone.

 

Acetone will dissolve the old glue. It will also dissolve silk, turning this cleanup step into a blue sticky ‘hot mess.’

 

But a little patience and perseverance yields a clean microphone case.

 

Warning – Skeleton Shot! I’ll betcha that you have never seen a microphone like this.

 

We found some sheer blue silk fabric for the wind screen. This brighter blue is not historically correct, as ‘Victoria Blue’ (Pantone 2756) was specified by the factory. However, this blue matches the Shure nameplate and badge.

 

This is a test.

 

Fabric is glued to the top and bottom as well as sides of the front half of the microphone enclosure. We will now fabricate a soft pillow to allow the fabric to be ‘blocked’ into place as the glue dries in the front half of the microphone enclosure.

 

This soft pillow will be fabricated from Oomoo. Yes, the silicon mold-making resin will be just the thing.

 

Equal parts by volume are mixed.

 

The mix is poured into the front half of the microphone. A plastic sheet protects the microphone shell from the casting material. The Oomoo silicone mold material won’t hurt the microphone shell, but I don’t want to risk contaminating the microphone shell and possibly compromising the glue adhesion later.

 

And here is our pillow!

 

We don’t want the pillow to deform the fabric, so these high points are removed by hand with an Exacto knife.

 

Here is the finished pillow inside the microphone shell.

 

And here is the fabric, glued and blocked into the microphone shell.

 

Time to reassemble. The moving joint is reassembled and the cable from the base to the enclosure is reinstalled.

 

The spring around the cable is held in place with the set screw, as we discovered earlier. This is a nice view of the fabric in the back shell of the microphone. This piece is just a flat rectangular sheet stretched across the back, so it’s easy to glue in place by hand without a block.

 

The joint is back together. No lubrication is necessary as the graphite washers are doing their job.

 

A smooth, firm grip at the joint is established before installing the lock nut.

 

This is what the spring protecting the wires is supposed to look like.

 

The microphone element works, but I couldn’t resist taking a look at the technology behind US Patent 2,237,298. The hemispherical shell on the back of the element helps establish the cardioid pickup pattern of the element.

 

I fabricated new foam vibration dampeners, which are installed in three places. The matching transformer assembly goes back where it belongs.

 

The microphone element is reinstalled where it belongs and wired in. This is now a working microphone. The covering on the microphone element is actually the same material used for vintage silk stockings a.k.a. nylons.

 

A set of four matching screws are fitted and finished to hold the two halves of the microphone enclosure together.

 

Pretty spiff!

 

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Kustom Defender 15H Amp Head Gets an Output Transformer and Tubes

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Mark’s future SIL picked up this little guy from the ‘friend’ he loaned it to, but it was mute when he got it back. Could the Unbrokenstring Crew make it audible again?

 

This unit is a very simple guitar amplifier, with two power settings.

 

The rear panel has some good functionality, including a DI out and a loudspeaker impedance selector.

 

And, of course, we have the Name, Rank, and Serial number, plus a couple of QC stamps!

 

The output transformer is on the left and the power transformer is on the right. The input or high voltage side of the output transformer is shorted, reading about 6 ohms. This should be about 10,000 ohms.

 

The date code on the output transformer says that this part is not old enough to fail. I speculate that it was made China-Cheap.

 

These specs are really useful, because the new output transformer can be sourced so that these ratings and connections can be matched.

 

And here is our new part. It is a little bigger, so it will be mounted at a right angle to where the old transformer was mounted.

 

The new transformer is bolted in. The wire color on the new transformer matches the wire color on the old one. This is too easy!

 

A drop of LokTite thread locker is added to the bolts to keep everything where it belongs.

 

New tubes are necessary as the old ones had cooked and were not anywhere near matched. This amp uses a novel circuit to split the phase of the audio signal driving the power tubes, so these tubes need to be matched.

 

The amp is working and has passed all the final tests! And it doesn’t sound bad!

 

The four hour burn-in starts after the top cover is installed. For a simple amplifier circuit, it does a good job of fighting against the silence.

Oh, and don’t loan your stuff out.

 

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Gibson ES125 Tune-Up

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Dr. John has collected this beautiful ES-125 (a Gibson Electric Spanish guitar with an MSRP of $125 back when it was produced) but it sounded as if it were underwater. Could the Unbrokenstring Crew toss it a life saver?

