Month: March 2019

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