[sdiy] HP 54602B selling on eBay.. DC vertical offsets on some channels or what ??

Florian Teply usenet at teply.info
Thu Apr 8 10:10:46 CEST 2021


Am Thu, 8 Apr 2021 02:23:43 +0200
schrieb cheater cheater <cheater00social at gmail.com>:

> guys this guy is just looking for a used scope
> 
> as long as it's not 15% out it's gonna be good
> 
Here I fully agree. I just wanted to point out that the all too common
view that calbration is some sort of magic assurance that any piece of
equipment is 100% free of issues is severely flawed. It is a very good
indicator that there probably is no obvious defect, but not a perfect
one.

And indeed, even if the scope was out by 15%, it's not necessarily
junk. It always depends on what you intend it to do. If you just want to
see the shape of a waveform in order to distingish between sine and
sawtooth, it doesn't matter if the numbers come anywhere close to what
has been spec'ed.
If you do relative measurements between two signals (direct amplitude
comparison, phase shift, you name it), even many very obvious
errors don't play any role.

And if you know that, say, amplitude readings are consistently off by
about 22%, then you can take that into account and work around it if
absolute numbers are required.

> just make sure he doesn't get something that's absolute shit
> 
> personally i'd go for a tek not a hp for scopes, hp scopes are just
> alien to many people i know
> 
Over here in central europe, Phillips scopes occasionally show up at
pretty decent prices as well. If cosmetics don't matter, there was a
time where the case color was changed from brown to light grey. From
what I've heard, quite a number of these share common subassemblies, so
even replacing faulty boards seems to be not too difficult between
devices of similar vintage even across different models.

But you probably don't want to go into fixing partially broken gear
before being able to use it...

HTH,
Florian

> On Wed, Apr 7, 2021 at 11:55 PM Florian Teply <usenet at teply.info>
> wrote:
> >
> > Am Wed, 7 Apr 2021 10:08:02 +0200
> > schrieb Roman Sowa <modular at go2.pl>:
> >  
> > > If you don't mind 15% error on a scope then it's fine, I get your
> > > point. It's easy to check a multimeter just by comparing with
> > > another one to see if it's way off. And everybody has at least
> > > few of them. Checking if a scope is more or less within specs is
> > > not that easy especially if old ebay scope is the only one in the
> > > lab.
> > >
> > > Calibration means it was fully checked and nothing's broken.
> > > Taking that to synth terms - once I have serviced some big
> > > polysynth, and it was quite OK when finished, but because not so
> > > perfect as I wanted, I went through meticulous 2-days long full
> > > calibration procedure. And only then I could find that there were
> > > still 2 or 3 faults, including a need to replace vintage chip
> > > being out of specs so much that it was impossible to adjust with
> > > trimmers. 
> > And here we enter "that depends" regime of things: I do agree that
> > usually the customer expects that equipment which has been
> > calibrated underwent rigorous testing and is performing to specs.
> > But this expectation is not always met in reality. In the end, it
> > all depends on what has been agreed on. Unfortunately, quite often
> > in the field of calibration strongly believe that it's so obvious
> > that they don't even consider actually discussing their
> > expectations but rather assume that there is only one possible way
> > of doing things. One example: At work we had for years some of our
> > microwave network analyzer calibrated only up to 18 GHz because the
> > calibration lab couldn't go higher. That's pretty pointless when
> > you're regularly use that equipment between 30 and 67 GHz and base
> > pass/fail decisions on the measurement result. But the guy who was
> > taking care of calibration before me didn't know (or didn't care).
> > Luckily I noticed before one of our customers did...
> >
> > First and foremost, calibration is just a comparison with a known
> > value. Calibration in strict metrological sense does not involve any
> > adjustment of the equipment being calibrated.
> > Think along the lines of a voltmeter which consistently gives
> > readings, say, 13 volts high. Or the abovementioned example of a
> > scope with 15% error. Calibration only gives you the knowledge,
> > that indeed on day X (the time of calibration) the readings were
> > off by whatever the actual amount of error is. But, assuming the
> > error is constant and known, you can correct your readings for it
> > to get a much better idea of the true value of what has been
> > measured. One example here would be one of our precision resistance
> > thermometers at work. It reads okay at room temperature, but the
> > readings are off once we deviate from room temperature, and have
> > been that way for at least thirty years. There are no trimpots or
> > similar inside the instrument which could possibly be adjusted. But
> > we have sufficient data to reliably predict the error and correct
> > the readings for it. Still during last calibration the calibration
> > technician refused to sign the calibration certificate at first
> > because the equipment was not meeting specs and he couldn't adjust
> > it. It turned out that at the cal lab where he worked before
> > handled things differently... Especially in precision metrology,
> > people quite often ask explicitly that adjustments are NOT to be
> > done. 
> > > All I'm saying, there's not much that can happen in multimeter to
> > > make it lie and not just go dead. But in a huge device like
> > > vintage scope with crazy high voltages, heat, airflow, early
> > > technologies stretched to their limits, and who knows what else,
> > > there's so many things that may drift in time.
> > >  
> > Umm, well, that depends on what you consider "lying" in that regard.
> > In recent years at work, I have seen quite a number of multimeters
> > which seemed to be working okay but nonetheless were not meeting
> > specs in various and non-obvious ways:
> > a) Readings were off by more than 10% in just one particular range
> >   a.1) across the whole range. Could be user error (using too small
> > a range for a measurement and therefore altering part of the
> >   measurement circuitry, mostly due to thermal overload)
> >   a.2) for values above a certain threshold. This was a nasty one to
> >   find: The multimeter in question had a hybrid resistor network as
> >   switchable voltage divider, which was supposed to be hermetic.
> >   Through a combination of factors this one developed a sort of
> >   conductive filament with a spark gap which shorted part of the
> >   resistor network when specific conditions were met: high humidity,
> >   voltages above a certain threshold and of course the right range
> >   selected. Even calibration didn't show this.
> > b) readings are off occasionally, but often okay. I had fractures in
> > such a hybrid voltage divider network, which caused open circuits in
> > parts of the network or not, depending on how the equipment was
> > bumped when placed on the desk, causing readings to be either 11%
> > high or good. c) all readings consistently of by a significant but
> > small percentage. That's quite often a reference drifting over time.
> > d) all readings off by a fixed amount, regardless of range. Could be
> > parasitic thermocouples due to corrosion and thermal imbalance in
> > the equipment...
> >
> > And there's probably more potential issues even simple multimeters
> > can have which are not obvious, and some of it even need a bit of
> > luck in order to be found during calibration, even if it is
> > performed to specification.
> >
> > HTH,
> > Florian
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