Peavey - Unbroken String

Peavey MKIII Bass Head Needs Un-Smoked

December 23, 2018October 29, 2018 by latchaw

This solid state Peavey Bass head is also capable of handling mixer, equalization, and preamp roles for public address, monitor, and other sound reinforcement roles. But power amp quit! Can the Unbrokenstring Crew help?

The AutoMix function that Peavey developed has been discussed elsewhere in the blog. Lots of EQ knobs here!

Note the graphical EQ and bi-amp capability.

Woah! An Instrument System! Ooh. Aah.

On the rear panel is the power switch and speaker connections.

Peavey mixes and matches front panels (inputs, preamp, eq) and rear panels (ps, power amp) to build different heads.

As is the case with many pieces of electronics, the City of Los Angeles Fire Department approves this unit!

Name, rank, and serial number, please.

The blue circuit board at the top is for connections to the power transistors. The I/O connections are to the left and the power supply filter caps are seen here.

More blue boards at the top, for power transistors. Driver transistors are found on the square heat sinks. Do you see the problem yet?

This circuitry is all preamp and tone circuitry.

This sucker got HOT!

The worst damage was to components that were near the root cause. They burned because the transistor on the aluminum heat sink suffered an internal short circuit.

The heat of the electrical fault was high enough to melt solder, which happens around 650 deg. F.

A matched set of driver transistors were installed and the circuit board cleaned. The destroyed components to the right have been removed and will be replaced.

The new parts are mounted just above the circuit board. We can get flame-proof resistors now, unlike when the unit was built with in the 1970s.

More collateral damage was found on one of the blue boards. This solder trace acted as a fuse at its narrowest point.

The circuit board is now cleaned up and the gap is bridged with a bit of 16awg solid copper wire.

Some of the power transistors were shorted as well, so all of them are now replaced with a matched set of eight.

These parts are still made by ON Semiconductor, the heir apparent to the Motorola semiconductor product line.

The electronics are back together. The filter capacitors are original, but are still in great shape, so they remain in service.

And, of course, after all the components and circuit boards are in flames, the fuse finally does its job. Of course.

With a new fuse, the electronics are connected again and initial tests begin!

This unit is back on the air! This unit is almost hifi sound quality, with endless bottom end. Good Job Peavey!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey MX Combo Amp Rescued from a Bad Amp Tech

December 23, 2018May 14, 2018 by latchaw

This combo amp had lived a hard life and had finally quit. Grandpa wanted his grandson to get the amp fixed so that they could jam together again. Could The Unbrokenstring Crew bring this unit back to life?
We begin with a quick tour of the rear panel. The ground switch is a tip of the hat to the Old Days of two wire AC.

The foot switch plugs in where the REMOTE SWITCH jack is coming loose. This gets fixed.

I’m surprised that this hadn’t ripped loose. The whole connector wil be replaced.

Name, Rank, and Serial Number, please!

What have we here? We found grandpa’s stash.

Let’s get this line cord wired correctly. Do you know what’s wrong?

The black wire goes under the brass screw. “Black on brass will save you ass.” You’re welcome.

The reverb tank connects to the main circuit board with this connector.

Even after all this time, the high voltage capacitors are still charged. Woah! This is my discharge wand at work.

Our first mystery… where does this nut go?

This is a fuse. No, you think that it is a piece of 16AWG wire, but it is a fuse. Or, it is where a fuse goes.

And here, someone was tired of the fuses falling out of the holders, or what was left of the holders.

The heat from the flow of current has wreaked havoc on this solder joint.

This probably smelled bad when it was hot.

Now that the introductions are out of the way, we need to start replacing this nonsense.

These are commercial fuse holders. These will replace all of the preceding nonsense.

The plan will be to install these new fuse holders at a spot in the circuit where they will be functional, yet out of the way.

The new fuse holders are held down with a screw. This hole is where the screw goes. Here goes!

Another hole is drilled for another fuse holder.

This hole is in the center of a trace. We won’t miss that copper. Much.

Insulating nylon nuts and bolts are used to keep the new fuse holders in place.

The traces in the burned circuit boards are replaced with this Teflon-covered wire.

Everything is now stuffed back into place. Not too shabby, if I do say myself.

Turning our attention to the rear panel, your sharp eyes may recognize this connector as a MIDI female panel connector.

To keep the connector hardware in one place, some of this Thread Locker is all we need.

We have the original foot switch. It needs a new cable, with a connector to match what we just installed in the amp.

This MIDI cable will be repurposed to replace the cable on the footswitch assembly.

