Cool Sunn Practice Combo Amp Blows a Fuse

The fuse blew in this Sunn Stinger 20, but the new fuse blew as well. Then, it quit working for good. Cool Under Fire wanted this combo amp back in action. Could The Unbrokenstring Crew sort it all out?

    

Our patient still has that cool Sunn vibe after all these years, even if it doesn’t work.  That name badge is recognized by all.

    

Getting up close and personal to the front panel, we see an input jack and three tone controls.

 

Independent Gain and Volume controls show that this unit means business!  The mute button and headphone jack give this amp a family-friendly advantage over other inexpensive practice amps.

  

As is found on many guitar amps, the cabinet is sealed with a closed back.

 

Oh no!  The sticker says “DO NOT OPEN.”  What are we going to do?

 

We open it, of course.  How long has it been since you’ve seen a loudspeaker with a square magnet?

 

Obviously this is a four-ohm loudspeaker.

 

The steel chassis has circuit boards for the preamp functions, just behind the controls, and power supply and audio power amplifier at the rear of the unit.  Nothing appears out of order here.  No, wait!  Look here!

 

This wire has come un-crimped from the terminal, seen in the background.

 

We can just open this terminal up a bit, re-insert the wire, and solder it in place.  But the question remains:  Could this have been the reason that the unit blew fuses before it finally quit permanently?  I don’t think so.

 

While we’re waiting on a copy of the Sunn Service Bulletins for this amp to come via email, let’s take a minute to clean this unit up.

 

The controls hold the front edge of the circuit board in place, and a couple of screws hold the back edge steady.

 

The headphone jack is entirely isolated from the chassis of the unit.  Even though the chassis is wired to the green wire safety ground in the AC cord, taking measures such as this makes the UL Certification easier.

 

The input jack is shielded from interference with this metal bracket.  This kind of additional shielding is almost never done on inexpensive amps…  this Sunn is definitely a Cut Above!

 

The switches and controls are easily cleaned now that they are easily accessible, as shown here.  The Unbrokenstring Crew NEVER forces cleaning fluid around the shaft of the potentiometers as a cleaning procedure, because dirt and old lubricant is forced inside the control.  It cannot end well.  Sorry, StewMac.

 

With all the hardware out of the way, it is a trivial matter to clean up the face plate.  Gibson Guitar Pump Polish is pressed into service for this step.

 

Reassembly also involves tightening the woodwork.  Over time, wood shrinks (even in humid South Texas) so most amplifier cabinets will develop buzzes and rattles as they age.

 

Maybe if we work quickly, the “Do Not Remove” police will not catch up to us and put us in jail.

 

The Service Literature arrived!  It states that the next-higher ampacity of fuse should be used in this unit;  There was an error at the factory wherein units in this serial number range had an inadequate fuse installed!  This little amp has run for hours with no issues!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

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