Solid State Amps

Peavey Mark III Bass Head – Busted Controls

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These sturdy old bass heads turn up in pawn shops and Craigslist from time to time. They soldier on for years, sometimes making music for decades. This unit came in for some minor repairs and a Million Mile Checkup. Let’s get to work!
This unit has LEDs for the power indicator and a clipping indicator, or something called “Compression.”

 

From the school of ‘crank it up and rip the knobs off’ we have a knob that has been ripped off.

 

Most of these Peavey heads usually consist of a preamp, mixer, and/or EQ assembly behind the front panel, and a rear panel that holds an amplifier and power supply.  The transformer is bolted to the case in the middle.

 

We need to get to the circuit board, so all the knobs and nuts come off.

 

This unit was built in the era of the Plastic Potentiometer Shafts.  Grrr…

 

A few screws keep the panel and circuit board flat.

 

More screws go into the magnetic parts holder.  We are almost there.

 

Here, at last, is the circuit board.

 

Potentiometers that stand off the circuit board like this are sometimes called ‘spider’ pots.

 

These other controls are fine.  The values 10K and 50K refer the resistance, and the letter ‘B’ implies that the taper is linear.  An ‘A’ letter implies an audio taper control.

 

The Alps company made these in Brasil.

 

Here is the new replacement part, with a metal shaft!  If this were my unit, I’d replace them all with metal shafts.

 

The plastic shaft broke off inside this knob.  A few minutes with an Exacto knife is all it took to reclaim the knob and cut my thumb..

 

The new control is soldered into the circuit board.  The factory workmanship on this assembly is pretty good.

 

Back together it goes.

 

With the knobs in place, you can’t tell that anyone has been here.

 

One last look as we reassemble the unit…

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey Bandit Combo Amp Refurb

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David found this fine old Peavey combo amp in a pawn shop for almost next to nothing. Could the Unbrokenstring Crew resurrect it?
This saga begins on a cool Saturday morning in the Guitar Center Pasadena parking lot.  If you look closely, you can see right through this unit.  Literally.  What are we getting ourselves into this time?

 

Electrically, this unit is mute.  No sound comes from it.  But, as the story is told, this amp languished in the barn for years so we have to expect the worst.

 

Name, rank, and serial number, please.

 

This part number goes to a Peavey Spider loudspeaker.  This is gonna be fun, as you shall soon see!

 

The grille is off.  This poor thing is a mess.

 

If a rat gets hungry enough, it will eat anything.

 

Perhaps it was not tasty enough, as the rats left plenty of bits of uneaten loudspeaker cone behind.

 

The brunt of the rat nasty-ness was taken by the reverb tank cover.  We need to clean this up right away!

 

The spring reverb tank is in perfect condition.

 

The tank cover did its job and kept this reverb tank in a pristine condition.

 

Pledge furniture polish contains solvents and emulsifiers that are excellent for cleaning Tolex.

 

The nice lemon-y scent is just the thing to counteract the barn smell.

 

The bottom of the unit is a mess, though.

 

The amplifier electronics are on the bench.  The unit is upside down.  The reverb tank connector is to the left.

 

The bottom cover comes off.  We need to go over this unit with a close eye for damage due to humidity and dust.

 

Perhaps moisture (rat urine? mouse-ture?) has seeped under the solder mask and attacked the copper on the circuit board.  Everything gets a bath.

 

The top side of the electronics is pretty filthy, as we might expect.

 

The circuit board is free of the chassis.  Everything is being scrubbed with water, alcohol, and compressed air.  The switches, jacks, and controls are cleaned, flushed, and lubricated.

 

Tracing signals, this guy is bad.  This IC is replaced from stock.

 

I want to clean up around the cases of the transistors.  The heat sink is at zero volts while the transistor case is at +70 volts or so.  The dirt may bridge across the insulator.  Not on my watch!

 

These power transistors are just fine.  They are also super-rare.  Luckily, I won’t have to replace these.

 

But they sure do clean up good!

 

The transistor on the left is already installed.

 

We use Wakefield heat sink grease on power devices at the Unbrokenstring Shop.  Old Skool.

 

The original mica insulator is reused.

 

This is actually quite messy, but is a thing of beauty when I do it.

 

Both sides of the washer are coated with compound.

 

The whole stack is installed as shown.

 

While we are here, some of the solder joints could use some attention.

 

This amp is now delivering 75 watts into 8 ohms.  This chassis is Good To Go!

 

Back in the case it goes.

 

These jacks are on the rear of the unit.

 

The knobs press on.

 

This guy goes in the trash.  Wait?  Aren’t we going to recone it?

 

No.  We don’t have to recone.  Peavey Spiders are available without the magnet.  No tricky alignment issues here!

