[time-nuts] HP 5328 PSU nightmare... Or stupid engineer, you decide...
Ed Palmer
ed_palmer at sasktel.net
Thu Sep 10 05:52:16 UTC 2009
Hal Murray wrote:
> [variac]
>
>> If you don't have one, you can wire a light bulb in series with the
>> power cord. Use 40,60,100 watt - whatever you need.
>>
>
> Neat. Thanks. That trick wasn't on my list.
>
> I think you can get lower wattage bulbs (at 120V). I think I've seen 25W,
> but I'm not sure. Old style tungsten night-lights are/were 6W.
>
Yes, and if your junk box goes back far enough, you may find some 120V
Christmas tree bulbs! Not sure what their wattage is. Basically, use
the smallest 120V (or 240V - depending on where you live) incandescent
light bulb that gives you reasonable current / voltage in your sick
circuit. You can also use lower voltage bulbs on the secondary side of
a power transformer if that works better.
>> The Variac is preferred because you may find that everything looks
>> good until you hit X volts. The light bulb obviously isn't that
>> selective.
>>
>
> I'd expect a light bulb to work well. If the DUT needs X volts to
> demonstrate the problem, it will act as a high resistance at low voltage and
> low resistance at high voltage. Light bulbs are (very) non-linear in the
> other direction. If you connect the two in series, I'd expect it to be
> stable at a balance point with some current, hopefully enough to debug things
> without burning anything out.
>
True, but with the light bulb you'd miss the fact that everything looks
good below X volts. That could be a significant piece of info.
> Here is another variation ... I'm assuming you are chasing something like a
> short on the power rail, probably a dead bypass cap. Things get more
> complicated if it's only sick rather than a solidly short.
>
> If you have localized the problem to one board, power that board from a power
> supply with a current limit knob. You may want to solder some wires onto the
> board rather than using the normal connector.
>
> Crank the current up until you get enough voltage drop to be interesting but
> nothing is smoking. Poke around with a volt meter. If you have traces
> rather than a ground plane, a few amps and a reasonable meter will localize
> things. Even with a ground plane, it will get you pretty close.
>
>
I've got a Jupiter GPS receiver that suddenly decided to start sucking
silly amounts of current. Never was able to find the short on that.
I've got a better meter now, maybe I'll put it back on my list of things
to fix. Sigh.
> I think I remember dunking one board in a pan of Freon(?). It was some
> liquid that bubbled at the shorted cap. That was ~30 years ago. Maybe I'm
> dreaming and confusing things with another story. It does seem like a good
> approach. Are there any good Freon like chemicals available/legal these days?
>
>
>
Freon was nice because it was non-conductive, evaporated cleanly, and
wasn't flammable. I'm drawing a blank on what would be a good substitute.
Ed
More information about the Time-nuts_lists.febo.com
mailing list