SOLVED Maleficent Mids weirdness

Mike52

Active member
This may not be a troubleshooting thread at all, but its more more of a gap in my understanding. Bear with me. I hooked this up to an Auditorium and after the pedal failed to work in effects mode I put an audio probe to it and got signal pretty much from the volume pot all the way to C1. I was satisfied that the signal path could work, but it was choosing not to do so. Voltages looked very weird.

NE5532 (9.37v)
1. 7.48
2. 7.42
3. 7.48
4. 7.99 (!!!)
5. 7.28
6. 7.67
7. 8.49
8. 9.13

TL072 (9.37v)
1. 7.27
2. 7.30
3. 7.27
4. 0.00
5. 7.27
6. 7.28
7. 7.08
8. 9.13

None of that made any sense at all, and I wasn't totally trusting the Auditorium board so I switched over to the breadboard to test the power rail. (voltages are just different there, different supply). With 8.6v in I was getting 8.35v at VCC and 6.68v at VRef (!!). I duplicated the circuit on the breadboard itself real quick and it behaved normally, with Vref being half of VCC.

Eventually I noticed that the ground wire was stranded (and kinda floppy) while the +9v wire was solid core. I swapped out the ground wire for a solid core wire and all my voltages settled into where they are supposed to be. I took a quick reading of the TL072 before going to bed (it was past midnight, too late to try to get sound) and it was a very different story.

TL072 (8.11v)
1. 3.82
2. 3.82
3. 3.82
4. 0.0
5. 3.82
6. 3.82
7. 4.08
8. 7.84

Like I said, I haven't had a chance to wire this into the Auditorium and see if I get a signal yet so I may not have a problem at all. But I don't understand this behavior. I get that a bad ground is going to make things off, but it seems like the power rail is just math. Voltage dividers will divide the voltage no matter what. And I was getting ground before swapping the stranded wire out, there was continuity and the readings were consistent at various points. Can anyone explain this?
 
Oh, here's the power section. Pretty standard stuff, it looks like. Although C102 seems unusual, I don't recall seeing a second capacitors arranged that way before.


power.png
 
We don't see double filtering like C102 every day in pedal circuits but it's pretty common in consumer electronics. (Televisions, DVD players, etc)

There must be a connection issue of some sort... damaged trace, bad solder, loose IC in socket, etc.

If Pin 4 of the TL072 measured 0V then Pin 4 of the NE5532 should measure 0V as well. A bad connection is pretty much the only thing that could cause them to not be equal. Nothing on the Auditorium (nor any defective or incorrect component on the Maleficent Mids PCB) could cause that.
 
We don't see double filtering like C102 every day in pedal circuits but it's pretty common in consumer electronics. (Televisions, DVD players, etc)

There must be a connection issue of some sort... damaged trace, bad solder, loose IC in socket, etc.

If Pin 4 of the TL072 measured 0V then Pin 4 of the NE5532 should measure 0V as well. A bad connection is pretty much the only thing that could cause them to not be equal. Nothing on the Auditorium (nor any defective or incorrect component on the Maleficent Mids PCB) could cause that.
Also pin 5 of the NE5532 should have been Vref on the nose, but it was showing higher voltages. Grounding schemes seem like voodoo to me, so I was hoping someone could explain it. Like I said, since changing the ground wire to the same wire type as the input power, voltages settled down. I don't understand why that made such a difference. Again, it was making a connection previously.

PS: Just remembered pin 4 of the NE5532 socket wasn't soldered. That was one of those late night discoveries that I quickly corrected and forgot about. Amazing what you overlook. That explains the voltage on pin 4, but not sure it explains the weird voltage divider behavior.
 
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