“The Fuse Was Blown!” in this Mesa 20/20 Stereo Amplifier

The music died when the AC line fuse blew in his Mesa 20/20 stereo amp.  What are we gonna do?  Call The Unbrokenstring Crew!

JC’s rig was built around the amp, so of course it was installed first at a place of honor in the bottom of the road case.

Exploratory surgery is fairly easy in this unit.  Everything is accessible once the top black panel is removed.

Here is a little Tech Porn.  A lot of versatility is designed into this amp.

Every tube in this unit is original.  These were all checked and packed safely to the side.

Here’s what a blown AG3 fuse looks like, just in case you hadn’t seen one before…

With the tubes removed and the power applied gradually through a Variac, the problem persisted.  Here, one of the red wires that sends high voltage to one of the output transformers is temporarily isolated from the circuit.

And this is the output transformer at the other end of that red wire.  As power was applied, it began to get hot.  Do you see the little round dents in the insulation?

Those little dents are from these rubber bumpers that help support the wire bobbin of the output transformer.  Current leaked from high voltage winding to the steel core laminations of the transformer.  The head generated by the electrical current melted the nylon bobbin, further damaging the transformer windings and leading to a cascading insulation breakdown in the unit.

Mesa Engineering had a few of these output transformers left in stock.  Here is a fresh one, via FedEx.

The new part is the output transformer on the right.

Now we are going to dress the wiring back through the chassis of the amplifier in a manner similar to the way the old part was installed.  Since the chassis is about one and a quarter inch thick, everything is super-easy to work on!

Except when the cabling goes underneath the circuit boards.   A pair of round-nosed pliers are used to snake the wires where they need to go.

And we come out the other side.  Recognize the red wires from the earlier picture?

These three wires are the four ohm / eight ohm output leads.  These were trimmed to length and tinned.

If you look closely, the colors of the wire insulation is painted on the circuit board, where each output lead belongs.  However, this color information did not match the original configuration of the amplifier.  I’m glad I took all those pictures before I started removing wires!

Here is another view, showing the final lead dress.  The amp is back together for final test.

This is Channel B at 20 watts into an eight ohm resistive load, driven by a 400 Hz sinewave.

OK, we’re back together again.  Here we have another gratuitous picture of the front of the rig.

The back of this unit is not nearly as pretty.  JC said, “Fix this mess!”

So we stripped everything out and went back with one channel of the audio chain.

This cable was intermittent and was replaced.  These ends are crimped onto the cable and are not serviceable.  Several layers of black duct tape attests to the fact that I am not the first person to come to the conclusion that this cable had a problem.  Hint: Why not try fixing the problem right when you find it, next time?

This is all the AC wiring that did NOT go back into this rig.

The AC cables were shortened and these rectangular plugs were fitted.  These plugs were chosen because they could fit on the AC outlet strip without blocking any outlets.  The black tie wrap augments the strain relief.

The AC power strip was disassembled so that a longer AC cord could be permanently affixed.

Here is the AC wiring and one channel of the audio cabling.  The other channel was similarly wired and dressed.

One Happy Customer!

Thanks for reading all the way to the bottom!

David Latchaw EE



Why Are The Tubes Falling Out Of My Fender Deluxe Reverb Amp?

Ian of Jealous Creatures found a loose tube rolling around inside his amp. The Unbrokenstring Crew to the rescue!

We need to take a closer look (and find some Guitar Porn along the way!)  Off comes the partial rear cover.

The smaller tubes, all 12AX7s are held in with spring-loaded bayonet tube shields.  There is no hardware whatsoever to keep the tubes larger tubes in their sockets.  Houston, We Have A Problem!

While we’re at it, Ian wanted us to take a look at the RCA sockets that handle the pedal duties.

Now that I’m getting older, I tend to forget where I found things, like cables.  So I labelled them “I” and “O” to correspond to “REVERB INPUT” and “REVERB OUTPUT.”

I need the tubes out so that I can add retainer clamps to the sockets.

Let’s remove the chassis and take a tour of this fine amp.  This is the chassis under the rectifier and output tubes.

Everything is wired point-to-point.  Here’s the phase splitter and the end of the preamp tube string.

Here is the beginning of the preamp tube string.

Here is the front-panel pilot light.

A brass grounding strip is affixed behind the front panel controls.

These are the controls for the vibrato.  Note the black ground wire wound around the cable bundle.

We see the VIBRATO input jacks in the center of this picture.

Volume and tone controls are here.  This brass grounding strip is in the best shape I’ve ever seen compared to other units of this same approximate age.

Here we see the volume control and the NORMAL input jacks.

The power cord, accessory power socket, and ground lift switch.  This is probably a replacement power cord because it uses the international color code scheme.  American color codes are black/white/green.

