What I do to trace shorts on power busses is to apply some current from a few volt power supply with a limiting series resistor, then probe along the bus w/ a DMM or analog millivoltmeter. The smaller the measured voltage, the closer you are getting to the short. It's like a 4 terminal measurement, but requires little gear.
FWIW, -John ============ > Well, it will look like everything is shorted to ground if some major > power line on the PCB has a component that has gone really full sort. > You will only find this out either by un-soldering it (eventually) or > you use a high end ohmmeter that can measure really low ohms in 4-wire > mode with a kelvin probe. I use a HP 3456A but I'm sure there are > plenty of other instruments that fit the bill. Even a decent Fluke > meter won't work to show you this sort of problem as there are shorts > and very low resistance paths. When you can measure the resistance of > a PCB track your up and ready for this. > > Cheers, > Steve > > 2009/9/11 Douglas Wire - PUPCo Studios <[email protected]>: >> Hello and thank you all for chiming in on my topic; all excellent >> suggestions. I have been ¡°out¡± since I submitted that, so I just now >> was >> able to sit down and read through the last days digests. This is >> probably >> the 10th HP5328A that I have had to fix the PSU on, so it shouldn¡¯t be >> such uncharted territory! Funny thing about it; as I sort of eluded to >> and >> am now seeing 100% is that virtually everything is shorted ¡°dead¡± to >> ground, making any path from where the transformer leads the voltages >> out >> forward to a 0§Ù reading - that is why this is such a PITA; I was and >> still >> am having a hard time finding a starting point with ANYTHING that is not >> shorted to ground¡¦ I pulled all of the fuses and forgot that still >> allowed >> the 4500uF cap to see potential and had to listen to it sizzle. (How >> dumb >> was that???) Luckily I have a bunch of spares here to replace it with. I >> am >> guessing with how absolutely shorted out everything is, this is merely a >> case of me not seeing the forest for the trees; as something shorting >> every >> bit of the PSU shouldn¡¯t be that hard to find, darn it! I am going to >> pull >> the schematics and flow chart and go through everything one by one to >> try >> and find the ¡°end of the line¡± culprit (which there may be several in >> this situation) that are allowing a freeway like short to ground >> potential¡¦ I wanted to use one of the bench PSU¡¯s too to unravel this, >> but a quick run through with the 196 in the §Ù range tells me all that >> will >> happen is it will see the same short and current over limit will shut it >> down; just as the fuses and everything else failed when I plugged the >> unit >> in. Thanks for all of the suggestions. As mentioned, I have fixed nearly >> a >> dozen of these and never run into one this out of whack; which is why I >> posted. The offer to just swap for a working PSU board is tempting, but >> I >> really cannot afford anything at the moment and I should have nearly all >> of >> the components on this PSU laying around either in the parts bins or >> available from a scavenger board¡¦ Thanks again everyone, I will keep >> everyone up to date; it likely will be funny when I find the problem as >> something tells me it has to be a pretty obvious issue to short >> everything >> out together¡¦ It also looks as if someone has already swapped PSU >> boards >> in this judging by the solder joints and such as well as the coloration >> of >> the board is a slightly different color too¡¦ >> >> >> >> Warm regards, >> >> Douglas M. Wire, GED, FNA, >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > > > > -- > Steve Rooke - ZL3TUV & G8KVD > A man with one clock knows what time it is; > A man with two clocks is never quite sure. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
