Intermittent Ampeg SVT-200T Bass Head Problem

Billy complained that, while he was rockin’ steady with his bass line, the sound from this bass head would come and go.  Other people had looked at it and didn’t fix it.  Could the Unbrokenstring Crew look at it?  You Betcha!!

Let’s take a quick look around before we dive into this project.  The rear of the unit is dominated by a huge aluminum heat sink.  To one side is the output panel, which shows the considerable versatility this amp offers.

On the other side of the heat sink is this panel, handling the power in and out duties.

WARNING – TECH PORN  This interior view shows the power transformer in the lower left corner, the solid state full-wave bridge rectifier, filter caps, drive circuitry, and output power transistors.  Along the top is the circuit board for the front panel controls.

This view gives a more complete view of the heat sink and power transistors.  This layout is very clean and functional!

We quickly identified an intermittent internal switching contact in one of the input jacks.  Here, we are removing the knobs and nuts from the front panel controls in order to better access all the jacks and clean them.  If you look closely, you will see a piece of red felt glued to the face of the socket.  This felt keeps the socket from scratching the front panel.

From here, you can easily see the switching contacts on the input jacks.  The dimpled leaf touches the contact whenever no plug is inserted in the jack.  When a plug is inserted, the dimpled leaf is displaced away from the contact.  More often than not, a signal necessary for the proper operation of the circuit is routed through this contact.  The surface of the metal that makes this contact can easily be contaminated with dust or cigarette smoke and can become intermittent.  The jack closest to the camera was identified as intermittent during testing.  All the jacks in this unit will be cleaned, as they are all the same age and have been in all the same places as the one which is acting up.

The circuit board is inverted so that the liquid from the TechSpray can will run down into the contacts that are causing the problem.  The rag is a vain attempt to limit the size of the mess, but it does a pretty good job of catching the gunk flowing out of each jack.

More nasty proof of the validity of the diagnosis.  But if you compare how bright and shiny the metal is on the switches of the jacks to the earlier pictures, you can say that we are making progress against the Forces of Dirt!

I cut a strip of white typing paper (that’s copy machine paper for those of you born after 1980) that is just the same width as the switched contacts.  Each contact was cleaned and burnished by pulling the paper through the contact mating surfaces.  Most reliable switches and relay contacts meet with a ‘wiping’ motion that helps keep them clean.  These contacts just meet and separate when plugs are inserted and removed, and do not experience the same wiping motion seen in switches and relay contacts.  When I become king, I will have all these jacks redesigned, and I will have only clean, dry air circulated around expensive electronics.

Now we reassemble the head so that it can be tested.  Here is another one of my sockets with the red felt on the open end.  I learned this trick at the Space Center as a strategy to protect aluminum anodized Orbiter components.

We’re ready to retest on the bench.  Both input jacks, effect loops jacks, and output switching jacks will be tested.  After an hour at full power, the heat sink barely gets warm.

I wanted to show some detail of how the cabinet goes together.  This is a fabric-covered insert which goes in the front face of the amp head, beneath the control panel.  It is not held in place with screws at all, as you will see.

This panel is held in place with extreme super extra-heavy duty, industrial/commercial grade Velcro hook and loop fasteners.  You can see here that the loop fasteners are held to the filler panel with a screw.  Yes, it’s that strong!

I also want to document a literal ‘snag’ for you, in case you ever work on one of these.  See these screws that hold the carrying handle in place?  They do an excellent job of holding the handle in place.  However, they keep the internal chassis from sliding in and out of the cabinet freely.

Here’s why.  They are just long enough to stick down inside the cabinet.  When attempting to remove or install the chassis from the cabinet, be sure to back the two handle screws out a couple of turns so that they do not snag the chassis.

Here is that Velcro in action.

All done!  Billy intended to sell this amp after the Unbrokenstring crew repaired it, but he likes it so much that he will keep it instead.  This head sounds AWESOME with his six string bass.  This is a very versatile unit with a straight-ahead solid state sound.  And you can still buy them new!

Thanks for reading all the way to the bottom!

CONTACT – David Latchaw  EE