Weird power issue... new to me...

[studiostud]

Hey guys,

I'm trying to troubleshoot a tube screamer pedal that I built for a friend about 4 years ago and it stopped working recently.  I'm encountering a new problem that I haven't come across before.  When the power jack is wired up like normal, the positive lug only registers about 40-50mv.  I checked the plug on the wall wart and it registered fine at 9.56v and then I disconnected the wires from the power jack and measured across the + and - lugs with the wall wart plug in and it also registered fine at 9.56v.  But then I wired it back up thinking maybe something wasn't soldered cleanly and when I measured again, it was back to 40-50mv on the jack as well as the +9v input on the PCB.  Is the circuit somehow sucking voltage away from the power jack?  If so, how is it doing this?  I've troubleshooted many signal flow issues in my day, but power and GND problems still get me sometimes.  Any ideas would be helpful and appreciated.  Thanks!


Jeff

[StereoKills]

Seems like you have a short from V+ to GND somewhere. Did your build include a reverse polarity protection diode? If yes, check/swap that first.

[studiostud]

Thank you for the quick response.  I had a chance to look at it again this morning and I'm still stuck.  I don't believe there is a polarity protection diode.  It is the basic tube screamer layout from tonepad and the only diodes listed are the clipping diodes.  This was my first build years ago and so I even bought the pre-made PCB from Francisco.  I did a continuity check with my DMM between GND and any points in the V+ path and everything checked out there.  Then I did it with every other point on the board and the only ones that beeped were the points that had traces connected to the GND path.  Then I did a continuity check between the V+ lug on the power jack and any GND points in the offboard wiring and everything checked out there too...     

So still stuck...  any other ideas to try?

[ChanchoPancho]

1. If you put a IC base then remove IC and check the voltage.
If not then measure every pin voltage.
2. Measure every pin voltage in the transistors.
3. Check your Vb network (47uF + 2x 10k)

[StereoKills]

I'd check the transistors, possible they have failed and are conducting much more than they should.

1. If you put a IC base then remove IC and check the voltage.
If not then measure every pin voltage. Hard to check voltages when there's a serious short.
2. Measure every pin voltage in the transistors.
3. Check your Vb network (47uF + 2x 10k)

[studiostud]

Well I figured it out.  The problem turned out to be more of an embarrassing noob mistake than anything.  When I built the pedal initially, I knew much less of what I was doing and I soldered in the IC socket backwards (although the IC was in there correctly since I followed the layout when I put it in initially.)  Well fast forward to now, when I do know more about all this, when I switched out the IC (which I think was the initial problem of why it wasn't working cause I've fixed the IC on it before that was blown by a bad power source), I didn't think to check the orientation and when I put the new one in, I followed the notch marking on the socket instead of checking the layout.     oops.

Thanks for the help.  I probably wouldn't have caught that if I hadn't spent more time snooping around the trannys and IC.


Jeff