Muroidea signal drop

[CleanSocks]

So I had this crazy idea of putting two gain pots on a Muroidea with a footswitch to choose between the two pots, so I can toggle between two gain levels. In theory, everything should be fine - I breadboarded my two-pot switch, got the bi-color led to change appropriately, and everything else looks to be done correctly (I tested the resistance for the pot legs and all).

But, now that I’ve put it all together, I’m barely getting a signal at all from the pedal when it’s on. When the pedal is bypassed, I get my clean signal no issue. When I turn the pedal on, the main LED lights up. And with an audio probe, I was able to trace my signal backwards to R6 no problem. But, between R6 and C7 there’s a huge drop in volume. This drop persists up to pin 6 on the OP07, and then when I check pin 3 there’s just no signal at all.

I’ve been reading around the forums, but no one’s seemed to have my exact issue. All my pots are insulated with dust caps. I can’t find any strange grounding/contact issues. As far as I can tell, all my components are the correct values.

On the IC, I’m getting the following voltage reads from each of the pins:
Pin 1 = 9.24
Pin 2 = 1.99
Pin 3 = 2.75
Pin 4 = 0
Pin 5 = basically 0 (my DMM is showing like 0.1mV)
Pin 6 = 1.819
Pin 7 = 9.36
Pin 8 = 9.36
I’m still new to reading voltages from an IC so I hope I did that right… but the negative lead on a ground, and put the positive at each pin.

I’m at a bit of a loss here… I burnt out one of my bicolor LEDs, and it seemed to improve the situation a little (but only in the sense that it got a little louder, but still not what I’m expecting, and turning the gain knob up got this high pitched squeal).

Can anyone think of any ideas of what could be going wrong? Happy to provide more pics if that’ll help.

 

[jesuscrisp]

Your IC voltages are all wrong.
Double check all the values and reflow your solder joints, then check it again.

 

[Feral Feline]

Ahhh I don't see how you could possibly be getting low-to-no sound in front of R6 and "normal" levels after R6.
There's nothing else in the signal path between C7 and R6; remove either that cap or resistor and you've got zero signal.

What do you mean by "Normal" anyhoo? Normal it sounds like a Rat, or Normal it sounds like clean or ?
The signal enters the IC at Pin 3, if you've got no signal going in at pin 3, how can you have any signal at all further downstream at C7/R6?


At any rate, your C6 looks broke off and that compensation cap, if the OP07 is anything like the LM308, is critical for the IC to operate correctly.

 

[Feral Feline]

I'm a big fan of swapping pots, and I see you've used a 4PDT; for future reference, you can use just a 3PDT.

It'll be even simpler to 2knob the gain compared to the Vol in the diagram, cause the Rat's gain pot has lugs 2&3 tied together:



You could simplify further from my diagram to just run one wire from the PCB's GAIN-PAD 1 to one pot's lug 1 and then put a jumper between both pots Lug 1. Signal out in the diagram would of course go to the PCB's GAIN-PAD 2.

A bi-colour LED could replace the two in the diagram and depending on if it's common-cathode or common-anode wire up the appropriate CLR(s) for the bi-colour LED.




I'd also suggest with all the extra offboard wiring going on in the build, insulate at least one leg of your main bypass LED to prevent shorting its legs together.

 

[CleanSocks]

Ahhh I don't see how you could possibly be getting low-to-no sound in front of R6 and "normal" levels after R6.
There's nothing else in the signal path between C7 and R6; remove either that cap or resistor and you've got zero signal.

What do you mean by "Normal" anyhoo? Normal it sounds like a Rat, or Normal it sounds like clean or ?
The signal enters the IC at Pin 3, if you've got no signal going in at pin 3, how can you have any signal at all further downstream at C7/R6?


At any rate, your C6 looks broke off and that compensation cap, if the OP07 is anything like the LM308, is critical for the IC to operate correctly.

So for the R6/C7 thing, I’m using an audio probe I put together. I had a loop going through the hot of the audio probe, I connected the ground to the ground on the pedal, and then I sent the normal pedal output out to my amp. When I touched the hot to the output jack itself, I got a normal-sounding output, as if it was bypassed (because it literally was). I traced this normal sound backwards through the circuit. On the side of R6 that’s closer to the output, it sounded fine. On the side of R6 closer to the IC, it was lower. On Pin 3, no sound whatsoever.

