Troubleshooting 'shorted' power rails

And RF oscillation creators (only in circuits designed by other people, naturally). "Amplifiers oscillate, oscillators won't".

At 0.5V my fault would only suck milliamps. Remember, it was a set of diode junctions.

At least two of the rails so affected had 100's of mV saw-tooth waveforms on them, some sort of UJT-ish parasitic relaxation oscillators being formed somewhere.

Not mine to fire. But it was Digikey's wrongly-labelled bags that occasioned the problem. Digikey has committed the sin of being so good that people don't think to double-check them.

Cheers, James

30 years ago, I tried to TDR a 10 layer board with multiple power/ground planes. Didn't have much luck locating the short, but I did find that I needed more bypass caps.

Borrowed a thermal imager and found the short in short order.

One issue was normalizing emissivity. Somebody suggested spraying the board with foot powder. That worked GREAT. Problem was that I couldn't get it off, so the board was unusable.

If foot powder worked then there must be something else that would. Maybe a sheet of some kind of paper on top. Put some fans under the board to suck the sheet down.

the version I heard 40 years ago is even more pessimistic..

Amplifier oscillate. Oscillators oscillate on more than one frequency

I use a variation of that technique, power the board with a fixed voltage and watch the current as you warm suspect parts by wafting the soldering iron about near them. I've often found problem parts like this

- usually power and ground pins swapped but occasionally blown or faulty parts.

I once managed to get a processor, with swapped power and ground pins, to work by disconnecting them all, making a copper foil ground plane on top of the chip and connecting all the power pins with nice thin bare copper wires. Once was enough !

MK

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