T Rex NeoComp

Another T Rex pedal with problems. This is a VCA-based compressor, and it was really noisy, like white noise with some low-frequency popping and crackling. Fortunately the noise was there in both effected and bypassed modes - this meant that everything was more or less working and that the source of the problem could be isolated to parts of the circuit common to both.

On the inside there are some similarities with the previous T Rex pedal, there is a CD4013B and momentary footswitch for latching bypass, so there are probably some JFETs nearby. MC33174 quad opamp. The compression is done with a THAT 4316 which seems like a nice chip for pedal applications. It's sold as an "analog engine" and contains a VCA and the RMS detector. The QSOP package will probably turn people off. An LM393 comparator is used for something, but I have no idea what for.

Main PCB.

Noise problems like this are usually caused by a bad semiconductor or a missing or failed capacitor. I like to find the source of the noise with an oscilloscope and then remove parts to see if the source disappears. Opening the circuits isolates things into two sub-circuits and narrows things down. In this case I ended up removing nearly all the ICs to figure it out. I started tracing out the circuit as well but solved the problem before I got very far with that. The SOT23 transistors in the corner of the board are driven by the 4013 and are connected together in the same way as the switching JFETs in other T Rex pedals ADD LINKS One of them showed a lot of noise when probed with the scope, so I replaced it with a J201 and that killed the noise. The originals are marked "G5   7" which I can't identify, but for switching it seems a J201 will work just fine.

Switching FETs are in the top right corner, faulty FET has been removed.

Apparently this noise problem is a common failure with T Rex pedals. Swap the JFETs.