 

When the magnet was removed from the old loudspeaker, the magnetic gap was cleaned and then sealed shut with tape.

 

Once the old magnet is aligned, three screws hold it in place.  We’re done!

 

This is the new loudspeaker, seen from the front.

 

A rear view shows us the cleaned cabinet and reverb tank cover.

 

The loudspeaker looks new, because it is new.

 

The grille was cleaned up and is seen here, reinstalled.

 

The refurbed chassis slides in from the rear.

 

These washers go underneath the trim plates thru which the chassis screws extend.  Don’t leave these out!

 

These big trim pieces are metal, coated black.

 

And these, boys and girls, are how the chassis is suspended inside the cabinet.

 

Installing this trim piece is all that is left to do.

 

David is really tickled with his like-new Peavey Bandit!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

 

Marshall AVT50X Combo Amp Has Issues

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The customer was headed to the recording studio with his favorite amp.  However, our patient has two issues. When the unit operates, the fan is noisy and the output signal comes and goes when you tap around the headphone jack. When you don’t hear the fan anymore, the whole thing has quit. Can the Unbrokenstring Crew make this work AND make the fan quiet at the same time?

This hybrid combo amp has a lot going for it. Besides being Marshall loud, the unit is light-weight and has some moderately cool features such as a lead and rhythm channel, solid state reverb, a 12AX7 preamp tube for that tube sound, a headphone output, and a CD input so that the aspiring bedroom rock star can play along with their favorite real rock star.

 

An unexpected bit of excitement is, this is actually a Bletchley-built unit, factory-built for export to the United States.

 

Removing the chassis from the cabinet is straightforward.  Here is a view from behind the front panel.  The power switch is to the left.  The headphone jack and the CD input jack are in the center.

 

The front panel controls are those green vertically-mount potentiometers.  The input jack is on the right.

 

Looking inside the rear panel, we see the IEC power jack.  The power transformer and heat sink dominate the center of the chassis.

The rear panel jacks on the rear panel have their own circuit board.

 

We need to remove the main circuit board, as The Unbrokenstring Crew has quickly identified many intermittent solder joints that have caused this unit to quit.  We are going to be giving the soldering iron a workout today.

 

These pictures that follow document where the cables plug in.  All of the connectors are polarized.

 

The blue wires go to the final amplifier, which is a high power integrated circuit.  Power for the amp is carried by the white wires.

 

These cables carry the output signal to the rear jacks.

 

More cables.  The white wires are the AC mains cables.

 

These cables go to and from the power transformer.

 

The knobs and now pulled and the retaining nuts are removed from everything.

 

The main circuit board is free!

 

Now that all those knobs and jacks are out of the way, we can give the front panel a good cleaning.

 

The Gibson Guitar Pump Polish does a good job of removing grime and fingerprints.  And it smells nice!

 

The headphone jack has a wide circuit board footprint.  Here is the replacement jack compared against the output jacks.

 

The original shield from the bad headphone jack is moved onto the new part.  We’re changing the headphone jack because the preamp output signal goes here and either passes thru to the output amplifier, or goes to headphones.  When NOT using headphones, the signal to the output amplifier was intermittent, because of the dirty switched contacts on the original jack.  These cost less than a dollar so replacement is faster and better than cleaning the original.

 

The headphone jack is 100% now!  Because it’s brand new.

 

Some of the controls needed to be changed out.  The new parts arrived today!

 

Have you ever seen a four-terminal potentiometer?

 

These are all changed out.

 

Next, we need to look at the noisy fan.  This is the power amp / heatsink / fan assembly.

This is an inexpensive 12vdc fan, similar to the ones used in personal computers.

 

Perhaps we can oil this guy and shut him up.

 

Well, that didn’t work.  It’s worn out.  Off it comes.

 

We can salvage the electrical connector and install it on a new fan.

 

The new ‘silent fans’ came as a pair, so this heat sink is going to get two fans.

 

This fan is oriented to move some air across a power resistor on the main board.  Because I’m an Engineer, that’s why.

 

The other fan is oriented the same way as the original fan.  IMHO this makes a little more sense than the original setup.

 

This is the top view of the final setup.

 

When removing cables, a lot more than the cable came loose from the circuit board assembly.  See the hole?

 

This pin used to be soldered to the board through that hole.

 

These pins are tin plate over steel.  They have a larger thermal mass than the other components on the circuit board, and therefore MAY have not been at a high-enough temperature long enough to make a good solder joint.

 

These pins will be re-tinned with tin/lead solder.  The entire circuit board will be reworked to make it NON-lead-free.

 

This stuff is rosin-activated solder flux.  This enables good ‘wetting’ of the tin/lead solder joint.

 

This is a beautiful NON-lead-free solder plate over steel.  The whole amp gets this treatment.

 

Lots of those pins came loose when the cables were removed.

