Ampeg

Ampeg SVT6Pro Bass Amp Repair

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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
281-636-8626

Dropped Ampeg SVTIII Pro Bass Amp Quick Fix

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This Ampeg hybrid amp was dropped.  And of course, it quit working.  And, of course, the guy who borrowed it was the guy who dropped it.  Can the Unbrokenstring Crew get it going again before the friendship gets damaged?

The exterior of the unit is in great shape, because it was in a hard sleeve that absorbed most of the drop.

The handle is now useless.

The rack ears (those aluminum angles) broke away from the sleeve.

The rack ears are pop riveted into the sleeve.  The guy who dropped it said that he would fix all that… just repair the electronics.  Got it!

A quick tour of the front panel, from left to right.  The midrange frequency selector has five discrete positions, which moves the center frequency of the midrange tone control up and down in frequency.

The graphic equalizer is switched in and out by the button below the mute button.  It took me a few seconds to figure that one out.

And here we are at the right end of the front panel.

This panel handles the internal switching and foot switch duties.  The fan and filter are to the left of this picture.

The name, rank and serial number dog tag is here.  Interestingly, this unit has binding posts as an option for speaker connections.  More goodness from St. Louis Music!

The toroidal power transformer dominates the interior view.  The emitter bias resistors rate their own heat sink, in the path of the cooling fan air.

All of the tubes in the preamp were brand name except this one.  Which tested fine and works well in this circuit.

The real issue with this amplifier is that the jolt of the drop unseated this cable connector.  This is easy enough to fix.

This amp is operating at 450 watts into 4 ohms.  I think we fixed it!  And the owner will be none the wiser!

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626