Big Muff Pi Pedal Refurb and Update

Matt owned this pedal FOREVER and needed to put it to work in the studio for a project.  But, the foot switch had disassembled itself years ago, and the audio jacks were worn out.  This pedal needed an update to the 21st century, adding compatibility with the distributed power in a pedal board.  Could the Unbrokenstring Crew make this happen?

The knobs and controls were in great shape.  Beyond the audio jacks, switching, and power, the unit works great.

With the top cover removed, we see the VERY heavy gauge frame that takes the force from the foot switch.  Yes, stomp boxes get stomped on!  The LED mounting scheme is obviously some Soviet military hardware.

The top view of the circuit board show the ‘in’ and ‘out’ jacks.  The originals are plastic and completely shelled out.

The circuit board is more Soviet military goodness.  Clearly, this board was ‘hand taped’ and not laid out with an automated.CAD program.  I’d call this ‘one step beyond hand wiring.’

Note that all the wiring insulation is white.  This harness started out as ribbon cable, and each wire was pulled out of the ribbon as it was wired to place.

A modern epoxy-sealed foot switch is trial-fitted.  This one is triple-pole double-throw on/off to handle both the required functionality of the old switch and to implement ‘true bypass’ when the effect is deselected.

Here is the user-side of the foot switch.  We need to be sure that the mushroom button is installed as high as possible to simulate the height of the old foot switch.

For those of you who know your ‘one-line’ electrical symbols, you know that these two terminals are ‘no connect.’  What?

These ‘no connect’ terminals are used to mount the current-limiting resistor for the LED.

So the current-limiting resistor is moved to its own spot in the wiring harness.  Yes, that is clear heat shrink tubing.

More ‘one-line’ electrical symbol goodness.  The body of the switch is phenolic.  It is badly cracked and ready to shatter.  Had I been able to find the missing switch hardware, this switch could not be returned to service anyway.

And for those of you who understand the Cyrillic alphabet, this pic’s for you.

The input and output jacks will be desoldered.  The gray plastic is quite brittle after all these years.

Now that they have been removed, we can complete the schematic.  Those jacks have switches in them that need to be analyzed in isolation.  The switching function is not necessarily what Western manufacturers utilize in their jacks.

The printed circuit board is temporarily reinstalled so that we can check the fit of the new jacks.

The new jacks are genuine Amphenol units, professional grade, as they say.

The terminals are bent slightly inward to clear the internals of the pedal.

The stereo jack functions as a power switch, disconnecting the ground to the 9v source when the plug is disconnected.  Both jacks need to be oriented in such a way to minimize mechanical interference with the circuit board.

The original jack escutcheons really dress up the jacks!

All new wiring was made with silver wire with a white Teflon insulation, matching the original SovTek wiring harness.

The new switch is installed.  Unlike the original foot switch, this switch is wired as ‘true bypass’ when the effect is off.

The edge of the circuit board was trimmed away to allow clearance to the body of the new Amphenol jacks.  The trace that was cut is for the sleeve terminal, which is duplicated in the array of remaining pads.

This pic shows the orientation of the notch in the circuit board to the body of the new connector.

A new 9v power jack is added between the two in/out jacks.  This jack is compatible with the ‘Boss’ pedal power “standard” with negative in the middle and 9v on the sleeve.

To reassemble the pedal, the top frame goes on first.

The cover goes on next.  This is beginning to look like a pedal again!

Matt would use the 9v power jack in the studio, but the functionality of the pedal with a 9v battery remains unchanged.

Here is a look at the finished unit, updated with steel jacks and the 9v pedal power capability.  This unit works well!

Thanks for reading all the way through!

CONTACT – David Latchaw EE
281-636-8626

 

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