 

This instrument is in collectable condition, with all original hardware. The finish is finely-checked as you would expect a seventy-year-old musical instrument to be. A new hand-wound pickup was included in the instrument case, if the original one was defective and could not be easily fixed.

 

Years of oxidation and skin oil had made the neck sticky, particularly when the humidity is high (which is all the time in Houston.)

 

The sticky finish ends at the head stock, which implies that the finish is OK but the skin oil is the culprit.

 

Here, fine polishing compound is mixed with Dr. Duck’s Axe Wax to rub out the finish and remove the oxidation.

 

Next, we will look under the pick guard to investigate where the underwater sound is coming from.

 

This pick guard is shaped in such a way that it holds all the controls, and only a hole for the ground wire to the bridge and a slot to clear the pickup is needed in the sound board to electrify this instrument.

 

The ground wire to the strings appears to be a piece of lamp cord. The solder joint around the ground wire did not alloy to the ground wire between the pots, but slides up and down the wire.

 

This ceramic cap is the tone cap. It bleeds off high frequency to ground under the control of the tone pot.

 

This tone cap is marked 0.02uF at 50 volts.

 

On the capacitor tester, the value is correct.

 

However, the dielectric is very leaky, which would probably change things in the tone circuit for the worse. This is probably where the ‘underwater’ sound comes from!

 

Some high quality film capacitors are retrieved from stock.

 

These are the same value, 0.02uF, but are rated at 400v in case the guitarist plugs the instrument into a wall socket. At least the capacitor will survive. The player, not so much…

 

Dr. John lives about seventy miles away. As each change was made, a sound file of the instrument was emailed to him to monitor progress.

 

A free copy of ProTools First and Ableton Live came with the interface, which will amazingly run pretty well on this old rack-mount controller PC that I have on the bench.

 

John decided that the new pickup didn’t add anything to this fine old instrument, so it remains in its original condition as of seventy years ago (with a new tone cap, of course.)

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Marshall JCM900 Tune Up

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This wonderful old Marshall JCM900 lives in a recording studio. It was due for a set of tubes and a million-mile checkup. Could the Unbrokenstring Crew refresh this head and resolve the tiny issues that had arisen over the years?

 

In simple terms, this head has two channels that share a common tone stack, effects loop, and reverb tank. The amount of reverb, as well as the gain and volume, are independently adjustable.

 

Name, rank, and serial number, please.

 

The effects loop is accessible from the back. This unit is recording-friendly, with outputs for ‘wet’ and ‘dry’ signals.

 

The Business End. This amp can be switched to 50 or 100 watt output power.

 

Two fuses are used in the high voltage plate supply for this amp, which is a nice touch and will add something to the story later. IEC mains power socket and a line fuse rounds out the rear panel.

 

These power tubes have pushed billions and billions of electrons around, and some of those electrons have interacted with the inert gas inside the glass envelope. Do you see the frowning face in the upper insulator? The brown scorch mark is his beard.

 

These great tubes have delivered a long service life and are now just about worn out.

 

Interestingly, Marshall delivered these heads with 5881 tubes, a military 6L6. Later 6L6GCs dissipate more power and take higher voltages. You can read Internet posts regarding the battles between Marshall in England and American importers; the latter changed the tubes on new amps to 6L6GCs because they believed the 5881s would not last through the warranty period.

 

And here we have the reverb tank.

 

A walk through the bottom of the unit shows us the output transformer. The red and black leads to to the reverb tank.

 

On the left is the preamp circuit board containing the input jack, tone controls, and signal switching. The tube sockets are discretely wired, and on the right is another circuit board handling the effects loop jacks.

 

More views of the preamp board on the left and the output jacks on the right. Tube sockets are in the middle.

 

At the lower right side of the output circuit board is the power supply power resistors, rectifiers, and fuses

 

The large blue items are the filter capacitors. These are in excellent condition and will not be replaced today.

 

The power transformer and power switches are mounted directly to the chassis.

 

This blue control sets the idling current (bias) for all four tubes. The current splits thru R28 and R29 to manage a pair of tubes each, part of the 50W/100W power control circuit.

