Voltage in signal path?

[Kroars]

Alright, I’m stumped. It’s been a while since I couldn’t debug a build. I’m hoping one of you kind folks could help. I built two AionFX DZ4 (VH4 Preamp), one works great one does not. Did the usual continuity checks and everything checked out. Traced signal path with the schematic and started probing. Everything was fine until pin 2 of IC1. It was very, very faint signal as compared to the working build that had a strong signal on pin 2 of IC1. Started taking voltages and found I’ve got 12 volts running through nearly every pin of IC1 and IC2 and IC3….. Swapped out all IC’s for new ones, same issue. Checked the anode of D1 on both builds and the working one shows 0.0001 and the non working one has 10.51v at the anode of D1. I see no obvious shorts and after a few hours of probing, visual inspection, taking voltages and swapping components- I am at a loss. Hoping someone could pitch me an idea. Could it be that two of the traces are shorting spilling 12v into the signal path? Any help is appreciated.

https://aionfx.com/app/files/docs/dz4_preamp_documentation.pdf

 

[Cybercow]

I would try the following . . .
1) Lift one leg of D1 & D2 and test each. Replace if needed.
2) Remove all the three opamps (ICs 1,2 & 3) from the circuit and test the voltages at VA, VB, VC & VD and -VA, -VB, -VC & -VD to verify they are correct. (VB, VC and VD should all be the same voltage. VA would be just a touch higher.) Pin #4 of IC 1, 2 & 3 should read the same as VB, VC & VD respectively. And pin #8 of IC 1, 2 & 3 should read the same as -VB, -VC & -VD respectively. If any of the voltages are off, isolate and correct.
2) If all the voltages are correct, with the the 3 opamps still out of the circuit, power down and check for ANY continuity between the the audio pins (1, 2, 3, 5, 6 & 7) of all 3 opamps and a) ground, all b) 4 positive rails & all c) 4 negative rails. If any continuity is found between those opamp audio pins and Gnd, the + rails or - rails, isolate and correct.
3) If no continuity is found between any of the opamp audio pins and Gnd, the + rails & - rails, don a pair of magnifying lenses and under a strong light visually inspect both sides of the board for any possible cracks and/or scratches across any of the traces. If a crack or scratch across a trace is detected, do a quick continuity check across it. If a continuity test across a scratched trace is detected, create an appropriate jumper. And during the close visual inspection, look for any hairs, lint or other detritus that might be creating a short anywhere on the PCB.

D1 is supposed to cary the positive VA rail at its cathode and D2 is supposed to carry the negative -VA rail at its anode. Where D1 & D2 meet, at pin #3 of IC1A, should be zero volts.

That's all I have for now. At least, that's how I would start.

 

[Kroars]

I would try the following . . .
1) Lift one leg of D1 & D2 and test each. Replace if needed.
2) Remove all the three opamps (ICs 1,2 & 3) from the circuit and test the voltages at VA, VB, VC & VD and -VA, -VB, -VC & -VD to verify they are correct. (VB, VC and VD should all be the same voltage. VA would be just a touch higher.) Pin #4 of IC 1, 2 & 3 should read the same as VB, VC & VD respectively. And pin #8 of IC 1, 2 & 3 should read the same as -VB, -VC & -VD respectively. If any of the voltages are off, isolate and correct.
2) If all the voltages are correct, with the the 3 opamps still out of the circuit, power down and check for ANY continuity between the the audio pins (1, 2, 3, 5, 6 & 7) of all 3 opamps and a) ground, all b) 4 positive rails & all c) 4 negative rails. If any continuity is found between those opamp audio pins and Gnd, the + rails or - rails, isolate and correct.
3) If no continuity is found between any of the opamp audio pins and Gnd, the + rails & - rails, don a pair of magnifying lenses and under a strong light visually inspect both sides of the board for any possible cracks and/or scratches across any of the traces. If a crack or scratch across a trace is detected, do a quick continuity check across it. If a continuity test across a scratched trace is detected, create an appropriate jumper. And during the close visual inspection, look for any hairs, lint or other detritus that might be creating a short anywhere on the PCB.

D1 is supposed to cary the positive VA rail at its cathode and D2 is supposed to carry the negative -VA rail at its anode. Where D1 & D2 meet, at pin #3 of IC1A, should be zero volts.

That's all I have for now. At least, that's how I would start.

Thank you kindly!! I’ll report back this evening.

 

[Kroars]

I would try the following . . .

Alright, here’s what I found with all IC 1,2 & 3 out of circuit:

1) Both D1 & D2 read the appropriate 0.58v
2)VA, VB, VC & VD are all 12v
3) -VA, -VB, -VC & VD were wonky. They were all the same. Although, when I first tested -VA the voltage went from -2 or so and steadily dropped down to nothing. Now -VA, -VB, -VC & VD seem to be settled at -0.006v. I’m assuming that’s not where it should be

Edit: I do have continuity between pins 1&2 on IC1 and IC3. Not IC2. Also, there is continuity between pins 1&7 on DC1, the 12v power converter. Could it be a faulty unit? Looking at the power supply schematic, pin1 goes to ground and pin 7 is COM.

Thank you again for your help!! It is very much appreciated!

