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

cheater cheater cheater00social at gmail.com
Thu Apr 8 02:23:43 CEST 2021


guys this guy is just looking for a used scope

as long as it's not 15% out it's gonna be good

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

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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