 

The Unbrokenstring Crew are big fans of DeoxIt products. Here, we have sprayed a little D100 into the cap, and then soaked a pipe cleaner in the solution.

 

The pipe cleaner works well to clean and recondition each individual octal tube socket contact.

 

We will also wipe off the pins on the bottom of each tube.

 

So with the tubes installed and operating into an 8 ohm resistive load, we set the idle current for one pair of tubes. But the two sides don’t match.

 

Here, I’m using my good Fluke bench meter to confirm that one pair of tubes is idling at 50 milliamps, while the other pair is idling at about 41 milliamps or so. Both meters are in good agreement with the values measured, but I’ll stay with my good Fluke to investigate the situation.

 

Plate current causes heat to be dissipated in each tube. The V1 and V4 tubes are about 114 degrees C. while idling at about 41 milliamps.

 

The V2 and V3 pair are a little warmer. These tubes are idling at 50 milliamps. The temperature difference confirms the validity of the different idling currents… but why are they different? They share one transformer winding. We paid big money for matched tubes (which, when swapped around, make no difference…) More work!

 

Remember seeing separate fuses for plate current on the back of the amplifier? Checking voltage drops in the entire plate circuit, we see that this fuse drops about 0.2 volts across it more than the other fuse. Does that tiny voltage drop make any difference?

 

The fuse for the V1/V4 pair of tubes measures over half an ohm (meter zeroed for test lead resistance.)

 

This is the other fuse, for the V2/V3 pair plate circuit.

 

This fuse measures a tiny bit smaller resistance from end to end. Does this actually account for the higher current?

 

Sure enough, those voltage drops and differences in resistance accounts for about 10mA difference in plate current. New Fuses, Please!

 

While we’re at it, we will clean the fuse caps with DeoxIt, just as we did with the tube pins.

 

And the fuse holders will be similarly cleaned. (Hint – these pipe cleaners are perfect for cleaning other hardware besides your tobacco pipe.)

 

This line filter capacitor is scorched by a power resistor that was pushed up against it, perhaps a result of rough handling during shipping.

 

Components that are used on AC power require all sorts of safety certifications, which this part has.

 

I could probably leave this part in the amplifier, but film capacitors are cheap and if this were my amplifier, I would want it taken care of in a proper manner.

 

So here is the new line capacitor. The power resistor will be moved away from this guy when it is installed.

 

The filter capacitors in the bias circuit were also replaced, while troubleshooting the plate current imbalance.

 

Of course, replacing those parts requires access to the bottom of the circuit board.

 

While we have the circuit board up and out of the way, we can catch a glimpse of the discrete-wired tube sockets. This is a much better way to wire vacuum tube sockets, rather than solder them to a printed circuit board IMHO, because the tube sockets expand and contract much more than the circuit board material, whereas the discrete wire can just flex with the expansion and contraction.

 

This little bit of trimmed wire was stuck on the bottom of the circuit board. This will be no issue unless it comes loose, which it might do just as you are ready to go on stage and start the set.

 

Now this amp is running like a clock. The waveform represents the voltage across eight ohms driven with 110 watts, with a 440Hz sine wave injected into the input jack.

 

The chassis goes back into the case. I removed the power tubes for this step because I didn’t want to risk breaking anything in case I got stupid. The red and black cables to to the reverb tank.

 

Everything is checking out!

 

The sheet metal rear panel is much easier to align when the unit is face-down on the bench.

 

Zenith televisions were advertised with the slogan “The quality goes in before the name goes on!” After a four hour burn-in, the sticker is affixed on the output transformer side of the rear panel.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE

281-636-8626

New Luna Ukulele Is Bent Out Of Shape

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Sophie’s aunt wanted to do something to further her niece’s continuing education and eventual career in music therapy.  This nice Luna ukulele was a thoughtful gift.  Unfortunately, there were many dead notes.  Could the Unbrokenstring Crew bring those notes back to life?

Inside the bag is a ukulele, some picks, a tuner, and an instructional DVD!

 

The words for “Peace” in forty-four languages are engraved into the soundboard of this instrument. All you hippies will recognize the peace sign in the sound hole.

 

Sure enough, there are several notes on the fret board, near the nut, that are muted out.