And for C6, I was following what someone said in this thread about the OP07 not needing the 30pF cap. I thought that was causing the problem, so I popped it off. I can try putting it back in if you think it needs to be there.

 

[CleanSocks]

I'm a big fan of swapping pots, and I see you've used a 4PDT; for future reference, you can use just a 3PDT.

It'll be even simpler to 2knob the gain compared to the Vol in the diagram, cause the Rat's gain pot has lugs 2&3 tied together:


View attachment 37707
You could simplify further from my diagram to just run one wire from the PCB's GAIN-PAD 1 to one pot's lug 1 and then put a jumper between both pots Lug 1. Signal out in the diagram would of course go to the PCB's GAIN-PAD 2.

A bi-colour LED could replace the two in the diagram and depending on if it's common-cathode or common-anode wire up the appropriate CLR(s) for the bi-colour LED.




I'd also suggest with all the extra offboard wiring going on in the build, insulate at least one leg of your main bypass LED to prevent shorting its legs together.

That’s all super helpful - thanks for that!

I think the reason I needed the 4PDT was because my bicolor LED only has the two leads. I couldn’t figure out a way to switch the hot/ground on a 3PDT. I’ve got some three-lead bicolor LEDs on the way for another build tho - maybe I’ll swap it out and give your diagram a try, just for simplicity’s sake.

 

[Feral Feline]

So for the R6/C7 thing, I’m using an audio probe I put together. I had a loop going through the hot of the audio probe, I connected the ground to the ground on the pedal, and then I sent the normal pedal output out to my amp. When I touched the hot to the output jack itself, I got a normal-sounding output, as if it was bypassed (because it literally was). I traced this normal sound backwards through the circuit. On the side of R6 that’s closer to the output, it sounded fine. On the side of R6 closer to the IC, it was lower. On Pin 3, no sound whatsoever.

And for C6, I was following what someone said in this thread about the OP07 not needing the 30pF cap. I thought that was causing the problem, so I popped it off. I can try putting it back in if you think it needs to be there.

Okaydokay. That all makes more sense now.

I probe differently!

I'm not familiar with the OP07, I have a couple but haven't used them yet. I just thought it'd probably be like the LM308 — if ChuckDBones says it doesn't need the compensation cap, that's fine, it doesn't need it.
It also now makes sense why your output is lower on the C7 side of R6, because you're introducing the signal source from your probe and it then has to get through that resistor ergo it'll be lower on that side to the output.

Here's how I probe, no idea if it's how it should be done but here's what I do:

Send a looped signal or generated tone through the pedal from the very beginning of the circuit, the probe is connected as output — so instead of plugging the amp/headphone-amp into the output, the probe is plugged in its place. Starting at the beginning of the circuit I touch the tip of the audio-probe to key points in the circuit from input working towards the output. For example If I jump from the input cap where I've got sound to the output of the IC and it's not what is expected, then I know the problem is between the input cap and the IC out and I work my way back until I get a good signal again. If the IC output is fine, I keep working through the audio path to the output...


The way I look at it, to mash a metaphore, you're probing the cart before the horse.

 

[CleanSocks]

Okaydokay. That all makes more sense now.

I probe differently!

I'm not familiar with the OP07, I have a couple but haven't used them yet. I just thought it'd probably be like the LM308 — if ChuckDBones says it doesn't need the compensation cap, that's fine, it doesn't need it.
It also now makes sense why your output is lower on the C7 side of R6, because you're introducing the signal source from your probe and it then has to get through that resistor ergo it'll be lower on that side to the output.

Here's how I probe, no idea if it's how it should be done but here's what I do:

Send a looped signal or generated tone through the pedal from the very beginning of the circuit, the probe is connected as output — so instead of plugging the amp/headphone-amp into the output, the probe is plugged in its place. Starting at the beginning of the circuit I touch the tip of the audio-probe to key points in the circuit from input working towards the output. For example If I jump from the input cap where I've got sound to the output of the IC and it's not what is expected, then I know the problem is between the input cap and the IC out and I work my way back until I get a good signal again. If the IC output is fine, I keep working through the audio path to the output...


The way I look at it, to mash a metaphore, you're probing the cart before the horse.

Probing the cart before the horse is exactly what I was doing - it’s amazing what you can convince yourself is the right way to do something with just a little bit of stubborness!

