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

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

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

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

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

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

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