[time-nuts] hp5328A PSU stupidity...

[J. Forster]

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 <contact at pupcostudios.com>:
>> 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,
>>
>>
>>
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>
>
> --
> Steve Rooke - ZL3TUV & G8KVD
> A man with one clock knows what time it is;
> A man with two clocks is never quite sure.
>
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