Even probing it from the beginning though, I’m still losing my signal right around Pin 2 of the op amp. I have a clear signal right up to Pin 3, but after that it disappears completely. I’ve tried reflowing all my solder joints, and I tried swapping out the OP07 with both another OP07, as well as a TL071. Same result, no signal. I’m at a bit of a loss here - I don’t think I damaged the board, since the dip socket was one of the first things I soldered and I did it super quickly (no chance for anything to burn or melt).

Any other ideas?

 

[CleanSocks]

Your IC voltages are all wrong.
Double check all the values and reflow your solder joints, then check it again.

I reflowed all my solder joints, and double checked my values. No significant change from before. This may be a dumb question - the DC mode on my DMM is the right one to use, right? When I try using the AC mode it beeps at me and just displays “0L” on the screen. And I’m doing it the right way, but putting black on ground and red on each pin, right?

 

[jesuscrisp]

DC is correct to check your DC voltages.

If you're already at it, could you check the voltage at either side of R11 and R12? Your IC is severely misbiased according to your measurements, which is probably why you get the huge signal drop.

 

[Feral Feline]

A couple of thoughts...

- If a bit of wire or solder somehow got under the socket between the board (and its pads), it could short-circuit without showing visible traces of the short.

- Pin 3 is the input into the op-amp, while signal is lost at pin 2 already, pin 2 being part of the feedback loop that the gain pots are attached to. I suggest a complete unboxing of the circuit, and rebuild it stock (just tack in the wires from the one GAIN pot to the PCB). Get the basic circuit working, then add in your dual-pot mod.


I suspect something is awry with the gain-pot mod, which might also explain the mis-bias.

...- I don’t think I damaged the board, since the dip socket was one of the first things I soldered and I did it super quickly (no chance for anything to burn or melt).
...

When you reflowed the solder joints did you reflow the IC-socket? Maybe you have a cold solder-joint in the socket (from doing it "super quickly").

 

[CleanSocks]

I suggest a complete unboxing of the circuit, and rebuild it stock (just tack in the wires from the one GAIN pot to the PCB). Get the basic circuit working, then add in your dual-pot mod.


I suspect something is awry with the gain-pot mod, which might also explain the mis-bias.

It appears that it was the dual-pot mod causing the issue. I didn’t go so far as to repopulate the entire circuit, but I did undo the dual-pot mod and put in a single gain pot. This appears to get it to work - I’m able to get a distorted signal through the pedal (the distortion seems a little weak for a RAT but perhaps there’s something else wrong as well).

Now that I know the circuit should be mostly fine, I’m going to reattempt the dual-pot mod, following the diagrams you provided before. I got some three-lead bicolor LEDs so I’ll give the 3PDT a try. Hoping I can get it this time!

 

[CleanSocks]

When you reflowed the solder joints did you reflow the IC-socket? Maybe you have a cold solder-joint in the socket (from doing it "super quickly").

Yup, reflowed everything. I had the same thought and made sure to reflow the IC as well.

 

[CleanSocks]

DC is correct to check your DC voltages.

If you're already at it, could you check the voltage at either side of R11 and R12? Your IC is severely misbiased according to your measurements, which is probably why you get the huge signal drop.

I got rid of the dual-pot mod and now both R11 and R12 are reading ~4.8V on the non-ground sides.

My full IC voltages are now:
1- 9.6V
2- 4.8V
3- 4.4V
4- 0V
5- 0V
6- 4.8V
7- 9.65V
8- 9.6V

 

[fig]

There shouldn't be negative voltages.

 

[CleanSocks]

There shouldn't be negative voltages.

Oh those aren’t negatives - just poor punctuation choices

 

[fig]

Oh, that's a tilde...sorry poor eyesight!

 

[CleanSocks]

I GOT IT

It was my original dual-knob setup. Whatever I was doing with the 4PDT, it wasn’t right. The 3DPT with a three-lead bicolor LED was the ticket.

Thank you all so much for your help!

 

[Feral Feline]

There shouldn't be negative voltages.

Don't be so, you know, negative, man.

Oh, that's a tilde...sorry poor eyesight!

It's just a hyphen or a dash. Maybe should've been an n–dash, or even an m–dash, as both are longer, but you know, everyone has their own style.


"That's not a tilde,

This is a tilde"

You misplace your reading glasses again, Fig?