From left to right, we see the fuse holder, power switches and the output/speaker jacks.

RCA jacks for pedals and reverb tank connections.

And this, ladies and gentlemen, is the end of our chassis tour.  Back to work!

Ian said that the pedals were intermittent.  The connectors were not broken, but I removed them anyway to scrub them down with contact cleaner.  Here, a cotton Q-tip has been resized to scrub the bore of the RCA jack.

The same technique was used to apply a thin coating of Blue Shower protectant and lubricant inside and out.

The sockets were unscrewed and new tube base clips were installed.  These are from the same OEM as supplies Fender and Peavey.  Dunno why they weren’t installed in the first place… maybe CBS was cutting costs?

The RCA and its washer stack goes back home where it belongs.

After tightening the nuts, we’re soldering everything back the way it was.

This should wrap up everything electrical that has to do with the rear panel.

I hit the RCA jacks assigned to the reverb tank using the same process as I used on the pedal RCA jacks.

All the quarter-inch jacks were cleaned and lubricated.

The front panel jacks received the same attention, using the cleaner and lubricant process.

More input jack love.

This is a little better view of what’s actually happening during the cleaning cycle.  Note all the gunk on the Q-tip.

This beautiful Fender-branded 5U4G rectifier is the first tube to slip into the tube base clips.

Your turn.  This is a Ruby-branded tube.

This is a JJ/Tesla tube.  We’re ready for final test.

Wow, this thing is loud!

Thanks for reading all the way to the bottom.  With all the Guitar Porn, this is the longest post yet.  I hope you enjoyed reading today’s post as much as I enjoyed working on this unit and documenting this little trip down Amplifier Avenue!

David Latchaw EE


New Seymour Duncan Pickups for an ESP LTD

Experimenting with various pieces of gear is one aspect of the guitar hobby that appeals to most players.  This Indonesian-built ESP would benefit from some after-market modifications.  Here We Go!

This is not a bad guitar, and is surprisingly easy to play despite it’s flying-Vee shape.

What the customer wants, the customer gets!  These will fit right in to the guitar without any modification.

As you can see, this guitar is strung through the body.  If you look closely, you can see that the string saddles are all going the same direction.  From a practical point of view, this doesn’t make much difference as long as the intonation can be set properly, but many an Internet flame war has been waged over the ‘correct’ saddle orientation.  Spare me.

These are the original factory pickups.  They work fine, and will be saved and returned to the customer.

The main work will happen underneath this cover.  All of the wiring is terminated here.

The original electronics are just fine.  I’ll cut the tie-wrap and sort out where the existing pickups are wired.

Both pickups are wired directly to the selector switch.  The red cable is the neck pickup.

Note that the trim screws are different length.  Knowing which screw goes where will be important when putting everything back together.

We are liberating the the neck pickup and trim ring.  Note the red insulation.

The cable to the bridge pickup is covered in black insulation.  Note the common passage that both cables use on their journey to the wiring cavity, accessible from the back.

Houston, we are clear the launch tower.

The original pickups were stored in the same container in which the new pickups were delivered.  And, because I’m forgetful, I made a note about which one was which.

I clipped the wires in order to remove the old pickups.  The solder sucker is clearing away the terminal where the neck pickup was wired.

Now we’re cleaning up where the new bridge pickup will be soldered.

All the shields were wired together and attached to the body of the selector switch.  Here, I’m cleaning this up so that a new solder connection to the shielding braid can be made.

With all the hardware off, this might be a real good time to clean everything up, don’t you think?

That’s better!  Oops, I did it again…  Guitar Porn!

The Seymour Duncan pickups were wired for split coil operation, but we’re not using that option today.  Thus, the two halves are soldered together, as shown here.

Heat shrinkable tubing insulates and protects the splice.

The original pickup trim rings are cleaned up and polished.

Can you guess which one is the neck pickup trim ring and which one is the bridge pickup trim ring?

With a little patience, the new pickups are integrated into the trim rings using the springs and screws provided.

Here, I’ve snaked the neck pickup cable through the body of the guitar.

And as we saw earlier, the cable passes through the bridge pickup route and onward into the electronics cavity.

I’ll use a couple of the trim ring screws to temporarily keep things in place.

Same with the bridge pickup.

The new cables were soldered where the old pickup cables were originally installed, and a fresh tie-wrap will keep things under control.  We’re done here.

Now that the guitar is closed up, it’s time for a little Fret Love.

We’re all set up and ready to play!

Jacob does a final test on the electronics and guitar action.  He was pleasantly surprised that such a radical-looking guitar could be so easy and fun to play!  Good Job, Unbroken String Crew!


Thanks for reading all the way to the bottom.  More Guitar Porn to come!

David Latchaw  EE