Month: October 2014

Gallien-Kreuger 1001RB Bass Amp Input Jack Upgrade

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Broken input jacks are a pain in the “ear.”  The replacement of switches and jacks is the bread-and-butter for the bench technician.  Let’s see if the Unbrokenstring Crew can take this otherwise-mundane repair to the next level!

Looking inside, we see a big toroidal power transformer on the right, a power supply unit in the center, and a big heat sink with cooling fan on the left.  These are the main components in a Class D (switching) power amplifier.  Tone-shaping and low-level audio tasks are performed on the circuit board behind the front panel and around the periphery of the main circuit board.

At this angle, we can see that our objective is to get the long blue circuit board behind the front panel out of the chassis where we can replace the input jack.  The power switch is the silver box to the right and is not part of the blue circuit board.  It comes out first.

We turned the chassis around and went searching for a screw driver that was ‘just right’ to properly fit the Phillips-head screws and with a handle to supply enough torque to do the job while allowing us some space over the power transformer.

Now, all the knobs come off.  All of them.  Two screws holding the face of the three-pin XLR jack come out, too.  With the hardware completely out of the way, we will have an opportunity to easily clean up the front panel.

If you look closely at the jack in the middle of the picture, you can see that it is cracked.

I am not EVEN going to ask what happened to the effects in/out jacks.  Houston is a humid climate, but humidity and mold did not cause this!  Not to worry… we’ll change these out too as they are the same part as the input jack.

The blue corrosion is copper sulfate.  This is commonly seen around copper that has been exposed to the out-of-doors, or perhaps in this situation, the not-quite-out-of-beers.  Note the D15 and D16 LEDs on the right.  More on these later!

The owner mentioned that he had sent the amp back to GK for an input jack replacement.  I’m not impressed with their workmanship.  The brown gunk is solder flux.  Usually, solder flux is not conductive, but the dust and moisture it attracts is.  And, as you can see, the input jack finished breaking.  The threaded part fell off and is somewhere on the floor now.

Here is the carcass of the old input jack, free from the circuit board.  The circuit board has been cleaned up with alcohol and wiped clean.

OK boys and girls, listen up all you GK Amp repair persons.  This post will save you money and increase your profits.  Two new jacks lie on either side of the original (broken) GK input jack.  The ‘factory’ GK jacks are available online for prices that range between $5 and $13 each.  The ‘factory’ jack is identical to the Amphenol part seen on the left.  I will leave it as an exercise for the graduate student to write down the Amphenol part number and the Mouser part number that are printed on the label on the left.  As an added bonus, the jack on the right is identical to the GK input jack except it has a small metal tang that is intended to make electrical contact with a metal front panel.  If you look closely at the jack on the right, you will see this tang.  This is just an FYI…  you want the jack on the left for most GK applications.  Did I mention that the full retail price for this jack, in single quantities, plus Texas sales tax, is less than one dollar?

Here’s the New Jack City being soldered in.  We’ve now begun the reassembly process.

The effects jacks were also removed and the circuit boards cleaned up.  Here you can see some pesky copper sulfate remaining to be cleared out.  Whatever the contaminate was, it was water-soluble as it wicked under the blue solder mask on the circuit board.  No beer I’d drink…

The less-than-one-dollar Amphenol jack comes with nicer hardware than the factory GK jacks.  Here, you will see a nice hard rubber washer behind the beefier nut.  The rubber washer doubles as a shock-absorber and a lock washer.  The original GK jacks had no washer at all, but rather, a flat washer was molded into the base of the nut.  The new nuts are tightened finger-tight e.g. as tight as you can get them without using tools.

Again, really a nicer jack.  And, did I mention that these were under a dollar, full retail price?

This is another picture of the XLR jack, with the mounting screws.  Have you spotted the clean front panel?

Knobs.  Lots of knobs.  And they all go back on in a particular orientation.  More knobs…

Everything is back together, including the rack ears.

Last pic.  Here are the new effects loop jacks, and the neat power indicator.  If you check out the earlier picture of the blue corrosion on these jacks, you will see two LEDs stacked one behind the other on the circuit board.  This is an easy way to make a multi-color indicator.  When the power comes on, the LED is red.  Then when the power is stable, the indicator switches to blue.  Very cool-looking!  This is the only way, in my experience, to build a multi-color indicator that includes blue with common colors.

Thanks for reading all the way to the bottom of the post!

CONTACT INFORMATION – David Latchaw  EE

281-636-8626 voice or text

Noisy Volume Control in the Peavey XR-600B Mixer Amp

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This honest, hard-working powered mixer lives in a music room, where all the musicians can plug in and hone their craft.  After all these years, the volume control has become dirty and intermittent.  The Unbroken String Crew realizes that this repair will be a wonderful opportunity to show an Old School method for repairing the part, not replacing it.

