>What can one do to avoid getting a lemon or broken
>or poorly calibrated, or whatever broken, oscilloscope
>at the local hamfest ?
As others have suggested, a seller who is willing to give you a
money-back guarantee is one possibility. Expect to pay for this privelege.
Otherwise, Be Prepared. If you are planning to look for a scope,
bring enough stuff with you to test one -- a signal generator of some
sort with at least semi-accurate amplitude and frequency, a 10x scope
probe, and a BNC cable (for direct 1x input).
The signal generator need not be a commercial item, and its frequency
and amplitude need not even be variable. A 1 kHz square wave at TTL
level (5 V p-p) is enough to get a good idea of the basic functioning
of most any scope, and can be built with almost nothing in a small
box that runs on batteries. Note that most scopes have such a signal
available for adjusting probe compensation, so you may need only a
probe and a power outlet.
If you want to get really fancy, build a little box with a 1 MHz CMOS
crystal-controlled clock generator and some decade dividers and
attenuators to generate signals from 1 MHz down as far as you want to
go at levels from 5 V p-p down to however low you want to go. Or
bring your choice of commercial signal generators (for example, a Tek
TM-50x cabinet with most any FG5xx, PG5xx, or SG5xx module).
All of this assumes that a power outlet is available, as one almost
always is somewhere at a hamfest. If your signal generator requires
AC power, bring a cube tap in case there is only one outlet. Bring a
"ground adapter" in case the outlet you find is not three-prong.
However: If you are not confident of your ability to tell whether a
scope is working satisfactorily given such a tool, then you need your Elmer.
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