Gretsch Contro Fuzz Debug

Gretsch Controfuzz Boost-Distort pedal plugged in on the bench in front of an amplifier — Gretsch Controfuzz (Model No. 7798) back on the bench after the fix, September 2023.

This write-up documents a debug of a vintage Gretsch Controfuzz Boost–Distort pedal. The fault: when the back panel was closed, the pedal would short out the distorted signal and pass the dry signal when the pedal was turned off. Resoldering the switch and input connections did not fix it. The cause turned out to be mechanical — the battery clip was touching the newer switch lead whenever the panel was screwed down.

Status (September 2023): fixed. Battery moved and the clip covered with electrical tape; the pedal works with the back panel closed.

Pedal

Field	Value
Model	Gretsch Controfuzz, Model No. 7798
Controls	Boost, Distort; footswitch bypass
Op amps	Two type 748, each with a 25 pF compensation capacitor
Clipping	Pair of 1N4001 diodes
Power	9 V battery

Schematic

From the schematic sheet: the Controfuzz is a variant of the op-amp-driving-diode-clipper type of distorter. The distortion is run at high boost all the time in the first op amp, then subtracted from the dry signal in the second op amp. The Distort control sets how much distortion is mixed in, and the Boost control sets the overall level when the effect is switched in. Both op amps are type 748, which need the 25 pF compensation capacitor to be stable.

Download the Gretsch Controfuzz schematic (PDF)

Reference

Now I notice that the Contro Fuzz circuit is not all that different from the Expanda Fuzz.

Gretsch Expanda Fuzz schematic with bass, mid and treble op amp sections and a distort and boost section — Expanda Fuzz schematic, for comparison with the Controfuzz.

Diagram of an op amp with passive shunt diode clipping — Op amp with passive shunt diode clipping — the topology the Controfuzz is built around.

Debug log

9/10/23

Resoldered several different connections on the switch and inputs.
When I close the pedal, the back panel will short out the distorted signal and pass the dry when the pedal is turned off. Maybe it’s shorting the power?

Inside the Controfuzz enclosure showing the circuit board and colored wiring — Inside the enclosure: circuit board, switch and jack wiring.

9 V battery sitting inside the pedal next to the circuit board — 9 V battery sitting next to the board.

9/14/23

Experimented with what order to tighten down the screws. Discovered the lower right, above the battery clip, affected the signal. Pulled the battery out and put it to the left — it worked.
Kept pressing on the middle of the panel and found out the battery clip was touching the newer switch lead. Put electrical tape over the clip and it works perfectly now!

Controfuzz back panel with four rubber feet — Back panel, off the pedal.

Battery clip on the inside of the back panel covered in black electrical tape — Clip covered in electrical tape.

Result

With the clip insulated, the back panel can be screwed down without the clip touching the switch lead, and the pedal works normally.

Controfuzz closed up and plugged in, knobs set — Closed up and plugged in.

End of the Controfuzz showing the OUT and IN jacks — OUT and IN jacks (2016 photo).

Photo gallery

Select an image to view it full size. IMG_47xx–IMG_50xx are earlier photos from 2016.