The unit is a wonderful example of intelligently-packaged, simple product that just works.  Four screws, and we’re into this unit.

These two circuit boards carry all the front-panel controls and interconnects.

Most of the interior of this unit is empty space.  Plenty of room for the reverb pan and power transformer on the bottom.  The amplifier is seen on the far side, supported by the rear panel.

The objective today is to clean up the ‘MAIN’ pot, in the lower left hand corner of the big front panel.

The nuts on the front panel controls along with the nuts on the quarter-inch jacks hold the front panel together.

Everything is hand-tightened.  A socket and my hand allows enough torque to get the nuts loose.

Here’s what’s behind the front panel.  Now the real work begins!

I’ve desoldered the volume control.  What I intend to do is to rebuild this control, rather than replace it.  By rebuilding this control, we can preserve the original markings, some of which are useful when proving the age and authenticity of guitars and amplifiers.

I’m using a colorful rag to catch all the small parts that always seem to fly away.  First, we’re prying open four fingers that hold the back of the control on the body.  Yes, I know I shouldn’t use cutters as a pry…

The back of the control is on the left.  Inside the control we see the central shaft.  On this shaft is a rotor, which is the part that turns with the central shaft.  The actual resistor is the dark circle facing the rotor.

We’re looking at two pairs of sliding contacts.  The large pair on the left, towards my hand, run on a circular metal contact ring.  The overlapping sliding contacts to the right actually touch the resistor itself.  These sliding contacts must be clean and dry.

Do not attempt this at home!  Leave this to trained professionals!  Seriously, a pink eraser is a quick and effective abrasive tool for cutting the crud off the sliding contacts.  But we need to be super-careful here because these sliding contacts are very delicate.

The larger sliding contacts touch this circular metal contact ring.  The part that I’m holding in my hand is the actual center leg of the control that solders to the circuit board.  This ring has a lot of surface oxide and contamination, and is probably why this control was noisy and intermittent.

And here is the circular ring without surface oxide and contamination.  We have now begun our path back to a repaired control.

Moving our attention now to the resistor, we see that this ring is actually not too dirty.  However, this is very delicate, so we will go slow here and Do No Harm.

All I’m going to do here is lightly burnish the resistor and set it aside.  The cotton swab shows what came off the surface of the resistor.  I think we’re good here.

Plenty of lubricant remains in this unit.  Here, I’m wiping away the surface of the lubricant to remove dust and oxidation.  What is under the dust and oxidation is perfectly usable, and will be left in place.

The clean lubricant goes wherever there are moving parts EXCEPT on those sliding contact surfaces.  You will recognize that this passage is where the volume control shaft passes through the body of the control.

Everything goes back together where it belongs.  I’ll hold this together with one hand while I reach for the pliers.

The four fingers are crimped back into place.  This portion of the job is complete.

The control is soldered back into the board where it originally came from.  And, yes, my mama told me not to end my sentences in a preposition.

Everything lines back up, including the newly-resoldered control.  We’re about done here!

We’re back together.  The master volume works perfectly.  All the channel strips are good.  All Systems Go!

 

Thanks for reading all the way to the bottom.  I hope you have seen enough of the process to gather an understanding of what might be involved in repairing a control versus replacing it with a modern unit.  I discussed using vintage parts to repair my Gibson GA-30 amplifier, and made the decision on that project to use modern parts.  Likewise, in the previous post about tearing down the Shure 555H microphone, the switch was repaired instead of replaced.  So keep the repair process in mind when working on vintage gear that may be dated from EIA codes stamped on controls.  Gear may lose value when non-original parts are installed.

 

Contact Info – David Latchaw EE

281-636-8626

Shure 555H Microphone Rebuild

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Another well-worn road warrior from Dr. Shoen is on the bench for service.  The foam is shot.  The on/off switch is always in the ‘on’ position, regardless of what the switch says.  The Unbroken String Crew goes to work!

The slide switch is under this cover.  I’m using a rag on the desk top to keep the small parts from bouncing away.

When the switch is in the ‘off’ position, the signal line is shorted to ground.

Speaking of small parts bouncing, this sliding contact is supposed to be inside the switch.  What’s going on here?

Here is an internal view of the switch wiring cavity. The spring is a flexible conduit of sorts, protecting the wiring that goes up to the head of the microphone to the cartridge itself.  The yellow and black wires allow the user to switch off the microphone by grounding the audio signal.  Or that’s the way it’s supposed to work.

