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


Yamaha G240 Classical Guitar Refurb

Donny mentioned that this beautiful 1970s era Yamaha needed a set of strings.  Turns out, it could use a little TLC.  The Unbrokenstring Crew steps up to the plate to bring this classical guitar up to speed!


Intended as a student guitar, these wonderful old Yamaha guitars rival many medium priced guitars found today.


The serial number is hand-stamped on a soundboard brace.


At the time, we considered Made In Taiwan as junk.  Nowadays, that was the Good Stuff!  How times change…


Obviously the nut is unglued, but note that the rosewood fingerboard had been painted black, to give the appearance of ebony.  Such was the snobbery found in the classical guitar world back in the 1970’s.  My first classical guitar sported a black painted fingerboard, and it looked a lot like this after years of playing.  Nothing to be ashamed of nowadays.  In another fifty years, we’ll be playing guitars with synthetic Richlite fingerboards, and consider rosewood fingerboards as ‘high end’ and collectible.


This diamond file is removing the old glue and truing up the gluing surface on the neck.


The diamond file is working on the end grain of the fingerboard.  That little cavity under the nut is not for a truss rod, but rather appears to be a slot for a stiffener of some sort.  Classical guitars seldom use truss rods to counteract string tension, as nylon string tension is about a third of the tension created by steel strings.


Any glue remaining on the nut itself comes off using the mill file.


Some alcohol on a rag cleans off residue on the nut.


Time to clear out the old DNA and tune up the string slots in the nut.


This is a serrated wire used to clean orifices on cutting torch tips.  They are often re-sold for many times the price by companies servicing the luthier craft.  Just get yours from the car parts store or a good tool store.


The tip cleaners come in many sizes, so just use a micrometer to select the correct size for your application.  Here, I’ve moved to the B string.  The rest of the slots are clean and smooth, so I’m done here once this slot is smooth.


Looks good!  We’re ready for some hide glue.  But how do I clamp it down?


How about using a couple of the old strings, brought up to tension, to keep the nut in place for a day or so?


This guitar was missing the saddle.  Here, I’m shaping a piece of Vietnamese water buffalo bone on the belt sander.


The water buffalo bone is incredibly hard, a natural material, and a renewable resource as opposed to ivory.  Some of the new synthetics are good, but I have this in stock and I love grossing out the other Unbrokenstring Crew members.  Bone? Dead animals?  Yuck!


The Yamaha saddle is about 0.100 inch thick.  These blanks are about 0.140 inch thick to start.  After a few minutes, I’m down to 0.137 inch.


We are at 0.131 inch.


Down to 0.121 inch.


Can you read this one?  0.115 inch.


Almost there!  We’re at 0.103 inch thick.


As George Bush would say, we have destinated!


The trial-fit part of our program is complete.  Note that the saddle is a little long.  There is a method in my madness here.  Let me introduce you to a real luthier who can explain what I’m trying to accomplishing here:

Q: Why do the British like Lucas refrigerators?

A: Because they like warm beer.

Seriously, the last thing I want to use on a fine musical instrument is lubricant that oxidizes and turns into gum.  Synthetic lubricants don’t oxidize, and so make a better choice when metal-to-metal lubrication is needed.  Such as in tuning machines.  This Lucas product is thick enough to stay in place when assembling engines.  This lubricant will stay in place on a musical instrument.


I need to install some strings in order to set the action at the twelfth fret.  Before I do much more with these tuning machines, I will lubricate them and set the gear mesh so that the guitar will stay in tune yet the tuners will run smoothly.


I re-used a couple of the old strings to accomplish the string height setup.  One of them has a ball end!


Stupid Me.  The old strings were worn and did not have a constant cross section.  Thus, I got weird, inconsistent readings.  I’ll use a new string.


Same with the high E string.  A new string is pressed into service in order to complete the setup.


The top edge of the saddle will be rounded off in order to remove any sharp edges that may cut the strings.  A rounded top edge also allows a single point to support the string, resulting in better sustain.


Now that we have the saddle to the proper dimensions, every surface will be polished with a high speed buffer.


The bottom edge of the saddle blank gets special attention.  Here the blank is set level with the jaws of the vise.  The jaws will act as a guide to keep everything straight and true.  A polished, flat surface against the bridge will permit the best transfer of energy from the strings to the soundboard.  The variation in color in this piece of bone is due to the changes in orientation of the grain of the calcium in the bone.  This gives an almost opalescent appearance to the saddle.


The top of the blank is polished, yielding a hard surface to support the strings.  Note that this end of the blank is curved.  This marks the orientation of the saddle; the rounded end points in the direction of string 1 (high E.)


The faces of the saddle are polished as well, mainly for appearance.  Note again the presence of the calcium grain.


Time to string it up.  Here you can see the wedge shape of the saddle, which allows the user to set the action height.


All laced up!  The free end of the adjacent string is secured in the loop of the next string over.  I’ve never had to retie a string or tie an additional knot when tying the string ends off in this manner.


She plays beautifully and the intonation is perfect!

Thanks for reading all the way to the end!

David Latchaw  EE


Fender Princeton 112Plus Jacked-up Input Jacks

Worn input jacks are the bane of players and the bread and butter of amp technicians everywhere.  The Unbrokenstring Crew is ready to Slay the Bane and get this wonderful unit back on the air!


Most commercial equipment manufacturers sell their gear world-wide.  The European Union (and other areas of the world) have stringent requirements regarding electrical isolation of the consumer and any electrical parts of the equipment, including the grounded chassis.  Thus, synthetic insulative materials are pressed into service for input jacks, although the longevity of that material is nowhere near the longevity of steel.  Such is the world we live in.


We’re taking a quick tour of the rest of the front panel.


These jacks aren’t in much better condition.  The synthetic material just degrades over time.


We’re documenting some connections here, so that we can disconnect them to remove the amplifier and then get them properly connected during reassembly.  The reverb tank lives in the bottom of the amplifier cabinet.


More documentation, and a little Tech Porn!  Loudspeaker connections are shown here.


The circuit board is typical of Fender gear from this era.


These screws hold the output module to the heat sink and thence to the aluminum cabinet of the amplifier.


Oops, I did it again!  A little tech porn, this time the high voltage fuse which protects the high voltage transformer.


I believe that this is the date when this amp was built.


With the circuit board free from the chassis, we have our objective in sight!


Here is a quick view of the other front panel jacks.


Here are our new jacks.  Next, all the nuts holding the controls will be reinstalled.  I have spared you the details of replacing the jacks and the soldering and all that.  If you want to see that process up close and personal, check out the Gallien-Kreuger jack repair in an earlier blog posting.

Here’s Jacob doing a checkout:


This unit is 100% electrically.  New knobs are on order, but I’ve returned this amp to the owner as-is.  Later, I made a trip to his place to install the knobs and audition the amp.  As it turns out, he is quite the Jazz Cat and is really enjoying this amp.

Thanks for checking out my blog post!

David Latchaw EE