Probably A Simple Fix

ProCo Rat 2

ProCo Rat 2

I sniff out a rat while I attempt to repair a true icon in the guitar effect world; the ProCo Rat 2.

 

I Smell A Rat

As a guitar player, you'd have to be living under a rock not to have at least heard of the ProCo Rat. For good reason, the Rat is to distortion pedals what the Tube Screamer is to overdrive pedals.

Originally released in 1978, the Rat was designed by Scott Burnham and Steve Kiraly, who at the time, were repairing and modifying existing distortion pedals for ProCo Sound based in Kalamazoo, Michigan.

Not satisfied with what was already on the market, they decided that they themselves could design a far superior product; which, of course, become the very first version of the Rat.

The Rat is a relatively simple design, utilising a variable gain stage powered by a single op-amp with a pair of diodes clipping the waveform at the output in order to create the distortion. This is followed by a passive tone control (a variable low pass filter), and finally, a JFET buffer to ensure signal integrity.

The Rat on the workbench with the back of the enclosure removed to reveal the PCB inside.

The Rat on the workbench with the back of the enclosure removed to reveal the PCB inside.
The solder side of the Rat's PCB.

The Patient
Another auction site score, this time the previous owner had stated that the footswitch was very inconsistent in its operation. On occasion, it engaged the effect and other times it didn't. Sometimes it turns on the status LED light but the signal would cut out. Other times it would engage the pedal but the signal was barely audible. They said that they had tried using some electronic contact cleaner to clean the switch but to no avail.

The solder side of the Rat's PCB.

This was confirmed once the pedal arrived and I was able to plug it in and test it for myself. After trying some contact cleaner of my own (just to be sure), I was able to determine that contact cleaner wouldn't resolve the issue.

Bypassing the footswitch with a few jumpers, however, I was any to determine that the circuit itself was fully functional and the problem lay indeed within the footswitch itself.

A close-up of the installed questionable footswitch.

A close-up of the installed questionable footswitch.
The footswitch removed from the PCB.

This is not surprising as one of the most common faults that occur in guitar effects pedals is the footswitch failing in (in his best Debbie Harry voice) one way or another. Mechanical parts like switches are rated by the manufacturer for a certain number of cycles (usually found of the datasheet of the part in question). What does that mean exactly? The switch should be able to be pressed X amount of times before it is expected to fail. This number is usually in the tens of thousands, 30,000 cycles being common for this type of footswitch. Of course, mileage does vary. Depending on the quality of the switch and how it is treated over the course of its lifetime.

The footswitch removed from the PCB.

The unique thing about the PCB of the Rat is that footswitch and the potentiometers pass through the actual PCB from the component side of the board to the solder side. The footswitch itself uses a combination of three separate bare wires to connect to the PCB in addition to three insulated wires (two red wires and one yellow wire), two of when connecting the switch to the input and output jacks and an additional wire to the PCB. There is also an insulated jumper installed across two of the lugs of the footswitch.

The new footswitch installed with its bare wire connectors.

The new footswitch installed with its bare wire connectors.
The insulted jumper soldered into place.

With the potential for things to get a little confusing, I was sure to take a number of reference photos of the wiring scheme to refer back in case I needed to when installing the new footswitch. So with the old footswitch removed, it was time to prepare the new one for installation.

Firstly, I installed the bare wire connections to the switch before securing it into place on the PCB and soldering the new connections. Next, I removed the insulted jumper from the old switch and installed it on the new switch.

The insulted jumper soldered into place.

Finally, I soldered the insulated wires into place on the switch and the pedal was ready for testing.

I made sure to rock it before I boxed it, a useful pedal building adage that ensures everything is working as it should before one installs the PCB back inside the enclosure. And rock it did! No more loss of signal quality in bypass mode or when the effect was engaged; no issue with the status LED either.

The final wires soldered into place.

The final wires soldered into place.
Ready to rock again.

So Was It An Easy Fix?
Yes. The previous owner had done the hard work for me and installing the new switch had its quirks due to the wiring schemed but all-in-all, it was indeed another simple fix.

 

Ready to rock again.