refurbish

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

MIJ Fender Precision Bass Gets a New Nut and Setup

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Lisa’s marvelous Fender P-Bass needed attention.  Some of the open notes were dead, and the electronics needed some attention.  Could the Unbrokenstring Crew sort it all out?
I just love the pale yellow finish.  Except for a string, everything is here.

 

Yes, it really is Made In Japan.  Back in the day, ‘made in Japan’ was another word for cheap imported junk.  Nowadays, this is some of the better stuff, particularly in guitars.

 

Name, rank, and serial number, please!

 

An electrical test shows that we have no output.

 

A quick look under the hood does not reveal an immediate problem.  Hmmm…

 

Oh, this is it.  The ground point for the whole unit is this potentiometer body.  However, the ground wire to the output lead does not connect to the potentiometer body anywhere.

 

With that fixed, the remaining ground wires are cleaned up a bit.

 

The bridge ground wire made an intermittent connection to the bridge.  We need to remove the green corrosion.

 

OK, the electronics are now all up to snuff, and actually look pretty nice.

 

While we’re here, we’ll tighten the output jack and potentiometers a bit.

 

The knobs go on now.

 

Final test is performed with a signal generator and another bass pickup.  The signal generator excites the windings in a bass pickup from an Aria Pro II bass, which will be featured in a future blog post.  The test pickup is brought near the instrument’s pickups, and the magnetic field carrying a test tone is coupled into the instrument’s electronics.

 

The dead open notes are traced to a cracked nut.  Here, we’re cutting the finish around the old nut so that it can be removed cleanly.  The Exacto knife gets a new blade for this operation.

 

The old nut comes out in two pieces.  The crack expanded until the nut broke in two.  That’s why we’re replacing it.

 

Here is the new nut that the customer wanted installed.  Good stuff!

 

Oops.  Houston, we have a problem  This new nut does not fit the neck.

 

The new nut is just a tiny bit smaller than what is required for this neck.  What gives?

 

We can clearly see the difference in the sizes between the old nut and the new one.  This neck is the width of a five string bass, but it was delivered as a four string bass from the factory.  So, we will make a custom nut for this instrument.

 

A Tusq blank is radiused to match the radius of the fretboard.  I’m using an Exacto knife as a scraper.

 

The Tusq blank is cut to rough length with a fine saw.

 

It doesn’t take long to slice through the Tusq material with this blade.

 

This is a saw blade set that I use for sawing fret slots and general fine work on wood.

 

The blank is now shaped on the disk sander.  A piece of birch plywood serves as a raised table that can be placed very close to the abrasive surface of the disk, necessary when shaping small parts.

 

The blank is now pretty close to the rough shape we need.

 

The first trial fit shows that we haven’t cut it too small, yet.

 

This is a little better.  The ends are flush and smooth with the edges of the fret board.

 

In AutoCAD, a drawing is created showing the cross sections of the four strings and the width of the fret board in actual size.  The distance between the edge of the outside strings and the edge of fret board, established by factory specs, is drawn, and the position of the outside strings fixed.  We then subtract the diameters of the four strings from the width remaining.  This result represents the space between strings, which shall be three equal spaces.  This establishes the center lines of the inner strings.  The spaces between the strings are the same, not the center-to-center distance.

 

But, to cut the string slots, we need to know where the edge of the fret board is, and where the center lines of the strings fall.  These solid lines represent that information.

 

The lines which represent the centers of each string are transferred to the nut.

 

A shallow file cut is made at each string center.  Here, we are checking these cuts against the template.

 

These shallow cuts represent the eventual center of each string.

 

These cuts were made with a triangular mill file.  Nothing special, but accurate enough.

 

Here, we’re polishing up the sides and faces of the nut, in preparation for gluing the new nut in place on the neck.

 

The nut depth is established by the fret height plus a constant which is established by Fender (and can be adjusted a bit by a good luthier, like me, for best play-ability.)  This is the Secret Sauce of making an instrument a great instrument.

 

The slot depth is now established by this stack of feeler gauge shims.  They are held in place with rubber bands wrapped around the back of the neck.  I’ve taped off the head stock so that I don’t scratch it up with the end of a file.

 

When the file touches the stack of feeler gauges, continuity will be detected by this multimeter, and it will beep.  This is another check of slot depth, besides my eyeballs.

 

Here, the slots are cut.  With a little cleanup and polish, this will be a good nut!

 

The nut is all done and polished.  Looks good!

 

The action on this instrument at the twelfth fret is pretty high…

 

We have a metal neck shim between the neck and body, made from a piece of the machinist’s feeler gauge of the proper thickness to reestablish proper neck geometry.  The metal shim is the hardest practical material for this purpose, with an accurate thickness, and better mechanical stability and hardness for greatest vibration transfer between the neck and body than a guitar pick or a piece of business card.  This results in the best tone.  And a set of feeler gauges are less than five bucks.

 

A quick adjustment gives us just the right amount of neck relief.  (Sharp-eyed readers will spot the fact that the strings are off in this picture.  This is the only pic I took of the truss rod adjustment, setting the neck flat while the neck shim was being sized.  Who cares if my pics are out of chronological order?)