 

Name, rank, and serial number, please.

 

I am not sure what this number is…

 

The fret rocker shows a very tiny difference in fret height, when checking between adjacent frets.

 

However, a straight edge reveals that the neck is back-bowed. The tape just keeps the machinist’s scale upright so I could take the picture.

 

I recorded the string height for all posterity. This is not far from right for a ukulele. Some authorities say it’s too high, others say too low. Whatever.

 

The fret wire height is not adequate to support a fret level job. The back bow is just too much. Yes, the back bow is more than 0.040 inch on each end of the fret board!

 

My guess is, the fret board will need to be reshaped. Here, I’m recording the width of the fret wires.

 

Concert ukuleles are tuned A – E – C – G, with the bottom string, sometimes called String 1, the highest pitch. This G string diameter is about 0.022 inch.

 

The C string is about 0.030 inch.

 

The E string is the largest, measuring about 0.035 inch.

 

The drone string is tuned to A above the G. This string measures about 0.025 inch.

 

Interestingly, the fret board has about a 20 inch radius, while the nut and saddle are absolutely flat. The Luna Guitars Web site specs this instrument with a flat fret board, too. I’ve decided to re-flatten the entire fret board. The nut must come off. Here, I’m cutting the finish so that the nut can be removed cleanly.

 

I love whacking musical instruments with a hammer. I find it strangely satisfying.

 

The saddle slips out of its slot. You can see that there is no radius in the saddle at all.

 

The Smoking Gun. There is not enough string tension in the world to straighten this neck. It also has a twist. It doesn’t matter that this instrument has no truss rod because it wouldn’t help.

 

Visually, we can see the wavy fret board and a clear radius. How did this instrument leave the factory?

 

Let’s get the tuners out of the way.

 

I made this fret remover from an inexpensive set of end nippers from Harbor Freight.

 

These frets over the body are easily removed.

 

Before the woodworking begins in earnest, let’s tape everything off.

 

Some cardboard protects the entire soundboard.

 

The strategy is to flatten the fret board on the belt sander.

 

This little belt sanding station came from Harbor Freight.

 

Some of the safety covers were removed to enable the instrument to set flat on the sanding belt. Do not attempt this at home, kids!

 

The eighty-grit sanding belt begins to make an impression on the fret board. This fret board appears to be rosewood, but the Web site says that this instrument is all mahogany. Dunno about that.

 

With a twist that bad, we can easily inspect our progress.

 

Now I am wondering what I got myself into.

 

Back to it! Many thanks to my wife Glenda for taking these pictures.

 

Serious amounts of sanding dust are produced, so we are outside today.

 

Another check shows that we are not there yet.

 

The sanding belt is doing its work.

 

What is it going to take to get this straight?

 

Sanding dust is going everywhere. No scorch marks yet!

 

I am pleased that the fret markers are still intact.

 

Very light pressure is used now to clean up the surface.

 

Now we’re getting somewhere.

 

Continuing on, producing sanding dust like crazy.

 

Now I’m thinking that I need to be careful not to go too far.

 

Most of the fret board is flat. There is still some fall-away over the body of the ukulele, which is OK with me.

 

Last few strokes on the belt.

 

The twist is gone and most of the fret board is absolutely flat. I thought that the noise of sanding would drive the cats away, but we see Jack on the bottom step in the lower left corner of this picture.

 

The luthier’s scraper shows that the fret board is flat.

 

The sharp scraper is an excellent tool to finish raw wood.

 

The fret slots at fret six and seven are almost gone. I really don’t think that this fret board is mahogany.

 

Fret slot ten is almost gone.

 

One end of the fret slots over the body IS gone. Yes, it was that bad.

 

My fret saw was set to 0.054 inch, which is the depth of the new fret tangs.

 

Every slot was taken down to about 0.055 inch.

 

These short sections of small fret wire are perfect for this instrument.

 

Each fret was installed, and nipped to length after installation. The long ends were the pieces at the end of the short strips that weren’t long enough to fill another fret. They get nipped off separately.

 

After nipping, this file embedded in a block of nylon files the fret ends 90 degrees to the fret board. Moving the file to another slot allows the fret crowns to be filed to a 60 degree angle to the fret board. I love eBay!

