Quote:
I figured it was a short based on what cobaltmute told me in my thread - the LED lights up, but it flickers and there's only a tiny voltage on the 3V3 point while the VRM gets hot as hell. And if I can't measure resistances in caps to determine if they're busted, I'm unsure how to find the short, since pretty much all points prone to causing a short are caps connected to ground as far as I can see.
Here's some principles to guide you in troubleshooting:
1. The ground plane is the ground (sounds redundant, I know). In this case (sort of an unofficial standard), Ground is negative (-).
2. Traces that are not connected to the ground plane are of opposite polarity from the ground. That means they're positive (+).
3. Placing DMM probes connecting any two points of 1) the ground plane, and 2) a trace or a part of component connected to a trace - should measure some definite resistance, as dictated by the components in the path between the two DMM probes. I was going to say capacitors are exempt from this rule, but in actuality - you're trying to measure pads on the PCB vs. the ground plane. It won't matter if you measure a capacitor for resistance to the ground plane in this instance, because even a capacitor will have "some" resistance other than zero.\
3.a. To make this easy, pick a Ground point for one of your probes and simply keep it there. Anything connected to the ground plane should have zero resistance to this point. "OG" should suffice, for instance.
4. If you measure anything close to -zero- between two points as described in #3), then you have a short.
5. Try to follow the schematic for the GrubDAC:
http://www.diyforums.org/GrubDAC/GrubDACschematic.php. If the voltage regulator is getting super-hot, look for the circuit of the voltage regulator from the power-in point (the USB connection) to the regulator itself. That will be the circuit path for powering the voltage regulator. You may need to follow the output path of the regulator - from its output to the component that its supplying, but worry about 2nd, not 1st.
6. Once you have the schematic path pinpointed for your part in question (the regulator), try to make sense of the schematic with the actual PCB. Look at either the layout -
http://www.diyforums.org/GrubDAC/GrubDAClayout.php, or the board photos -
http://www.diyforums.org/GrubDAC/GrubDACboard.php, so that you understand how the circuit on the PCB goes exactly how the schematic shows it. IOW, if you've identified the path on the schematic for the voltage regulator, then look for the components in that path on the PCB. See if you can find the traces on the PCB that connect all of the components in that path on the schematic.
7. Measure the resistance of those component connections - with ground - all along that path that you've identified. The values are not important when reading resistance. What's important is whether you read any two points along that path, between a trace and the ground plane, that are -zero-. THAT'S YOUR SHORT.
Give it a try and let us know.