Ampeg SVT6Pro Bass Amp Repair

This Ampeg hybrid bass amp became intermittent during an extended gig.  The unit was ready for a million-mile checkup anyway, so it was time for this guy to visit the Unbrokenstring workbench!

Externally, this unit is in gorgeous shape!  Let’s look inside.

Swollen filter capacitors are never a good sign.

I’m cutting the hot glue away from the bodies of the capacitors so I can remove them.

The circuit board assembly needs to come out of the chassis. I printed these labels to mark the cables as I remove them.

I’m documenting where the cables go with pictures.  And you get a nice look around the inside of the amp!

More cables.

Yet more cables!  Note the fuse holder that can accommodate two different fuse sizes.

The various screws holding the chassis together are different lengths.  So, I’m documenting THAT with more pics.

Same view, different place, different screw length.  This stuff is important, you know.

The AC power port gets its own short screws.

Now, the circuit board is out where we can work on it.

These spacers belong between the sheet metal housing and the various rear panel jacks.

St. Louis Music (SLM) built a lot of stuff for major American brands.  What year was this printed circuit board fabricated?

I like to mark the solder joints to be desoldered with a little liquid rosin flux.  It assures that I desolder the correct joint, and a little extra flux helps the solder to ‘play nice’ during rework.

These are the bulged capacitors.

The replacement caps need to fit on the circuit board footprint.

We probably won’t have a height restriction with any new part, but it never hurts to check.

The replacement caps need to have the same lead spacing as the circuit board.

The site is prepared for the new parts.

These are modern, high-temperature, long-life replacement capacitors.

What’s this?  This rectifier has been very hot.  Those holes are probably for cooling.  However, there has been enough thermal stress (due to the unequal coefficients of expansion between the copper, the fiberglass, and the component itself) to crack the solder joints.

Here is a better view of the cracked solder joints.  Wow!

Whoever was applying the hot glue that day probably over-did it.  There is very little ventilation around these components.

I replaced both rectifiers, leaving a bit of space between the component and the circuit board.  The hot glue has been completely removed, to allow some air circulation.  This fixes everything!

The finished unit looks as it did before we started.  Now, it works as well.

Thanks for reading all the way through!

CONTACT – David Latchaw EE