 

Checking for flatness, these frets are ABSOLUTELY flat, which is not surprising.

 

The fret ends are shaped and burnished by hand, and the fret wires are lightly sanded. As with a classical guitar, the frets are not polished, but finely sanded in the same direction as that of the string.

 

The original nut was reused, and re-slotted to restore the original 0.060 inch string height. The nut was just right as it was. The fret board was oiled. New strings complete the job. All the notes are present and accounted for!

Thanks for reading all the way to the end!

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

Phonic 1500 Rack Mount Stereo Amp Repair and Update

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Billy was running this amp as part of his PA for years until one channel quit.  Could the Unbrokenstring Crew bring this inexpensive amplifier chassis back from the e-waste pile?
To some folks, Phonic and Professional do not belong in the same sentence.  But this unit has been working very well behind the scenes, for more years than many of its Brand Name Brethren have been working.

 

Simple controls on the front.  Cranked to eleven, of course.

 

It doesn’t get much simpler than this.

 

Billy asked to have an IEC power cord installed on this when (not if) we fixed it.

 

The Neutrik power connectors are a nice touch, almost required at higher power levels.

 

Inputs are either balanced quarter-inch TRS or XLR connectors.  The usual set-and-forget controls are found here.

 

The power transformer seems adequately-sized for this power level.  AC-to-DC duties are performed to the lower right, and one audio channel is seen between the transformer and the aluminum heat sink.

 

With the unit turned around, the other audio channel is found here.

 

The front panel controls are here.  These potentiometers will get a million-mile cleaning and lubrication.

 

The dark charcoal-colored ribbon cable connects signals to the two audio channels.  The flat ribbon is Just The Thing here, because it does not block the path of cooling air into the unit.

 

The electrolytic capacitors are bulged, which is not unusual for a unit that has seen this many years and this much use.

 

These electrolytics will be replaced.

 

These rectifier blocks are wired in parallel.  Yeah, Baby!

 

I was kinda wondering where the line fuse was located.  It’s under this bundle of cables.  Yes, that says 30A at 115VAC.

 

Before the unit comes apart, I need to document where all these cables go.

 

These cables need to go back where they started.

 

Behind the colorful bundle in the foreground is the circuit board that connects the rear-panel output connectors.

 

I need to remove the larger circuit board to get to the solder-side of the PCB.  The dirty little ribbon cable comes off first.

 

These output cables come off next.

 

And now, we begin.  There are about thirty screws that hold the circuit boards in place.

 

These little screws are everywhere.  Like that’s a bad thing…

 

More screws.

 

The heat sink is split into two sections, one for each channel.  They need to come loose from the chassis as well.

 

This aluminum block bridges the tops of the heat sinks to add strength and rigidity to this unit.  Nice!

 

OK, the main board is out of the chassis.

 

Interestingly, some other version of this amplifier uses more electrolytic capacitors.  In this version, the pads are jumpered.

 

Turning the board over, the solder joints to be cleared are ‘marked’ with some rosin solder flux so I can find them if I look away to grab the soldering iron and braid.  Yes, I’ve unsoldered the wrong solder joints in the past.

 

The old caps at the top of the picture are out and the holes in the PCB are cleared.

 

Meanwhile, back at the ranch, we will remove the captive line cord and add the IEC connector here.

 

This cord has 14AWG conductors in it.  A matching large molded IEC power cord will be supplied with this unit when it is returned to the customer.

 

Here is our new IEC male socket.  Some of these come with flange ears, but spacing on the rear panel is too tight to allow the use of one of those.  This one snaps into place.

 

This looks a little rough, but this is the approximate outline of the rectangular cutout for the new IEC connector.

 

While we are hatchet-ing on the rear panel, these magnets will catch any chips or bits of steel removed from the hole.

 

Here is the outside view of the first trial fit.

 

And this is what it looks like on the inside.  This IEC socket snaps into the hole, so the hole size needs to be right.

 

Here is the new IEC socket wired into place.

 

And this is the closeup of the finished installation.  That silver thing next to the CE mark is a ground point.  I temporarily removed the thumb screw while grinding on the chassis.  It goes back on the unit next.

