[time-nuts] hp5328A PSU stupidity...

Steve Rooke sar10538 at gmail.com
Fri Sep 11 13:37:36 UTC 2009


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 0Ω 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 Ω 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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