Peavy Decade Preamp DeadendFX loss of volume

[airvian]

Link to the build documents: https://drive.google.com/file/d/14GZfHa1M3ThQPYdIIEGSTHft33C3Eh2I/view?usp=sharing
So, the circuit works, but I get no volume when it is switched on. I built an audio probe to check for where i lose the  volume and found a sudden drop off after C26. I swapped it out for another ceramic 100pF but that didn't change anything. The level after pin 1 of IC4 seems to be at about the volume at the signal input. At pin 7 it is significantly louder, as it should be (I believe). Any idea on what could be at fault? My next guess would be a faulty IC but I don't have a spare on hand atm to swap it out. I'd have thought about jumpering C26 for a quick and dirty fix, but don't know if that would work.

[antonis]

Jumpering C26 should made things quieter..
(IC4A should turn into unity gain buffer..)

Check signal level on pin 3  (non-inverting input) to compare it with the one on pin 1 (output)..
(it should be about 12dB louder or about 4.3 times of higher amplitude)

[idy]

Good work whipping out the audio probe.

"Sudden drop after.." *usually* means not a bad component, but a short or near short at that "node." That cap (C26) is just to smooth off the highs and prevent oscillation. If it is much bigger than it should be (i.e. 100n instead of 100pf) it would turn that stage into low gain for most frequencies.

Also if R26 or 27 are off (or not soldered in) That would interfere. Check all components attached to pins 1,2,3. Check for shorts to ground, and for a path to ground for R27.

Bad components are rare. Since the first stage of the IC is working, I would look elsewhere. What about voltages for that IC? We want to see power on 8, ground on 4, and mid voltage on the other pins.

[airvian]

Jumpering C26 should made things quieter..
(IC4A should turn into unity gain buffer..)

Check signal level on pin 3  (non-inverting input) to compare it with the one on pin 1 (output)..
(it should be about 12dB louder or about 4.3 times of higher amplitude)

Thanks for the explanation. pin 1 seems to be louder than pin 3 so that checks out. Can't quantify it though as my probe is just sound and not graphic. It's just a pen with a sound generator and a tiny speaker with some switching for lights and different generator modes and volume.

[airvian]

Good work whipping out the audio probe.

"Sudden drop after.." *usually* means not a bad component, but a short or near short at that "node." That cap (C26) is just to smooth off the highs and prevent oscillation. If it is much bigger than it should be (i.e. 100n instead of 100pf) it would turn that stage into low gain for most frequencies.

Also if R26 or 27 are off (or not soldered in) That would interfere. Check all components attached to pins 1,2,3. Check for shorts to ground, and for a path to ground for R27.

Bad components are rare. Since the first stage of the IC is working, I would look elsewhere. What about voltages for that IC? We want to see power on 8, ground on 4, and mid voltage on the other pins.

R26 and 27 are within their given values. Path to ground of R27 is there. The components on pin 1,2,3 seemed fine, none of them had any shorts to ground. The IC voltages look fine. 4 goes to gnd, 8 has about 18V measured from pin to ground, rest have 6-9V. I reflowed all the solder of the components in this part of the circuit but there is still no change. Volume seems fine after pin 1 and at the leg of R26 that is connected to pin 1. Volume and sound at pin 2 is the same as on leg of R26 and C26 that is connected to pin 2. Volume at leg of C26 that is connected to pin 1 is lower but of the same sound as at pin 1. So no change yet. Any other ideas?

[idy]

Volume at leg of C26 that is connected to pin 1 is lower but of the same sound as at pin 1.

Maybe confused? How can the the volume at pin 1 be one thing, and the volume where pin 1 goes to C26 different? That is one place. Do you mean other end of c26? No that is pin 2...

Do you mean the volume at pin 1(and C26 and R28) is the same as at pin 3 but lower?

If something is bad after that 1k R28 that would kill the volume. If those diodes are shorted... worth pulling them if you aren't afraid of destroying them... just use diode check mode of meter.
Also double check the volume pot.

[airvian]

Volume at leg of C26 that is connected to pin 1 is lower but of the same sound as at pin 1.

Maybe confused? How can the the volume at pin 1 be one thing, and the volume where pin 1 goes to C26 different? That is one place. Do you mean other end of c26? No that is pin 2...

Do you mean the volume at pin 1(and C26 and R28) is the same as at pin 3 but lower?

If something is bad after that 1k R28 that would kill the volume. If those diodes are shorted... worth pulling them if you aren't afraid of destroying them... just use diode check mode of meter.
Also double check the volume pot.

Ok so R26 and C26 are connected to pin 2 on one side and pin 1 on the other. The sides where R26 and C26 are connected to pin 2 have the same Volume as pin 2. On the side where R26 and C26 are connected to pin 1 however, the connection of R26 has the same volume output as pin 1 but C26 does not. I hope that makes sense.
I have tons of spare diodes so I'll just rip them out and replace them now. Nothing to lose really...

[Govmnt_Lacky]

Ok so R26 and C26 are connected to pin 2 on one side and pin 1 on the other. The sides where R26 and C26 are connected to pin 2 have the same Volume as pin 2. On the side where R26 and C26 are connected to pin 1 however, the connection of R26 has the same volume output as pin 1 but C26 does not. I hope that makes sense.
I have tons of spare diodes so I'll just rip them out and replace them now. Nothing to lose really...

Check for continuity between Pin 1 and C26. Possibly have a broken trace or dodgy solder joint.

[antonis]

pin 1 seems to be louder than pin 3 so that checks out.

That's OK for IC4_A..

Ok so R26 and C26 are connected to pin 2 on one side and pin 1 on the other. The sides where R26 and C26 are connected to pin 2 have the same Volume as pin 2. On the side where R26 and C26 are connected to pin 1 however, the connection of R26 has the same volume output as pin 1 but C26 does not. I hope that makes sense.

No, unfortunately, that makes no sense..

One side of R26 & C26 are connected to pin 2 and the other one to pin 1..
(the only case for C26 & R26 exhibiting different volume levels is C26 to be disconnected..)

P.S.
Could you plz post some pics of your build (PCB both sides) 'cause that C26 issue puzzle me a bit..

[airvian]

pin 1 seems to be louder than pin 3 so that checks out.

That's OK for IC4_A..

Ok so R26 and C26 are connected to pin 2 on one side and pin 1 on the other. The sides where R26 and C26 are connected to pin 2 have the same Volume as pin 2. On the side where R26 and C26 are connected to pin 1 however, the connection of R26 has the same volume output as pin 1 but C26 does not. I hope that makes sense.

No, unfortunately, that makes no sense..

One side of R26 & C26 are connected to pin 2 and the other one to pin 1..
(the only case for C26 & R26 exhibiting different volume levels is C26 to be disconnected..)

P.S.
Could you plz post some pics of your build (PCB both sides) 'cause that C26 issue puzzle me a bit..

@Govmnt_Lacky was right... It was simply a connection issue as in C26 and R26 weren't connected on the pin 1 side, hence the volume drop as C26 was disconnected. I simply got out the copper wire and bridged C26 and R26. So the issue was probably down to something with the PCB not being right. as I reflowed those exact solder joints twice already. Thanks for the help everybody. The sound of the pedal is... well... noisy, as expected I guess 
Images are attached too. Don't judge the complete massacre I made of that PCB from all the parts swapping around C26...