 

To set intonation, we needed to work on the bridge.  Here is the underside of the bridge, probably not seen for decades.

 

The intonation screws were dinged.  Here, we are chasing the threads with a die to clean them up.  Yes, they are English/Imperial threads, not metric.

 

The bridge is tightened down and ready to go!

 

The moment we’ve all been waiting for!  Add strings, tune up, intonate, and play!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

The Korean Squire ‘Blackout Strat’ Build

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Matt’s unicorn was to again own a Blackout Strat like the one from his youth.  Could the Unbrokenstring Crew make an equal-or-better unicorn?
The body is a Korean Fender Strat with genuine wear.  The new neck is in superb shape.  This will be the basis for a cool project, particularly since modern parts are available to upgrade this guitar.

 

The neck comes off and will be updated separately.  Say ‘Goodbye’ to the Squire neck plate.

 

The dot inlays in this neck are really spectacular.

 

The pictures do not do justice to the spectacular mother of pearl in this fret board.

 

First, we’ll clean up a couple of handling skuffs that occurred while this project was coming together.

 

Tinted polyurethane, various grades of fine sandpaper, and a buff polish gets us back to where we belong.

 

A set of vintage tuners will be fitted to this head.  Here, we establishing the center line of the tuners.

 

Screws for the tuners will go where the scratches cross.

 

The pin vise is pressed into service to bore the screw holes.

 

This drill bit will be used as a gauge to verify the diameter of the tuner holes in the neck.

 

Sure enough, this neck was pre-drilled for 3/8ths inch bushings.

 

The bushings are fitted tightly into the neck.  This is essential for good tone, as the bushing supports the capstan, which is one end of the support structure that establishes string tension.  If these are loose, your strings won’t work very well.

 

Turning our attention to the body, we find that the ground wire was not properly soldered to the string claw.

 

The interior routes of the body were painted with a conductive paint.  This just won’t do for the Unbrokenstring crew.  This screw ties the conductive paint to the rest of the ground circuit.  We can do better.

 

The output jack is liberated from the stamped ‘football’ socket plate.  We will rework the wiring with heat shrink support in order to support the wires and make it more durable.  And the Unbrokenstring Crew will redo the soldering job, because We Own It if it fails.

 

The neck pocket must be square and clean.  Any debris or finish will interfere with the transfer of mechanical vibrations from the neck.  For maximum sustain, we need to take this pocket down to the bare wood.

 

This side view shows that the corners are square and clean.

 

Copper foil will be used to create the cavity shielding.  We’re starting with the hard stuff, the output jack route.

 

Bottom and sides are done.

 

All of the interior routes are lined with copper.  The seams are tacked together with solder.

 

This is the bottom side of the new pick guard, also made in Korea.  The aluminum foil must go.

 

All of the holes will be de-burred with this tool.

 

We will have a nice flat surface to which the copper foil will adhere without voids.

 

Foiling the pick guard takes just a few minutes!  Again, the seams are tacked together with solder to form a plane.

 

The foil is trimmed away from the edges with an Exacto knife.

 

This build will use these pickups.

 

This set will be a very cool foundation for this instrument.

 

The controls are mounted right to the copper.  True to Fender specs, we are using surgical tubing for pickup springs.

 

The pick guard assembly is done, complete with Orange Drop tone cap.

 

Here is another view.  All that is missing is the output jack wires and the bridge ground wire.

 

Here, the body is going together.  The output jack wiring and bridge ground wire is routed through the body and soldered to the pick guard assembly.

 

This workmanship turned out pretty.  New Old Stuff (NOS) push-back wire insulation is used to complete the vibe.

 

Everything fits!

 

The neck gets a little prep before the strings are installed.

 

Remember the Squire neck plate?  This is the replacement.

 

Meet the strings!

 

This new neck has a new nut, which needs to be slotted and filed.  String spacing is established here.

 

Here, the slots are taken to the proper depth, calculated beforehand.  The stackup of feeler gauge blades establishes the bottom of all of the string slots.  When each file touches the feeler gauge blades, we’re done.

 

The string slot depth is where we want it.  Next is to file off the top of the nut and polish it, which has been shown in other blog posts.  When this is finished, the strings will protrude just above the top of the nut.

 

Now this is beginning to look like a guitar.  This is set up as a ‘hard tail’ so no tremolo bar is needed, but one is supplied.

 

We lost Matt somewhere in the bowels of Guitar Center.

 

This message came into the Unbrokenstring Global Command Center after this guitar made it back to the rehearsal room:

“BTW been meaning to tell you, OUTSTANDING job you did with the recreation of Crow’s Fender Blackout Strat!

     “I think can say, without any hyperbole whatsoever, that just plugged straight into an amp with no tweaking whatsoever, that is the BEST SOUNDING GUITAR I HAVE EVER HEARD IN MY LIFE.

     “Completely outshines even the original Blackout he was trying to unbury.

     “We’re still absolutely dumbstruck by clarity and full tonal range of it. Truly amazing work, sir!

     “Without question, your finest creation to date.”

 

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626