Lightspeed (Mach1/Light Land) squealing at Highest gain setting

[megatrav]

Hey all,

Has anyone built one of these and experienced a high pitched squeal with the gain turned all the way up?

I build the Effects Layout version. Which has the option to build the Lightspeed or Southland.

The only thing I can think of is I opted to omit D6 and D7 so that per the instructions.
Should I have jumpered those?
Or should I try swapping a different value resistor or capacitor?

I also would like to mention that this pedals is so bright. I can't turn the tone control past 9:00. I would really like to try to roll off some of that high end and maybe get a little better sweep out of the pot.
Anybody tried anything like this?

Any advice or guidance is always appreciated.

 

[Boba7]

If it’s very bright and squeals, I think you may have an error somewhere, maybe a resistor or cap that’s not the right value.
Ive never built this circuit, but it shouldnt be that bright...

 

[megatrav]

It was C9!
I had a 1u instead of 10u

 

[Boba7]

It was C9!
I had a 1u instead of 10u

I don't see how changing C9 could affect the sound, but if it sounds good to you now, then congrats!

 

[giovanni]

C9 is part of the feedback loop of both op amp stages so it doesn’t surprise me that a wrong value will cause squealing - it probably causes it to amplify some noise from the power source at high gain.

 

[Boba7]

C9 is part of the feedback loop of both op amp stages so it doesn’t surprise me that a wrong value will cause squealing - it probably causes it to amplify some noise from the power source at high gain.

Right, I didn't get a good look at the schematic you're right!

 

[yazooligan]

I’m having this same issue with a Tayda Purple Plexi 800 I just finished. I’ll have to double check the component values.

Would probing them one at a time help me find the problem component?

 

[giovanni]

I’m having this same issue with a Tayda Purple Plexi 800 I just finished. I’ll have to double check the component values.

Would probing them one at a time help me find the problem component?

I don’t think so. The oscillation will manifest all over the circuit, not in a single spot, even tho one single component may be responsible. Checking all component values and checking for shorts is the best way to start.

 

[yazooligan]

I don’t think so. The oscillation will manifest all over the circuit, not in a single spot, even tho one single component may be responsible. Checking all component values and checking for shorts is the best way to start.

Just verified every single component in my build. I think this will be the last time I buy a PCB from Tayda. What a waste of time.

 

[giovanni]

It could be the board that's defective or poorly designed, but I suggest posting pictures just in case, somebody on the forum might be able to spot the problem.