Jim Dunlop Uni-Vibe Pedal from eBay Is Non-Functional

Joe wanted a new pedal board, so he used this occasion to get something he’s always wanted, a Joe Dunlop Uni-Vibe pedal. But the elation of the winning bid was replaced with the realization that this unit was defective. Could the Unbrokenstring Crew bring it back to life?

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The owner said that the LEDs were intermittent and no audio came through the pedal in any mode, even bypass.

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The Correct power supply was delivered with the unit, and it works fine. Note the +18v output.

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Jiggling the cables when plugged into the connectors seemed to have an effect, so we will begin by looking inside for mechanical issues.

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These knobs are guitar knobs with set screws… all metal construction here!

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The wrap-around case is very sturdy!

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Like most used pedals, adhesive from Velcro tape leaves a sticky mess. This is offensive.

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A little elbow grease and alcohol on a rag removes the sticky mess. That’s much better!

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Three circuit boards make up the internals of the pedal. The first board to come off has the optical ‘stuff’ in the silver box, used on many electronic tremolo designs. I think that’s how the rotating Leslie effect is realized, as well. We’ll check this out later.

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Connections to the next board are done with gold plated pins, mating with sockets containing gold plated pins. This avoids any problems with dissimilar metals.

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Interestingly, the parts inside the silver box were hand-soldered after the rest of the board was (probably) wave soldered. This makes sense, as the parts inside the silver box are probably not compatible with the heat of the wave solder process.

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The middle board has all the remaining electronics, such as power distribution, audio routing, and interconnection.

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The back side of the middle board has the wiring for the external control potentiometers.

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In true Leo Fender style, this metal bar serves as a washer for the controls, grounding everything together.

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While we’re here, we will clean this up. A sanding stick is just the thing to knock the rust off.

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This washer is steel, so a little lubrication is in order to keep it nice for the coming years.

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There. Shiny on both sides!

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Now that everything can be accessed, the power jack is cracked. Here is a new one.

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The holes in the circuit board are cut as slots, not drilled as round holes.

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This solder joint is cracked. This may have something to do with the lost audio, although this circuit is the ‘sleeve’ of the plug that attaches from the outside world. All of the rest of the solder joints on the connectors inspected under the stereo microscope and were cleaned up.

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The solder attachment points for those rear panel jacks are under this piece of insulating plastic, which separates the jacks from the metal bodies of the controls. This piece of plastic needs to cover all of the attachment points under the controls.

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Here is another view of the insulating plastic and the long washer holding the controls in alignment.

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A quick check shows another source of intermittent audio: the Pedal input jack. The electronics thinks that an external pedal is installed when it isn’t. These quarter-inch jacks have a built-in switch that is sometimes used to steer signals based on whether a plug is inserted. This contact pair was dirty. To clean it, a plug is inserted to open the contacts, a piece of clean paper is soaked with cleaner, the plug is removed, and the paper drawn through the contact pair.

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This stuff is really expensive these days, so I’m trolling for a corporate sponsor deal.

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The silkscreen printing on the case is VERY fragile. Most of the existing printing has come off.

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My Brother P-Touch label maker, loaded with clear tape, is just the thing to make some nice, new labels.

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As the pedal goes back together, contact cleaner is used extensively to clean up any junk and to give some assurance that the contact won’t open up again in the future.

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I just had to take a look at the insides of the optics on this pedal. Here is the silver box.

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The silver box reflects light from the incandescent lamp in the center and illuminates the four light-sensitive resistors spaced around it.

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We are checking the internal clocks, used for ‘Rate’ which are created by the middle board.

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Here is the optical system in action, minus the silver reflector box.

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Now that we’ve had our fun, the box will go back in its place.

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The hold-down wire is soldered on both ends to the circuit board, and the remaining slack is taken out by crimping the wire as shown. We don’t want this to move, as mechanical vibrations will be transferred to the audio chain.

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The first circuit board is done.

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Reassembly includes getting all the spacing washers between the jacks and the cabinet back in the right place. I see these left out when techs get careless or in a hurry.

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These jacks are secured by these plastic nuts.

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The whole jack is insulated from the rear panel to break ground loops (remember the long metal washer that we cleaned up above? They are connected to the chassis. These jacks are not…) and so they are made from insulating material. No wrenches were used to tighten these fasteners, but just finger-tight with a socket.

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Screws and knobs are back on!

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No RED LED on the right indicates that BYPASS is asserted.

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The RED LED shows that the EFFECT is active and the GREEN LED indicates that the ROTATING SPEAKER or CHORUS effect is asserted.

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The RED LED on the left indicates that the PITCH MODULATION or VIBRATO effect is asserted and no RED LED on the right tells us that BYPASS is asserted

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In this mode, the RED LED on the left indicates that the PITCH MODULATION or VIBRATO effect is asserted and the RED LED on the right tells us that the EFFECT is active.

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Thanks for reading all the way to the bottom!

CONTACT – David Latchaw EE
281-636-8626