Help me troubleshoot my Thermionic Deluxe!

[Song Naga]

Hi all, I've got a Thermionic Deluxe that needs some help. The current diagonis is that LED's light up and the clean bypass works but the output from the pedal when it's engaged is a floppy buzzing sound with no hint of the guitar signal.

I was hoping to trace the input signal through the various opamps until the signal crapped out, then troubleshoot the offending opamp and its supporting components, but instead I found that there is a buzzing sound right at the input of the very first opamp (the green point on the board and schematic excerpts below).





This has me somewhat confounded since there really is not much in the circuit before this opamp to go wrong... I measured the VREF point and it is 8.7V, and drops to about 8.3V as it goes through R2A. Maybe my VREF is wonky? Or maybe I have my offboard wiring wrong? Currently the pedal is removed from its case, so it's not a grounding issue with the case itself (which has tripped me up before). Can anyone suggest a next step to troubleshoot this? Thanks for your time!

 

[Song Naga]

Here are photos of my offboard wiring, in the event that I have fouled it up.

 

[JamieJ]

Yep it looks like your VREF is way off. Check the resistor values for R32 & 33. What voltage do you have on the + of C26 (22uf)?

 

[Song Naga]

Thank you for taking a look! R32 is 22k and R33 is 20k. Voltage at the + end of C26 is 4.84V, which is probably what VREF should be right?

 

[Robert]

Thank you for taking a look! R32 is 22k and R33 is 20k. Voltage at the + end of C26 is 4.84V, which is probably what VREF should be right?

Yes, but check again on Pin 1 of IC1. (The VREF buffer)

 

[Song Naga]

Seeing that C26 is connected to IC1.1, I measured that opamp. 4.84V is going into its positive input, but I've got 8.69V at the output of that opamp. That seemed pretty suss, so I swapped out the opamp for a fresh one and bam it was outputting 4.84V, looks like that opamp was bad. I do recall it was the one I had installed backwards at first powerup and got hot before I noticed it.

Suddenly I get audio out of the pedal instead of buzz! That was really easy, thank you for pointing towards the circuit that sets the VCC.

 

[Song Naga]

Assembled and still working! This is the first time I've burned out an opamp in a while that didn't produce blue smoke and obvious damage to IC. Really glad to know that the fix was prying one IC out of its socket and dropping another in. Thanks for helping me quickly identify the problem, I was dreading that this might be a long diagnosis since it's such a big old board!

 

[comradehoser]

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