We don’t need this connector. Instead, this end of the cable will be wired to the switches themselves.

The new cable is soldered directly to the switches Note the strain relief installed to the right of the picture..

This pedal is ready for action once again!

The amp is reassembled and is ready to go!

The four hour burn-in test is underway. I think we have rescued another vintage Peavey amp!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey 260 Booster Amp Needs a Boost

December 23, 2018May 2, 2018 by latchaw

A local church has a rack of Peavey gear that drives their public address system. This unit had failed. A phone call to The Unbrokenstring Crew was all that was needed to get this unit onto the operating table.
Unlike a traditional guitar amp or stand-alone amplifier, this unit takes high level signals and buffers it to the loudspeakers. They are often used as a means to fill in or expand the coverage of an existing system.

And the inputs are daisy-chain-able.

This is, of course, the volume knob.

Here is a tour of the rear panel. The power switch is ‘center-off’ and, on this unit, does not switch the chassis ground to connect to one side or the other of the AC power line.

No tour of a rear panel is complete without a high-rez pic of the power cord.

And the name/rank/serial number part of our tour.

The AC line fuse was popped.

This is, correctly, a five amp slow blow unit.

Removing the front and rear panels permit access to all the electronics.

The chassis is attached to the rear panel of the unit. Surprisingly, the power transformer is NOT fastened to the wooden cabinet, but is also attached to the rear panel.

These filter capacitors are good and will not be replaced.

This unit has a shorted rectifier. New parts were secured, and are shown here. Although only one rectifier is bad, the other three are the same age and have experienced the same abuse, so they will all be replaced.

The new rectifiers are installed.

Funny thing is, the new fuse looks a whole lot like the old fuse, except it is not all exploded and burned and stuff.

The output from my radio is amplified to a larger signal using my Marshall Stack, to drive the input of the amp at the proper level.

This was the point when the neighbors called the cops to complain of the noise.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey KB4 Keyboard Amp Disintegrating

March 27, 2018 by latchaw

This Peavey keyboard amp works intermittently, but the modular AC outlet has completely come loose from the rear panel. Can the Unbrokenstring Crew fix this?

This unit is pretty cool, with a built-in luggage roller and extendable handle as standard equipment from the factory.

The modular AC plug was covered in RTV rubber. Was this a user ‘fix’ or did it come from the factory this way?

Fortunately, all the electrical conductors were insulated. Otherwise, we would have sparks.

User controls are on the top.

The ground polarity switch is a throw-back to the days of two wire electrical cords.

A simple mixer is integrated into the unit.

Name, rank, and serial number, please.

The panel layout allows for some space for the handle. Good Job!

We removed the head from the cabinet,

Here is our intermittent. This power resistor had broken free from its solder pad.

A circuit board trace had broken. An Exacto knife clears away some of the solder mask to allow for a repair.

This crack was very small, so a good solder jumper is all that is needed here.

This unit appears to have been wet. Do you see the minerals left behind after the water evaporated?

Here is another little blob of mineralization. This may have been from solder flux residue left after the assembly was manufactured. Some fluxes turn white in the presence of water.

This screw was loose inside the grille. This screw holds the loudspeaker in place. This is not good.

Most of the screws were loose. While we have this unit on the bench, we should be sure that the loose screw is not a sign of a more sinister problem lurking with this unit.

Here is the loudspeaker in this unit. Nice!

My guess is, humidity has softened the baffle upon which the loudspeaker is mounted. These Tee nuts will be removed and new holes drilled in the baffle in different locations. The Tee nuts will be reinstalled and we should be Good-To-Go.

Some black nail polish will camouflage the new fastening hardware. You do have black nail polish, don’t you? Doesn’t everybody? Hint: This also makes good thread locker.

OK, now for the IEC power jack. This unit has mounting ears, so we won’t rely on friction or glue to keep it in place.

As an added bonus, this IEC jack has a built-in noise filter.

The hole in the chassis was enlarged to accommodate the new jack. This hand grinder is adequate for the job.

Yep! Just fits.

Now we will bore the steel panel to accommodate the mounting hardware. This will be SO much better than glue!

This doesn’t look too bad, does it?

All of the original wiring goes straight onto the new IEC jack. This is better than factory! Hot glue was apparently used at the factory to secure the switch and the old IEC jack to the rear panel. So that answers that question. Shame on you, Peavey!

Here is one last look of the internals before we reassemble the head. The mixer is at the bottom and the power amp and power supply is at the top.

This unit is literally ready to roll!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey Mark III Bass Head – Busted Controls

February 12, 2018 by latchaw

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.