Month: May 2017

Blackstar HT5RH Guitar Head is Dead

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This five watt head had gone nearly silent. The national retail chain that sold it told the owner to purchase a new one, because that’s their business model. Could the Unbrokenstring Crew repair this unit and bring it back to life?

At five watts, this head is bedroom-friendly, yet there are plenty of opportunities for distortion and tone shaping.

 

Interestingly, there are three options for outboard speaker cabinets as well as all of the usual in’s and out’s.

 

The back comes off with these screws.  But it appears to be stuck in place!

 

I speculate that when the unit was built, the Tolex glue was still fluid and squeezed out between the back and the frame, sticking the rear cover in place.

 

The chassis is held in with these screws.  No surprises here.

 

This is a hybrid solid state / tube unit, with a 12AX7 triode pair in the preamp section and a dual triode 12BH7 pressed into service as a push-pull tube output stage.

 

There are two different versions of the schematic available.  They can be easily identified by checking the number of conductors in that big ribbon cable that connects the rear panel circuit board to the main circuit board.  This particular unit uses the cable with 21 conductors.  Thanks to Armando Garcia at Mars Electronics who furnished this schematic!

 

In the foreground are the big heat sinks for the voltage regulators.  The rear panel wiring board is in the background.

 

Viewed from the other side, the main circuit board contains the preamp circuitry.

 

The preamp uses DSP techniques to create the reverb effect and the tone-shaping functions of this unit.

 

This is the preamp tube.  It’s fine.

 

This is the 5 watt output tube.

 

This part checked OK but it is a little weak.  We will continue to use this unit for troubleshooting purposes.

 

This is the pin-straightener from my tube tester.  The Chinese tubes have a little larger envelope than the JAN versions of these tubes, so the tube is a tight fit in the straightener.  If the tube envelope is too large, the pins can be straightened using the same tool, but the pins are inserted from the other side of the straightener.

 

So the output drive signal is split into an in-phase and out-of-phase copy, and applied to the grid of the output tube.  But the plates of the 12BH7 are stuck at +300 volts.

 

The primary windings of this output transformer are shorted to each other.

 

Let’s get this transformer off the chassis and take a closer look at it.

 

These Chinese transformers are usually not worth fixing, but this transformer is hard to find.  Some exploratory surgery shows us that the problem is deeper in the windings and not readily repairable.

 

So, out it comes entirely.  The search is on for a replacement.  The original manufacturer has no stock.

 

Hammond makes a versatile aftermarket unit that is available through distribution.  We can make this work!

 

The frame of the transformer is just a little bit larger than the original part.  One of the mounting holes is being moved.

 

Placing the magnet near the site where the drill is working helps keep those pesky metal shavings under control.

 

The new transformer is bolted in place and some Thread Locker is applied to the bolts to keep it in place.

Here the output leads are threaded through the insulating grommet in the chassis.

 

The input wiring is soldered into place, so the leads are trimmed to length.

 

To preserve the original wire insulation colors silkscreened on the circuit board, short pieces of the original wiring were left in place and butt-spliced to the new output transformer harness.

 

This unit is ready to test.  Pretty neat looking!

 

Surprisingly, this unit has a bias pot and a balance pot.  However, there is no commonly-available information available for the technician to set these.  So, I wrote this procedure:

BALANCE ADJUSTMENT

  1. Set DMM to mV range
  2. Affix DMM probes to TP6 and TP7
  3. Switch POWER ON; switch STANDBY ON after five minute warmup.
  4. Adjust BALANCE pot for 0V display on DVM
  5. Switch all power OFF

BIAS ADJUSTMENT

  1. Affix black DMM lead to ZD2 cathode (banded end) near Input Jack. This is a convenient GND.
  2. Affix red DMM lead to D20 anode (not banded end.)
  3. Switch POWER ON; switch STANDBY ON after five minute warmup.
  4. Adjust BIAS pot so that DMM display is 46mV.  This puts about 10mA of idle current through the 12BH7.