 

Now it’s time to reassemble.  Screws, anyone?

 

Glad I took all those pictures of where these wires went!

 

The unit is now reassembled.

 

Here, the terminal block outputs are tested at 250 watts.  So far, so good!

 

The Neutrik connectors are tested next at 750 watts per channel.  All is well!

 

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

Boss DD-20 GigaDelay Pedal Needs Direction

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Every function on this pedal works properly.  Except, the user cannot select menu functions without a lot of hassle.  And, the delay time can only be increased, not decreased.  Can the Unbrokenstring Crew decode the mystery and make this unit a little less of a pain in the rear to use again?

The unit turns on and works, but the delay time knob only increments.  This makes it almost impossible to select any function or mode unless the user spins the knob all the way around to wrap back to the beginning of the menu.  And there is, like, three hundred menu items.

 

Everything is accessible from the bottom.  The vertical strips are Velcro hook material, to hold the pedal in place on the pedal board.

 

The black plastic rectangle on the left is a battery box.  On the right is a stack of circuit boards.

 

Removing the circuit boards involves removing the nuts on these jacks.

 

The ribbon cables interconnect the two main circuit boards.

 

The metal piece around the jacks is really part of the noise shielding of the box.

 

We can tip out the bottom board now that the jacks and power adapter jack is free from the rear panel.

 

Interestingly, this little crescent moon-shaped piece is a bushing that makes a small hole out of a large one.  This may have been a design feature for another model of pedal, or, more likely, the result of an engineering change.  Or, my favorite theory, a design screw-up.

 

Back in the early days of electronics, stiffened cardboard was coated with wax to make insulating sheets called ‘fish paper.’  Nowadays, we have synthetic equivalents.  This insulating sheet separates the two circuit boards.

 

To gain access to the top circuit board, we need to remove the control nuts on the top side.  This is the knob for the delay select function.  The knob works fine, the function it performs does not.

 

This plastic bushing covers a larger hole and allows the knob to be recessed below the plane of the top panel.

 

These other controls are more conventional in their mounting scheme.  Those hex drive screws are for looks, and only hold the bezel down on the top of the pedal enclosure.  We don’t have to mess  with them today.

 

At long last, we are able to remove the upper circuit board.

 

We already have a pretty good idea that the rotary encoder for the delay function is not working properly.  To verify that, the unit is partially reassembled for electrical test.  The cardboard is more insulating material to keep everything from shorting out.  Kinda like home-made fish paper.

 

The unit is running and the oscilloscope can tell us if the two phases of signal are coming from the rotary encoder.  The answer is, one phase is missing.  Thus, the rotary encoder can only appear to electrically ‘spin’ in one direction.

 

So, it’s time for the old rotary encoder to come off the circuit board.  A soldering iron will melt the solder and this Solda-Pullit will remove the molten solder.

 

The new rotary encoder is installed, as shown.  This rotary encoder also has a built-in momentary switch that is actuated when the shaft is pushed down towards the circuit board.  Or, in use, when your foot taps the knob while playing.

 

Pretty nice workmanship, wouldn’t you say?

 

Re-assembly is the reverse of dis-assembly.  There are a lot of parts here, so this unit will go back together with the help of all the pictures we took earlier.

 

The insulative sheet goes here.

 

The bottom assembly goes here, with the requisite white cables.  Don’t forget to re-install the little crescent-moon-shaped piece where the connectors poke through the back panel.  I did, the first time.  Oh, yeah, and the metal shield, too.

 

Many of the Boss pedals use this barrel connector for power.  The shell of the pedal housing slide into the slots molded in the connector to firmly hold the power connector in place.  Note that not all barrel connectors have these slots.  If you ever replace a Boss pedal power connector, verify that the replacement part has the slots.

 

This is a little better shot that shows the proper orientation of the shield plate.  The little sharp fingers around the outside edge should point away from the middle of the pedal, so that they can ‘bite’ into the pedal housing.

 

The red and black cable carries power from the internal battery box that we saw in the picture above.

 

The unit is cleaned up and all the knobs reinstalled.  Ready for test!

 

All functions work flawlessly, PLUS the rotary delay time knob both increases and decreases delay time and allows the user to scroll up and down through the menus.  I think we’re done here!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626