 

Those bad solder joints go a long way in explaining why this amp became intermittent.

 

This amp was manufactured while the world was converting to lead-free solder.  So, the process guys were still learning what worked and what didn’t work.  These didn’t work.

 

All of the solder joints in this amplifier were reworked by removing the tin solder and reflowing with tin/lead solder.  The Unbrokenstring offers this service for those who wish to have the MOST reliable gigging and recording equipment.

 

Lead-free solder is NOT reliable.  Exemptions from the lead-free directive (RoHS) have been issued to automotive, avionic, energy/down-hole, and medical electronics precisely because it has proven to be unreliable.

 

Here is our new headphone jack again.

 

We are ready to assemble the unit.  Now where did I put all those knobs?

 

Four hours of continuous testing proves that this combo amp is in top shape!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Mackie 808S Powered Mixer Blows A Fuse

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This powered mixer lives two lives; During the week, it rides quietly in the back of the sound van; When the weekend comes, it works hard as the sound system on the main stage for outdoor festivals. After years of incredibly reliable service, the unit just stopped. And it already had a date on the main stage at The Lonestar Rally in Galveston for the following weekend.  Could the Unbrokenstring Crew see what happened and bring this unit back to life?

This unit was cold and lifeless on the bench.  But where is the main AC line fuse?  We need to get inside.

 

Mackie is top of the line name brand, nearly ubiquitous is the sound reinforcement industry and in recording studios.

 

Name, rank, and serial number, please.

 

These screws hold the front panel in place.  The lock washers are captive.

 

We note that some of the machine screws are a different length.  This is noted in the notebook.

 

The screws in the back of the enclosure have large fender washers around them.  We note this, too.

 

The front panel is free.  Ah, but we are just getting started.

 

A large ground cable and a smaller white signal cable is seen in this view.  These are taped to the side to get them out of the way.

 

At the other edge of the front panel is this relatively fragile flat cable.  This requires special care as these are easy to damage and pinch.

 

Next, the power section of the unit is removed from the case.

 

This is what it takes to handle 600 watts per channel of audio power.

 

And here at last, at the very back corner, is the AC line fuse.  This fuse has fatigued over the years and finally opened.

 

The actual fusing element inside most cartridge fuses is a soft metal strip, suspended at each end.  They can fatigue and fail open, even under normal use.  I believe that this is what happened here.  The green tool is a real fuse installer.  Because I’m OCD like that.

 

There appear to be no other problems with this unit.  All the screws going back into the right places for testing.

 

 

So we tested the unit with a little Lennie Kravitz.

 

This unit has operated at full power for four hours, as do all of the finished repairs at The Unbrokenstring Shop.  I am deaf.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

 

Fender Rumble 350 Bass Combo Repair and Modification

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I can’t help but think that this is not a real Fender, but “Fender Bass Amplification” is the nameplate on this unit, not the cursive Fender logo seen for decades.  This unit is very versatile, works better than most bass combos, but just screams “Designed and Manufactured In China.”

A lot of ‘motor noise’ came from one of these loudspeakers in this Fender Rumble 350 bass combo amp.  The owner said, while we were at it, could we wire this unit so that it could be used as an external cab, driven by another unit?  The UnbrokenString Crew said, ‘Sure!  Why not?’

 

If you listen closely, you can hear the warped voice coil rubbing against the magnet gap inside the bad loudspeaker.

Access to the inside of the cabinet is accomplished by removing the loudspeakers.  We are using an electric screwdriver to drill a pilot hole for a switching Neutrik connector that will allow this unit to be used as an external cabinet.

This Forstner bit is just the right size to clear the body of the connector.

Using the pilot hole, we can cleanly cut through the Tolex and into the wood cabinet.  The scratches in the Tolex were not part of this project.

Yes, this looks like a hole to me.

We will replace both Chinese loudspeakers with a matching pair of 200 watt ea. bass guitar loudspeakers.  The original loudspeakers were rated for 75 watts each, which is strange considering that they were tied to a 350 watt amplifier.  The bass loudspeakers have a different hole pattern, so we are drilling new locations for the Tee nuts.

These Tee nuts have little barbs that help keep them in place.

I am using this clamp to squeeze the Tee nuts into the drilled holes in the baffle.

 

Now we can begin wiring this unit up.  The black and white pair connect the two loudspeakers in parallel.

The red and black wires come from the power amp in this unit.  This Neutrik connector will disconnect the power amp when an external amp is connected to the cabinet at this port.  The soldering is done outside the unit.  BTW this is a Neutrik NL4MD-V-S.  The mating connector, an NL4FX, was supplied to the customer for his own wiring.

That looks pretty nice, in spite of the marks in the Tolex.

The new loudspeakers are in place and this unit is ready to test!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626