 

[PJS]

If you look on the schematic, pin 1 and pin 7 of the voltage converter are both connected to ground, so there should be continuity between them.

 

[Kroars]

If you look on the schematic, pin 1 and pin 7 of the voltage converter are both connected to ground, so there should be continuity between them.

Oh my lord, I completely missed that little guy right in the middle! Thanks

 

[Cybercow]

Alright, here’s what I found with all IC 1,2 & 3 out of circuit:

1) Both D1 & D2 read the appropriate 0.58v
2)VA, VB, VC & VD are all 12v
3) -VA, -VB, -VC & VD were wonky. They were all the same. Although, when I first tested -VA the voltage went from -2 or so and steadily dropped down to nothing. Now -VA, -VB, -VC & VD seem to be settled at -0.006v. I’m assuming that’s not where it should be

Edit: I do have continuity between pins 1&2 on IC1 and IC3. Not IC2. Also, there is continuity between pins 1&7 on DC1, the 12v power converter. Could it be a faulty unit? Looking at the power supply schematic, pin1 goes to ground and pin 7 is COM.

Thank you again for your help!! It is very much appreciated!

The voltages at the 4 outs for both sides of the voltage rails will be nearly identical. No worries there. I thought there might have been a slight voltage drop with the 10Ω resistors. But I realize they're just current limiters.

You've found the problem. Now to drill down to which part/s is/are bad on the negative side of the voltage supply, lift one leg of C33, C25, C27 & C29 and remeasure the negative rails. If they come back to about -12V, then one of those caps is bad. If the negative rails are still not where they should be, then DC1 is the prime suspect.

As for IC2 not having continuity between pins #1 & #2, it because it's not configured the same as IC1 & IC3. (See the schematic.)

And for what it's worth, DC1 has been selected with precision for its thru-ground construction. Meaning it will play well with DC bricks and negative center 9V wall warts.

Edit: Which part number are you using for DC1 and from where did you get it please?

 

[Kroars]

The voltages at the 4 outs for both sides of the voltage rails will be nearly identical. No worries there. I thought there might have been a slight voltage drop with the 10Ω resistors. But I realize they're just current limiters.

You've found the problem. Now to drill down to which part/s is/are bad on the negative side of the voltage supply, lift one leg of C33, C25, C27 & C29 and remeasure the negative rails. If they come back to about -12V, then one of those caps is bad. If the negative rails are still not where they should be, then DC1 is the prime suspect.

As for IC2 not having continuity between pins #1 & #2, it because it's not configured the same as IC1 & IC3. (See the schematic.)

And for what it's worth, DC1 has been selected with precision for its thru-ground construction. Meaning it will play well with DC bricks and negative center 9V wall warts.

Edit: Which part number are you using for DC1 and from where did you get it please?

Got it! I’ll try that and report back. I used the TEC 2-0922 from Mouser.

 

[Kroars]

The voltages at the 4 outs for both sides of the voltage rails will be nearly identical. No worries there. I thought there might have been a slight voltage drop with the 10Ω resistors. But I realize they're just current limiters.

You've found the problem. Now to drill down to which part/s is/are bad on the negative side of the voltage supply, lift one leg of C33, C25, C27 & C29 and remeasure the negative rails. If they come back to about -12V, then one of those caps is bad. If the negative rails are still not where they should be, then DC1 is the prime suspect.

As for IC2 not having continuity between pins #1 & #2, it because it's not configured the same as IC1 & IC3. (See the schematic.)

And for what it's worth, DC1 has been selected with precision for its thru-ground construction. Meaning it will play well with DC bricks and negative center 9V wall warts.

Edit: Which part number are you using for DC1 and from where did you get it please?

First off, thank you sincerely for your help on this one!! I wish I could buy you a beer!

So, I ended up checking the caps you mentioned. C33 & C27 were good. Although, both C25 & C29 were faulty. I replaced them and thought great, that was it! Went to check voltage on the negative rails and same issue persisted. I figured I’d run through the other handful of components coming off of pin 8 on DC1, based on your suggestion to check the caps. I was able to read the resistors without pulling them from the pcb and they were all good. As a last resort I checked the 10uH inductor (L3) and got a weird reading. Pulled it out, replaced it and BAM! -12v on -VA through -VD!!

Thank you again. Your time and knowledge is greatly appreciated.

Very Best,

Ryan

 

[Cybercow]

First off, thank you sincerely for your help on this one!! I wish I could buy you a beer!

So, I ended up checking the caps you mentioned. C33 & C27 were good. Although, both C25 & C29 were faulty. I replaced them and thought great, that was it! Went to check voltage on the negative rails and same issue persisted. I figured I’d run through the other handful of components coming off of pin 8 on DC1, based on your suggestion to check the caps. I was able to read the resistors without pulling them from the pcb and they were all good. As a last resort I checked the 10uH inductor (L3) and got a weird reading. Pulled it out, replaced it and BAM! -12v on -VA through -VD!!

Thank you again. Your time and knowledge is greatly appreciated.

Very Best,

Ryan

Good on ya. The idea was to approach it with a methodology. Shotgun troubleshooting is so inefficient and unrevealing. Using a processed method gives the troubleshooter a solid opportunity to learn. In my experience, inductors are so less prone to failure, it definitely would be last on the check list. Glad you found it.