refurbish

Behringer T1953 Preamp Refurb

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This preamplifier and microphone processor is in the audio chain of every hit produced by Majic$tyle Studios over in Houston’s 3rd Ward.  But it quit one day.  So the music quit.  Could the Unbrokenstring Crew help the Majic$tyle Crew bring back the music?
Like many recording facilities around Houston, the modest studio in the back room has a lot of history.  A lot of talent in the Houston Urban Contemporary and Hip Hop scene have stood in front of this box, layin’ it down.

 

This unit needs an overhaul.  A quick tour of the front panel reveals input level controls for two channels.

 

Phase and tone controls are here.  The tone control can be completely bypassed, which can be useful.

 

These inexpensive meters actually do a pretty good job of displaying dry level and processed (wet) levels for each channel.

 

The ‘warmth’ is another term for the wet signal, routed through a starved-cathode tube circuit.

 

Power input and device info is found on the back.

 

All of the inputs and outputs are on the back.

 

The cabinet screws are a different length.  This is noted.

 

The DC power management and toroidal line transformer are located in the center.

 

All of the rear panel wiring is done with a circuit board, with the wiring harnesses glued to the chassis.

 

We are missing some DC voltages, so our investigation will focus on this circuit board.  What a heat sink!

 

Toroidal transformers are the way to go in high quality audio equipment.

 

Behind the front panel, we see a pair of 12AX7 dual triodes living on this circuit board.  The incandescent light bulbs are burned out, but are for aesthetic purposes only.  We will have some fun with these!

 

The rest of the front panel circuit board is almost entirely encased in sheet metal, as shielding.

 

I documented these cables, in case I needed to disturb them.

 

Can you see the bulged electrolytic capacitor?

 

Hot glue is used to keep all the connectors connected and all the big parts from moving around.

 

With the wiring harnesses removed, this circuit board is held in place by screws that fasten the power semiconductors to the heat sink.  When those screws are removed, the circuit board assembly can be repaired.

 

Remember the bulged capacitor?  It leaked out the bottom.

 

This power supply will get a full cap job.

 

The electrolyte from the leaking capacitor has chewed away some metal from the circuit board and a couple of the solder joints.  This will need to be taken into consideration when the new parts are installed.

 

The blue jumper wire restores continuity on the corroded trace seen in the previous picture.

 

New caps are installed!

 

Here is another view of the new skyline of our power supply board.

 

Heat sink grease is used to insure thermal transfer from the power semiconductors to the aluminum heat sink.  This tube of compound will probably live longer than I will.

 

The tabs of the power semiconductors SHALL BE isolated from the heat sink.  Here, an ohm meter checks for isolation.

 

Those incandescent light bulbs are not part of the signal chain, but just serve as a back light behind the vacuum tubes.

 

These are 24v bulbs.  I could just replace them, but why would I pass up a little fun with just a stock replacement part?

 

The tip of the base of the bulb is actually a specially shaped bead of tin/lead solder, which holds one of the terminals from the filament of the light bulb.  So, I desoldered it.

 

One of the leads from the filament goes to the side of the metal base of the bulb.  So I unsoldered as well.

 

This is a better view of the base, showing the central terminal.

 

The glass bulb comes out.  Besides the broken glass, you can see bits of glue on the rag.

 

The plan is to add LED lights in place of the bulb.  This is the series limiting resistor for 24 volt service.

 

It hides inside the base, as shown.

 

These red LEDs are built on top of the resistor and are supported by the bulb base.  This is gonna be cool.

 

The meters are also internally illuminated.  These meters will get yellow LEDs.

 

The front panel will get a million-mile cleanup and polish.  Off comes the knobs.

 

These bushings keep all the knobs turning smoothly.

 

Now that the front panel is free, it can be cleaned and polished with Gibson Guitar Polish.

 

Removing the knobs is the only way to get the grime out from down around the knob shafts.

 

The clear windows are polished on both sides.

 

Behind the aluminum panel, we have rotary encoders for the wet level control duties.

 

All the low level audio is contained on this double-sided circuit board.  Any copper that isn’t signal is audio ground.

 

The switches are cleaned and lubricated.

 

With the front panel removed, this chassis is just floating.

 

Let’s take a quick peek underneath the audio shield.

 

Audio gain duties are handled by TL074 opamps.

 

Here are the microphone preamp chips.  These are surface-mount JRC4580s.

 

Here is the other mic preamp 4580.  OK, we’ve done our gut shots for the day.

 

Now let’s get these meters done.  Hot glue is everywhere.

 

Once these meters are removed from the front panel, they can be easily serviced.

 

The lens and front bezel come right off.

 

So far, we haven’t destroyed anything.

 

The meter scale just lifts out of the body of the meter.

 

Here is where we are so far, keepin’ it real.

