Tayda RAT Oscillation

[tfTom]

Hello everyone, it's me again. I've been working on a project that's been sitting for a while. I have a Tayda RAT clone I built a long time ago, it was my first ever build actually. The problem with this pedal is that it oscillates like crazy if the gain is turned up past 1/2 o' clock, you can sort of use the tone and volume knobs to 'tune' it if that makes sense. It's also oscillating even with no input which I thought was strange.

I tried doing my research and 2 things came up constantly: wiring and buffers. I opened up the case and the wiring was honestly horrendous so I thought that might've been the issue. I had a few vero boards lying around so I built a small 2n54557 buffer to put in the pedal as well. After tidying everything up, I turned the pedal on and unfortunately the oscillation was still there. I took the pots off so I could re-flow every component and make sure my joints were alright so I don't think cold joints are an issue. Everything was also sourced from Tayda.

I started probing the circuit and followed the schematic provided by Tayda. I poked around starting at the input all the way to pin 3 of the op-amp (TL071) with nothing. At pin 1 and 8 I noticed some sort of noise but it was very faint, barely audible. There was nothing coming from pin 2. Once I got to pin 6, there was a loud screech that could be heard until the end of the circuit. I also noticed that it could be heard on one side of C4 and the gain pot, but not the other, I'm not too sure if that's relevant or not though.

I wondered if the op-amp that was the issue, so I built this circuit to test it and it worked fine. (https://www.instructables.com/Simple-audio-amplifier/)
I'm not too familiar with RAT circuits so I'd have to look more into it to see how everything interacts together, but any pointers would be appreciated. Thanks!

 

[BuddytheReow]

Where did you get your opamp and Q1 (output buffer)? You could also have some suspect solder joints somewhere on your board. Post some pics of your build (both sides) and we will try to help.

 

[tfTom]

Where did you get your opamp and Q1 (output buffer)? You could also have some suspect solder joints somewhere on your board. Post some pics of your build (both sides) and we will try to help.

Both were bought from Tayda. Here's pictures of the board. Pots cover most of the back and I'm not home to resolder them right now lol

 

[BuddytheReow]

Can you confirm all components are their correct value? Minus the oscillation, does the gain knob work?

Sometimes the PCB layouts can be the culprit (not your fault), but let’s rule other pieces out

 

[BuddytheReow]

Also, take out C3. If I remember correctly, this part is not needed if using a TL071.

 

[tfTom]

Can you confirm all components are their correct value? Minus the oscillation, does the gain knob work?

Sometimes the PCB layouts can be the culprit (not your fault), but let’s rule other pieces out

Gain works fine, so does the tone and volume knob. I replaced every box cap, took out the footswitch PCB and wired directly to the switch, upped C3 to 100nf, took it out completely, and still no luck. I'm considering just switching the build to vero but I'll wait until the op-amps I bought (OP07s and more TL071s) get here in about a week or two to see if that does anything.

 

[jimilee]

Gain works fine, so does the tone and volume knob. I replaced every box cap, took out the footswitch PCB and wired directly to the switch, upped C3 to 100nf, took it out completely, and still no luck. I'm considering just switching the build to vero but I'll wait until the op-amps I bought (OP07s and more TL071s) get here in about a week or two to see if that does anything.

My guess it’s the trace routing.

 

[spi]

Also, take out C3. If I remember correctly, this part is not needed if using a TL071.

I'd keep it in. I don't think it hurts, and if you do come across a lm308 then you can drop it in.

Hard to tell from your picture, but it looks like your opamp might be upside down. Hard to see any markings on the opamp from the photo, but I think can see the small dot on the outer side of the board, but from the tayda picture of the pcb, it should be oriented toward the inner side. If this is true though, I'm surprised you get any sound at all, so this might just be a wrong guess.