Diagnosing A PCB Power Problem – Probable Short

[Big O]

It has been awhile since I have posted anything of substance on this board, but I am asking for some assistance on a pedal I built awhile ago, if someone would be so kind to provide some input.

The other day I picked up one of my pedals that I haven’t played in awhile.  I plugged the power supply cable into the DC power jack and the pedal wouldn’t turn on after depressing the footswitch.  True bypass works fine.  The last time I used the pedal it worked fine, but it has been a few months.

I got out my DMM and checked the voltage coming from the power input jack and it registered only about 0.7V, and then measuring on the PCB board at the power input, I got only approximately 0.4V.  Checking the isolated power supply I was using showed that approximately 9.6V was coming from the barrel tip, so the problem was not the power supply.

I then placed a 9V battery (output measured 9.3V) into the battery clip and still got unusual low voltage readings.  I also noticed the battery getting very warm and I immediately deduced that there may be a short somewhere in the circuit.  This was confirmed when I discovered that the LED on the power supply at the cable origin turned off as soon as I connected the other end to the pedal – this is supposed to be a failsafe of the power supply that indicated a short somewhere in the circuit.

There is continuity throughout the course of the DC power to the board.  Power jack to 3PDT PCB, 3PDT PCB to Power Filtering, Power Filtering to Main PCB power input and then to Charge Pump all demonstrates continuity.  The LED indicator for the pedal was not lighting up, but tested normal using a Peak Atlas DCA-55 semiconductor analyzer.

Below is the power portion of the circuit that I have tested for continuity so far.  Continuity is the path in GREEN. The only things I found that were unusual to me was that there was continuity across two of the power filtering capacitors labeled C11 and C7 as seen on the diagram below.  There was no continuity across the other capacitors (C8, C9, C10) as I would expect.

I was wondering if the power filtering capacitors showing continuity across them may be the source of the short circuit.

It would be greatly appreciated if any of the experts that contribute to this forum have a good method for finding the location of the short on a circuit board.  And do you think it may be the C7 and C11 capacitors that show continuity across them?

[brd]

I’m not an expert, but I would always suggest starting at the beginning. In this case it is the power supply jack, assuming the wiring is correct, you want to check continuity between the posts, however, because a short anywhere in the circuit would show continuity between the + and –  posts you’ll need to disconnect the ground wire from the jack, then with the power disconnected, check continuity between positive and negative posts. if you you have continuity there then the jack is shorted. Next I suggest a voltage drop test. basically you  use your DVOM set to DC voltage and place the probes on either side of each wire and component along the power supply of the circuit. If at any point you get a voltage reading close to 9V, that component or wire or trace has a short. To do this you will need to reattach the wire you disconnected from the PS jack and attach a power source. Use a battery so you don’t risk damaging a power supply. and try to be quick. First measure voltage from the power jack + to the Anode side of D1. it should be a very small reading maybe 100mV. That would show no voltage was lost across that wire. Next do the same for D1, move the red probe to the Anode side and place the black probe on the cathode side. This will be a slightly larger voltage drop because it’s a diode. Then move the red probe to cathode side of D1 and the black probe to C11 (check both sides), then check the trace between C11 and C7,  and continue to both sides of C7, etc… (follow the schematic). After the charge pump test any wires, traces, and components that connect -VA to ground. remember at any point you get near 9V you found your short.

I hope that helps

[Author]

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