 

If we peer inside the body of the meter, we can see a ‘grain of wheat’ lamp.  These are all burned out.

 

I will attempt to reuse the leads to each of the bulbs.  The burned out bulb is removed.

 

To get a little bit more room to move around, I’ve removed the meter movement.  Be very careful!

 

The meter bulbs are 12 volt units, and two of them are in series because only 24 volts is available.  This is the LED current-limiting resistor for one of the meter illuminating LEDs.

 

I purchased a bunch of these 1% resistors years ago from Texas Instruments (TI) and so these are probably collectors items.  Why not use them on this project?

 

Here is the yellow LED and resistor installed inside the meter body.

 

Now the meter face is reinstalled.

 

The clear lens is popped out of the bezel for cleaning and polishing.

 

Everything is reassembled and the meter movement is zeroed.  Ahhh, that’s done!

 

This shield was loose under the circuit board assembly that held the tube sockets.  What’s up with that?

 

Turns out, these standoffs were unscrewing from the bottom.  More LokTite, please.

 

This is going to take three hands.

 

Unfortunately, I only have two hands.  But The Unbrokenstring Crew can make it happen!

 

Let’s reinstall the front panel and do a final assembly.

 

The meters are gently torqued onto the panel.

 

The knobs and bushings are restored.  This is starting to look nice!

 

The red and yellow LEDs are doing their job.  Unfortunately, the .jpg engine in my phone camera is making the red LEDs behind the vacuum tubes look a little gaudy.  Oh, well.  More importantly, this mic preamp works well and is DEAD SILENT with no input signals.  I think the rebuild went pretty well!

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

Peavey 1810 Bass Enclosure Crossover Doesn’t (Crossover)

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Most of what you see in this post works well. However, a crossover network inside the speaker cabinet doesn’t work at all. Could the Unbrokenstring Crew sort out a solution that would put this cabinet back to work?
Here is some file old hardware from the 1980s.  This cabinet has an eighteen inch subwoofer underneath two ten inch loudspeakers.

 

Behind this panel is found a crossover network that routes the input signal to the proper destinations.  The crossover automatically routes the really low frequency stuff to the 18 inch loudspeaker and the rest is routed to the ten inch loudspeakers.  Or, you can specify which signal goes to which driver using the BI-AMP jacks.

 

The sheet metal screws hold this circuit board in place.  An inductor plugs into the pins in the center of the picture.

 

The input jacks are seen in this view.  The capacitor is part of the crossover network, and has been replaced.  As this is in a high powered audio network, the electrolytic capacitor is a non-polarized variety.

 

We can examine the circuit side of the printed circuit board to figure out the schematic for this assembly.

 

This inductor is cooked.  Unfortunately, this part is no longer available.

 

An examination of the inductor may give us some clues that we could possibly use to fix it and use it again.

 

This component can take the place of three different inductors, thus the three wires.  Any two wires yield a different inductance.  Unfortunately, the insulation is thoroughly cooked.  No salvaging this guy.

 

The functions of each ‘net’ on this circuit board is labelled with a felt tip marker.

 

For this particular model of crossover, the inductor wires we need to use are indicated by this inductor symbol.

 

This is an inexpensive crossover kit, with similar specifications.  We can harvest this inductor for use in the crossover.

 

Here are a few details, listed on the end of the box.

 

This inductor is has a laminated bar core.

 

We can remove this inductor from the circuit board and use it in the Peavey circuit.

 

The back side of the donor network is covered with some self-adhesive foam rubber.

 

The foam sticks really well!  But now we have access to the solder joints that need to be unsoldered.

 

The mechanical mounting scheme is VERY robust for this heavy part.  We can use all of this in the Peavey circuit.

 

These white plastic caps are handy to hold the mounting nut and to insulate the exposed iron core.

 

We will use the original circuit board as a template for finding and marking places to drill new mounting holes in the Peavey circuit board.  We can search around for a practical mounting location.

 

The mounting holes are marked.  Away We Go!

 

One end of the inductor is electrically wired here.

 

The other end of the inductor is wired here.

 

We will make lock the threads of the mounting screws with this stuff.

 

The thread locking compound is thin enough to seep into the threaded fasteners and lock them.

 

Now, we can reassemble the crossover network assembly.

 

The original inductor was mounted in the foreground.  This doesn’t look to tacky, does it?

 

One last look before it disappears into the enclosure.

 

The panel is ready for reassembly into the enclosure.

 

No.  Wait.  We need a new gasket between the I/O panel and the cabinet enclosure.  This self-adhesive foam strip material is just the ticket for this application.  Note the mitered corners.  Because I’m OCD like that.

 

Holes for the mounting screws are cleared with the Exacto knife.

 

OK, NOW is one last look at this assembly.  The wire pairs go to the loudspeakers in the cabinet.

 

This system is pretty awesome.  Everything tests out at full power.  Life Is Good!

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