[time-nuts] HP 5328 PSU nightmare... Or stupid engineer, you decide...
bruce.griffiths at xtra.co.nz
Wed Sep 9 21:10:24 UTC 2009
Neither the linear regulators nor the switching regulators in the 5328A have
any explicit current limiting circuitry other than that provided by the
pass transistor current gain and the limited current available from the
pass element driver.
The only protection against long term load faults is provided by the fuses
at the regulator inputs. However these fuses don't protect against
shorted rectifier output filter capacitors etc.
Brent Gordon wrote:
> I'm not familiar with this particular instrument, but a standard
> technique for linear power supplies is to hook it up to a variac. This
> lets you turn down the line voltage so you can do some measurements
> without smoking the system.
> Douglas Wire - PUPCo Studios wrote:
>> Good day everyone and thank you all for hosting this wonderful community
>> and allowing me to participate. I have several HP5328 with the “really-
>> nice” newer 10811-xxxxx Oscillators in them. I have found while I have
>> used the good old gold trace reliable HP instruments all of my life, these
>> units have been especially difficult. The first unit the 4500uF
>> electrolytic’s went bad and produced essentially a dead short; an easy
>> enough repair for me to not only track down in minutes, but it only takes a
>> straight bit screwdriver to fix in seconds!
>> Now our second unit has been giving me fits and while I would agree 100%
>> with one of the posts I saw here about how well HP did not only with their
>> schematics, but also the wonderful troubleshooting flow charts usually make
>> repairs on any of their old units a breeze. Sadly I have a unit here that
>> is giving us fits! It is a PSU issue and not related to the Motherboard or
>> any of the cards as I tested it with everything unhooked/ unsoldered and
>> still got the same result. It is quite similar to what we see when we get
>> an old HP unit that has a fried cap and is darn near creating a short to
>> ground, but alas I simply cannot find the problem (I am sure it is starring
>> me in the face is and I just can’t see it…) What I am seeing is super
>> high current flow through the R1 (I believe, but HP’s every unit I have
>> ever serviced had.47Ω resistor, NOT a 22-Ω as is stated in the
>> schematic…) that leads to F1. The troubleshooting is complicated by the
>> fact that unless I want to smoke that heavy duty, relatively close
>> tolerance resistor, I cannot even check voltages anywhere for it will blow
>> the fuse or if I put a slow blow to try and catch some measurements in a
>> second or two, well that is not very feasible either.
>> If I had to guess, I would say it has either a cap that has fried, outside
>> chance of a transformer issue, or the way this thing reacts, pretty well an
>> effective dead short somewhere, but I will be damned if I can find the
>> problem anywhere. I replaced the bad and 4500uF caps as well as the
>> rectifier, wondering if part of it had blown with no change in its issues.
>> One cannot follow the flow cart to much of anything other than boxes that
>> say look for a short, but so many areas one tests even on a perfectly
>> working unit come clear down near the zero Ω point even when they are
>> operating correctly.
>> I apologize if 1) this is not a clear email that anyone can easily
>> understand and 2) I almost feel embarrassed to ask anyone for advice from
>> their practical experience, for I feel as If I should easily be able to get
>> to the bottom of this in a matter of minutes with the wonderful data HP
>> provides us all for these old workhorses.
>> So if anyone has run into a problem such as this in the past where working
>> the flow chart only yields No, No, No -> check for shorts and has any
>> advice for how I might logically proceed, or what in fact you have found
>> out in dealing with a similar problem, it would be of great help, as we
>> need this in-service ASAP, but I guess I just cannot see the forest for the
>> tress in front of me or something here… Any advise, suggestions would be
>> greatly appreciated.
>> I would like to become a more active participant here with all I can
>> contribute, which hopefully soon should be a lot as I am doing some
>> innovative timing and generation processes that I am relatively sure the
>> outcome and data from derived from it could be of great benefit to the TIME-
>> NUTS userbase here. Thanks and don’t be too hard on me for asking (what to
>> me sounds like a stupid amateur question) but I am either too tired to
>> reason correctly, or it is just one of those very pesky problems, that
>> hopefully someone has identified before and might be able to enlighten us
>> over. I am begiinign to wonder if a voltage regulator might be responsible,
>> but I am at a loss at the moment and have not had enough sleep to properly
>> think this repair through… Thank you again everyone!
>> Warm regards,
>> Douglas M. Wire, GED, FNA,
>> PUPCo Studios, PUPCo Research Group
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