guitar circuit non isolated ground problem / short???

[thomasjjj]

hi guys, been building a few fuzz pedals lately and found a problem with a certain brand of power supply.

when the fuzz is on its own, or with a isolated multi power supply every things fine and dandy.

but, when its plugged into a multi power source where each output shares the same common ground / not isolated, its shorts the power supply out. im at a loss as to why this happens, as the input/output jacks are straight to ground, and the led is grounded through the chassis. only thing i can think is that ive soldered the led resistor to the cathode and that resistor goes to chassis round... could there be some voltage crossing due to a lower value resistor that means ground isnt 0v? a small voltage on ground / short here was the only thought the makers of the power supply had!

cheers, and thanks in advance for any help! thomas

 

[shrtrnd]

Somebody from your area will probably give you a good answer.
The way things work in the US is that there is a mains hot wire, a neutral return wire, and a ground wire.
What you're describing seems to me, to be (somewhere) a crossing/reversing of the hot and neutral wires.
I'd check to make sure each one of your main power inputs are matched hot or neutral.
Some equipment internal wiring shares the ground wire with some things in the chassis that (as you noted) are not isolated from ground. (Like input/output jacks).
I don't know where your problem is, but that seems like the most likely cause to my way of thinking.
Good luck, and I hope you get a more complete answer here from someone soon.

 

[brevor]

Some fuzz pedals using Germanium transistors have this problem, this is because they use PNP transistors and the polarity of the power connections are reversed. The "ground" of the Germanium pedal is connected to the positive of the power supply. Other pedals have the ground connected to the negative of the power supply. If used alone everything is ok, but if the 2 types of pedals are connected together it shorts the power supply.

 

[thomasjjj]

hi guys, thanks for the ideas.

unfortunately all the pedals are npn, center negative- which is a shame as this would have been the easiest option!

attached is my schematic, and only includes +vcc and gnd, so shouldnt be a problem, just struggling to upload a picture of my offboard wiring, will get it up soon as i figure how!

 

[shrtrnd]

If brevor's experience isn't your problem, I'd still be looking for an issue with a cross-over between earth ground and your circuit neutral. It may not even be on your schematic, it may be something plugged-in to your unit that you're not even thinking about. A couple of times I've been involved in troubleshooting where an input jack from some outside peripheral device was at a 'ground' potential different from the unit under test. When you use a DMM to troubleshoot, always use the same ground for all of your measurements, and then test the potential at each of your test points. Look for changes there.