Unicab no sound

[Rpschultz13]

Just finished Unicab, no sound. I checked all the resistor color codes, seem right. Then I removed the switch, jacks go in direct. Still nothing. Measured 9v at the rails pin 4 and 8 on all 3 TL072s.

What do I check next?

 

[temol]

What do I check next?

Show the wiring of the in/out jacks.
Use audio probe to check signal path.

 

[Rpschultz13]

I don’t have an audio probe yet. Building one…

 

[Rpschultz13]

There’s just not much to this pedal.
1 diode
2 electrolytic caps.
Could op amps be bad? I could swap them…

 

[jimilee]

You shouldn’t have 9v on pin 4 of your opamps; that’s ground. Check for bridges and reflow every thing regardless of how it looks.

 

[Rpschultz13]

Sorry. There should be 9v between 4 and 8. Like you say, 4 is ground on a single sided PS.

 

[temol]

Wiring looks ok.

I don’t have an audio probe yet. Building one…

Are you still building?

You could also measure the voltage between GND and the IC pins (1,2,3,5,6,7), but I think you will find the problematic place faster with an audio probe. Check the output of each opamp.

 

[Rpschultz13]

I’m making a Scientific Guitarist test box, still need a few components. How would I make a super simple probe. Done I need a cap or resistor in series?

 

[jimilee]

I’m making a Scientific Guitarist test box, still need a few components. How would I make a super simple probe. Done I need a cap or resistor in series?

Cap.

 

[Rpschultz13]

Cap value?

 

[temol]

Misc > Audio Probe : DIY Fever – Building my own guitars, amps and pedals

[DIY Fever] Audio Probe - very simple contraption that helps with debugging audio circuits

diy-fever.com

 

[Rpschultz13]

DC Voltages: chips in

IC1 bottom
1: 4.6
2: 4.6
3: 4.2
4: ground
5: 5.47
6: 5.47
7: 5.47
8: 9.2

IC3 middle
1: 8.6
2: 8.6
3: 8.6
4: ground
5: 4.5
6: 4.6
7: 4.6
8: 9.2

IC2 top
1: 8.6
2: 8.6
3: 4.8
4: 5.3
5: 4.6
6: 4.6
7: 5.5
8: 9.2

 

[Rpschultz13]

IC2 shouldn’t have voltage on 4

 

[temol]

DC Voltages: chips in


IC3 middle
1: 8.6
2: 8.6
3: 8.6
4: ground
5: 4.5
6: 4.6
7: 4.6
8: 9.2

IC2 top
1: 8.6
2: 8.6
3: 4.8
4: 5.3
5: 4.6
6: 4.6
7: 5.5
8: 9.2

Is there a continuity between IC2 pin 4 and circuit GND?

 

[Rpschultz13]

The followers all check out… pins 1,2 and 6,7 are all common EXCEPT IC2.2 which has some pots between 6,7 and reads appropriate resistance.

 

[Rpschultz13]

Pin 4 is common with ground on all 3 op amp sockets

 

[Rpschultz13]

I think I got it. I noticed that continuity between ground and pin 4 on IC2 was a fair bit slower than IC1 and 3 (they were instant). So I pealed back the Punch pot and reflowed pin 4 on the socket. It looked fine, but that seems to have done it. I now have sound. Hopefully when I re-wire the 3PDT it will all work as it should!

Thanks for all the help!
First time I've experienced a bad solder joint. So weird, it looked fine.