T Rex Mudhoney II

This is something like two slightly different Proco RAT circuits in one box. As received it only seemed to work with very heavily attenuated output volume. Maybe it was something to do with an electronic switching system.

Shown here without footswitches

The PCB photo came out very noisy, but the inside is just a CD4013 dual flip flop IC and 2 OP7 opamps driving some diodes to ground - I believe this is standard for modern reissue RATs. The red wire and replacement capacitor was a previous repair. The switches are held by rectangular cut-outs in the PCB, I like this method.

 

As well as being very quiet, the LEDs underneath the tone pots were not lighting up. I guessed that the 4013 was driving some switching FETs and it was dead.

First attempt at a fix was replacing the CD4013, as these are cheap and I can replace SOIC parts pretty quickly nowadays. No change. Probing the new chip I saw it was getting no power - I should have checked this first. I followed the traces from the 9V input outwards and they were hidden underneath the input jack. That had to be removed.

Input jack removed

Underneath was a trace carrying 9V that ran right next to the edge of the board and had been cut right through. I added a wire from to restore 9V to a power bypass cap at the 4013 and reinstalled the jack. Now the pedal lit up when engaged, and the left side (channel 1) was loud and sounded pretty great. The right side still had a very low output volume, much less than unity.

I hooked up a signal gen and tried to follow with an oscilloscope to see where signal was lost. Thi schematic was helpful. The opamp has outputting a 6Vpp signal but this was lost at the tone control, so the problem was probably somewhere in between. Testing the clipping diodes showed that one was shorted. I guessed and lifted one off.

One clipping diode removed.

This was lucky, that diode tested as a short out of circuit. This side of the pedal was now much louder, but less distorted than the other side as only half of the signal is getting clipped. These diodes are MELF packages and I can't find any identifying markings on them. The remaining diode measured around 0.5-0.6V on a DMM, so probably something silicon. Internet consensus seems to indicate that these pedals used 1n4148, and I had MELF 4148s at hand, so I used that.

My hand soldered replacement looks different to the reflowed diode. The good news is that it sounds the same as the other channel, and now both will get fairly loud. I forgot how nice a RAT can sound.