Unbroken String - Guitar, Amp & Music Instruments Repair

Fender Rumble 350 Bass Combo Repair and Modification

December 20, 2017 by latchaw

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

Crate Vintage Club 50 Amp Repair

October 31, 2017 by latchaw

St. Louis Music produced this handsome tube-based Crate amp. Robin said that this unit is ready for some service.

I just love the blonde Tolex and clean appearance of this unit!

The front panel reveals that this is a single input, two channel unit. The clean channel sports conventional treble and bass controls. Bringing the effects loop to the front panel is a nice touch! This jack is stereo, for in/out with one jack.

The gain channel has separate gain and volume controls, and a mid-tone control.

The foot switch is wired in parallel with the channel select switch seen above. Note the separate controls for reverb for each channel!

These loudspeakers are original and still in great shape.

Taking a tour of the bottom of the chassis, this unit has an attached line cord. The power transformer is affixed at an angle, as seen on the left. The grey wires go to and from the reverb tank.

Five minutes of visual inspection is worth thirty minutes of troubleshooting. Can you see the problem?

To remove the chassis, we need to unsolder the cables to the reverb tank. I marked this one so that it can be reinstalled in the proper place on the circuit board.
The loudspeakers are wired in parallel. This pic documents the wiring colors.

The chassis is now free of the cabinet. A quad of EL84 tubes serve as the class AB push-pull output circuit.

The preamp tubes, in the foreground, and all 12AX7s.

Removing the brackets on the output tubes is a little gnarly, though. This screw will be replaced.

Silicone foam cushions the envelope of the tube from the stresses of vibration and thermal expansion.

To do any soldering on the main circuit board, all the controls need to be disassembled. Marshall knobs, anyone?

We have three jacks to disassemble in order to liberate the electronics.

The primary and secondary wiring is easily disconnected from the circuit board, as shown here.

One more chance: Do you see any electrical issues?

We have the main circuit board well in hand.

Some glue was used to support the large mechanical parts. This stuff needs to be chipped away.

Here, we are checking the caps in-circuit with the Heathkit capacitance checker, just to verify that they are toast.

My only criticism of this amp (and others) is that the tube sockets are directly attached to the circuit board. The thermal expansion is more than the average solder joint can stand. Wire those sockets by hand; don’t mount on a PCB!

Here we are with new capacitors. All of the solder joints have been inspected and reworked as required.

With the unit disassembled, this is a good time to clean up and polish that gorgeous front panel.

The wrong sized fuse was installed. The Unbrokenstring has a complete stock of fuses, for such a moment as this.

Remember the broken screw? Here, we are drilling and tapping the chassis for a new screw.

The blue masking tape was deployed on the inside of the chassis to catch all the metal shavings.

The screw fix worked! As we knew it would.

The reverb tank wires are fished back through the hole in the chassis and refastened.

Everything has been reinstalled and we are ready to test.

This cover conceals the bottom of the amplifier chassis. The amp was a little noisy. Could some shielding help?

Aluminum foil tape is added to the cover. This will be electrically in contact with the chassis of the amplifier.

Aluminum shielding goodness at its finest! This is all on the inside, so no one but you will ever know that it’s there.

This Crate is very versatile – the clean channel is almost hifi quality, while the gain channel is a vast territory of sonic goodness to explore. And LOUD! Pick one of these up if you find one.

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey Stereo Chorus 212 Head Stuck In Mono

October 16, 2017 by latchaw

The owner of this amp was so anxious to get it fixed, he removed the chassis from this Peavey Stereo Chorus 212 combo and even helped load it! The amp plays, but only one stereo channel is operational. Can the Unbrokenstring Crew help?

A quick look of the front panel shows the inputs and the normal (clean) channel controls.

Did you catch the missing knob? The Unbrokenstring Crew will find a replacement, which is no small thing because these are no longer available new and are only available from spares or salvaged from a non-working unit.

This amp uses a digital signal processing (DSP) module to make all this audio magic.

Interestingly, the DSP module creates two unique audio signals (stereo) from a single mono input.

The ground lift switch is on the front panel, which is a nice touch from many other designs.

This dual OPAMP is non-functional. However, this is not the root cause of the missing stereo signal.

This is the DSP module in this amplifier. We have only one audio stream as an output from this device.

A piece of cardboard serves an an insulator so that the bottom of the DSP assembly can be probed.

Here is where the DSP processor outputs both audio as a stream of ONEs and ZEROs. We are OK here!

This chip processes the two DSP streams in a manner similar to a successive approximation analog-to-digital converter.

The bit streams are split using an analog switch. Here is the output of the switch assigned to the working channel.

And this is the output of the switch on the non-working channel. Time to change the switch!

Desoldering today is performed with a traditional iron, rosin flux, and copper braid. This keeps solder balls to a minimum.

The circuit board is cleared of excess solder and flux.

The new MUX chip is available through the usual sources.

With the tweezers, the new MUX chip is oriented properly on the circuit board pads. See the tweezers?

After soldering, the workmanship is inspected with this hand-held microscope. The high-powered white illuminator is on the left, and the black cylinder next to the big IC chips is the optical microscope itself.

Any remaining solder flux is removed with alcohol. The rag gives the dissolved mess somewhere to go.

Both channels are operating at full power. The knob is coming from an eBay auction. When it gets here this head is ready to return to the eager customer!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Bugera V22 Infinium Combo Amp Repair

September 28, 2017 by latchaw

This beautiful combo amp excels in all categories except one… no audio out. Could the Unbrokenstring Crew help?

Taking a walk-around, the rear panel tells us that this amp was built in Macao, PRC.

Good corporate branding…

The TRIODE/PENTODE switch is advertised as a ‘half-power/full-power’ switch. The LEDs labelled “Tube Life Monitoring” light up when the fuse in the plate circuit open, due to an internal short in the tubes.

Looking down from the end shows a handsome front panel with the Very Cool chicken head knobs.

The top view of the chassis shows us another good instance of strong corporate branding. I do like the protective cage over the vacuum tubes.

Here we get a good look at the bottom of the main circuit board. I am not a big fan of soldering tube sockets straight into a circuit board, but this seems to work here.

Most everything on this side of the circuit board appears to be OK.

I was documenting the location and orientation of these cables, in case we needed to further disassemble the unit.

These solder joints appear to be cold. We have found our intermittent ‘no sound’ cause!

Every other solder joint is removed, fluxed, then resoldered.

Ah, that’s better!

Ironic that just a couple of cold solder joints would bring this wonderful combo amp to its knees. This guy is 100% now!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Wireless Shure SM58 Battery Leaked

September 7, 2017 by latchaw

This microphone was part of the estate of an amateur journalist. As she moved into treatment and into hospice, there were other concerns besides removing the batteries from unused equipment. Could the Unbrokenstring Crew revive this unit and make it usable again?

This microphone is a basic SM58 capsule coupled to a battery-operated VHF FM transmitter.

These are often seen in many performance venues as well as in worship, news-gathering, and business settings.

The 9v battery has leaked. We will need to disassemble the unit to clean up the mess.

These VHF units are on frequency allocations that are shared with other services and may be subject to interference. However, these units work well if you are fortunate enough to be in a part of the country where interference is low.

The microphone capsule is built into the head of the microphone, which unscrews from the electronics. Connections between the capsule and the electronics are reliable regardless of how many turns are necessary to tighten the threaded bodies together.