Had the other ‘pole’ of the switch been soldered, the switch would have appeared to work properly. Hmmm…

In any case, we need to de-solder the switch so that the internals (including that sliding contact in an earlier picture) could be reunited with the rest of the switch, and the wiring will be replaced as we found an intermittent connection due to a broken wire inside the insulation somewhere.

This is a good view of what happens with a double pole, double-throw switch.  The pair of sliders work in unison to control two different circuits (double pole) and operate in two possible configurations (double throw.)  The folded tent-like structure is a spring which holds the sliding surface against the physical wiring contacts, which are the six metallic squares seen in the part of the switch in the upper right part of the picture.

Putting it back together, we have a better view of how the sliders relate to the physical wiring contacts.

Squeezing the switch together like this compresses those tent-shaped springs.  This pre-loads the contacts together to establish good contact.

Quickly, before I lose my grip, I gotta crimp those tabs back down so that the switch is held together again.

All four tabs are down flat, so this switch will be good for another million miles.

Let’s take the rest of the mic body apart now, to do something about the deteriorated foam.

A fine wire brush on the Dremel tool makes short work of the remaining foam. The dark lines are the old adhesive.

The front half of the shell cleaned up pretty well.  I’ll set this aside for now.

Here is the microphone capsule in its holder.  This all comes out to repair a broken wire as well as replace the windscreen foam on the capsule and the back half of the microphone shell.

The actual microphone capsule is held in the plastic holder with this big aluminum ring.

One of these wires were broken inside the insulation, so the wires will be replaced.  This pic was mainly to document which color went to which terminal.  I believe that the red magic marker indicates the ‘hot’ audio terminal.

The wire wheel made short work of the remaining foam in the back section of the microphone case.

For decades, technicians have used “Blue Shower” to remove corrosion and dirt from sensitive electronics.  Nowadays, more modern products have come to the market, and this is one. But I still keep my last can of Blue Shower in the chemical cabinet for those times when I need the Good Stuff!

Rather than spray the whole microphone with cleaner, I sprayed some into the cap, then will brush it onto the places that I want to clean up.

And this is my brush.  This brand of cleaner is a pale orange.

This is a good shot of the cleaner doing its work.  Compare the uncleaned pin in the background with the pins in the foreground that have already been cleaned.  And look at the junk on the swab!

The microphone is getting a new wire harness.  This is the back side of the connector in the previous picture.

To pull the wire harness and the protective spring through the rear shell, I’m using this old guitar B string.

This is a glimpse of the B string and the wire harness moving up through the body of the microphone.

The wires to the microphone capsule are blue, yellow, and black.  The switch is wired with yellow and black.

The microphone connector is now fastened into its pocket in the bottom of the body.

Now that the microphone connector is secured, the internal spring can be tensioned and its end secured under the set screw shown here.

Our switch can be wired up and put back where it belongs.  This time, I’m using both sections of the switch.

The foam on the microphone capsule is completely gone.  A small round piece of foam will be glued here.

Not perfectly round, but who will know besides you and I?

This foam comes in large sheets.  Here, I’m fitting a small piece of the foam in the rear shell.

This is what it will look like from the outside.  Now we’re getting somewhere!

Trim the foam to fit.  No rocket science here!

The rear shell foam is ready to be cemented into the shell.

The same approach will be used to pre-fit the foam piece that will be cemented inside the front shell.

This aerosol can is what my mother-in-law would call a ‘Free Methodist’ spray can.  It sprays adhesive everywhere.  To keep the application of cement under control, I’ll repeat the applicator trick that I used to clean up the microphone connector, and use an applicator to put the adhesive right where I want it.

Here, I’m using a disposable paint touch-up brush to apply the cement to the inside of the front shell along the same lines used at the factory.

This is starting to look like a microphone again!  This is the front shell.

By controlling the application of the adhesive, I didn’t have to worry about removing a bunch of over-spray.

This is a rubber plug that serves as a strain relief at the point where the internal wiring enters the plastic shell holding the microphone capsule.

This is about the final resting place of the rubber plug.  Now we can trim the rest of the wiring to length and solder to the microphone capsule.

The wires are soldered and we’re re-assembling the plastic shell supporting the microphone.

The body of the microphone is almost complete.

Four more screws to reattach the front shell, and we’re done!  Dr. Shoen loves the aesthetics of this microphone, don’t you?

Thanks for reading all the way to the bottom.  This was another long post, and I hope you see the attention to detail that the Unbroken String Crew can apply to your repair or restoration project.

Contact Info :  David Latchaw EE

281-636-8626