Recheck BALANCE and BIAS adjustments as they are slightly interactive, particularly if the internal sections of the 12BH7 tube are not matched.  Follow all the usual precautions of removing power and working safely around high voltage!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

JR Beck Six String Bass Blend Pot Adventure

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Billy said that he really liked this bass, but the blend pot was coming loose and something electrical was intermittent inside the instrument. Could The Unbrokenstring Crew look it over and make it stage-worthy again?

In literary circles, the term “vanity publishing” is often applied to authors who self-publish their own works. The JR Beck line is a ‘vanity-built’ line of musical instruments, wherein an individual approaches an instrument manufacturer to build instruments under a new name. This bass likely came into the world as part of a contract with a Korean manufacturer.

 

The neck is straight and the frets are level.  This is a player’s six string bass.

 

The head stock shape reminds me of an Ibanez profile.

 

The Grover tuners are really nice.

 

Someone has been here before.  This screw is too long for the hole.

 

Just how deep does the hole need to be?  It needs to be THIS long.

 

We will bore this hole down to the tape go get the length we want.

 

I can find no markings on the preamp electronics.  The rest of the wiring is a mess around the blend pot.

 

The duct tape did NOT come from Korea.  Why not get someone who can do this sort of work cleanly, like me?

 

The blend pot is a hack job.  The rest of the wiring is probably as it was from the factory.

 

Use more solder if you don’t solder it right the first time.  The solder has actually flowed inside the unit and mechanically interferes with the rotation of the wiper in one of the sections of the control.

 

We’ve removed the wires from the blend pot.  The cables with the green and yellow jacket come from the pickups. The cable with the white jacket wire goes to the active electronics.

 

Some RTV was used in an attempt to stabilize the front face of the blend control and keep it from moving when the knob was twisted.  What a mess.

 

As the control moved around, it scored up the wood body of the bass.  No pick guard on this unit.

 

To stabilize the wood and prep to mount a new control in a stable manner, a Forstner bit is employed to create a flat-bottomed ‘spot face’ in the wood cavity.  There is some risk of penetration, so we’re going slowly at this point.

 

A flat-bottom hole is bored without incident.  A steel washer will be epoxied into the body to fully-support the new blend pot.

 

The new blend pot comes with a couple of flat washers.  I’ll use a lock washer and another nut to set the height of the control so that the knobs are all installed at the same height on the face of the guitar.

 

A blend pot is a pair of “M” taper potentiometers, which allow one pickup to be faded out as the other side gets louder.  As the control reaches each limit of its travel, the other pickup is electrically ‘cut out’ of the circuit.  In a way, it acts as a selector switch at each end of the travel, with a continuous blend of the two signals in between.

 

The cross wiring is done with solid 20AWG copper covered with Teflon tubing.  The pickups are wired on the end.

 

This control has an index pin.  We will cut a slot in the steel washer to ‘catch’ this index pin and hold everything in place.

 

Initial tests showed that the signals from both pickups were literally ‘grounded’ when the control was set to either limit.  We need to install range-limiting resistors to each end of the controls so that the selected pickup is not grounded at the end of the potentiometer travel.  This is a blend pot, not a kill switch, after all.

 

It is an easy task to break the connection to ground and install a fixed resistor.  The proper value depends, in part, on the internal DC resistance of each pickup.  It will be easier to discover the proper value empirically.

 

The box with all the switches is sometimes called a ‘design box.’  The proper name is a ‘resistance substitution box.’  Various values of resistors are switched into the circuit until the desired result is obtained.  Here, we are choosing a resistor value for one side of the blend pot.  Too low a value attenuates the signal.  Too high a value causes hum.

 

Here is the fixed resistor for this side of the blend pot.  Note the use of the Teflon tubing to keep the signals under control.

 

Here is the new control installed in its new home.  I am satisfied with the electrical results of this experiment.

 

The original knob works nicely with the new control.  Note that the knobs are all the same height off the body of the bass.  Another thing I like about this bass is, look at that massive bridge saddle and end block!  The Koreans really know how to do it right.

 

Jacob takes the finished bass for a spin.  He wants one!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626