[time-nuts] HP 5328 PSU nightmare... Or stupid engineer, you decide...

[Douglas Wire - PUPCo Studios]

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