Obsidius (B3k) dies after briefly passing signal

[enron_valdez]

Im having an issue with the Obsidius Preamp - when I connect it to power it comes on momentarily but then dies while letting out a high pitch squeal. I’ve swapped around the CD4049, the tl072’s and the j201s with little or no change. The resistors are standing because of spacing issues but I’ve made sure there aren’t any touching. Any idea why it could be acting this way? Thanks!

 

[PJS]

It sounds like a capacitor is charging up and when it gets to full charge it is stopping the power supply. I would be checking the power supply caps - for orientation, for the correct value and also to make sure they aren't shorted to somewhere they shouldn't be. Also it might be useful to hook your multimeter to the power supply and then insert the power and watch what happens over time. Measure the power and also the VREF supply.

 

[enron_valdez]

I swapped out the power supply caps, no changes. Double checked the values, orientations. The VREF that’s between the pins of the center 072 comes out to 3.15 but this doesn’t change when the signal is there or once it dies.

I uploaded a short clip of what is happening because I’m not sure I’m explaining it well. Thanks for the help!

 

[PJS]

Don't go swapping out parts until there is some idea what the problem is. Does the oscillation occur at all settings of all controls or is it selective?

 

[PJS]

Also, where did you get your J201s? They are notorious for being faked in TO92 packaging

 

[enron_valdez]

The oscillation occurs at all settings, but changes character based on the settings. I got these j201s from guitarpcb.com. I’ve used others from the same batch in prior diy pedals without issue.

 

[PJS]

So, with level and drive all the way down you still get the tone? What happens with level down, drive down and then shift the mix from side to side?

 

[temol]

You can use an audio probe and check all the opamps one by one - that way you can locate if it's a general power problem or a specific place where something is going on. Check the outputs of the opamps for the audio signal. You can also measure the voltages on individual pins.

 

[Chuck D. Bones]

Board needs cleaning. Pot connections are suspect; insulated wire would have been better because the bare wires may short to something when you install the board in the box. You may have a bad solder joint on one or more of the pot leads, can't tell for sure. Can't read any of the resistors because they're standing up. Disk ceramic caps are always suspect. I'd replace that 220pF with a SM or MLCC. Are the 22pF caps MLCC?

Before you change anything, do a thorough visual inspection. Make sure all of the socketed parts are secure and none of the IC leads are bent under.