Re: Oscilloscope?

From Michael Caloroso
Sent Wed, Mar 4th 1998, 03:56

> Could those of you lucky enough to have 'scopes say a little about
> * what features are particularly useful/useless for synth play,

Get at least a dual trace.  If you're planning on troubleshooting
anything with CPUs, the four channel types with dual variable gain and
dual fixed gain inputs are good.

Choose your probes carefully, go for the ones made by the scope's
manufacturer.  10x probes reduce loading effects and are preferable. 
I've seen bad luck with cheap no name probes.

10Mhz bandwidth is good enough for general purpose analog
troubleshooting.  

In troubleshooting digital circuits, keep in mind that logic waveforms
are square waves made up of high frequency harmonics; insufficient
bandwidth will mask their true shape, and at worse, mask high frequency
ringing/overshoot/glitches that is kludging up your circuitry.  Go for
at least a 100Mhz bandwidth.

I could get into display modes, differential measurements, trigger
modes, coupling, delayed triggering, delayed sweep, proper grounding
techniques, avoiding improper loading, but this stuff fills a book.  You
can find these books at Barnes & Nobles and other fine bookstores. 
There might be online resources, I haven't made any searches.  Tektronix
& HP may provide primer books for novices; check for websites, I don't
know of any but I'm sure they're there.

> * which 'scopes are good buys, bad buys, worthless?

Tektronix and HP are good, I see them at just about any of the projects
I do for a living.  Nicolet is also very good but less common.  I can't
tell you the ones to avoid; maybe other AHers can share their
experiences with other brands that I don't know about.

Don't buy a tube based oscilloscope - you may be able to find them at
cheap prices, but if tubes go bad the cost will go up quick.  The
Tektronix tube scope variety require a special solder type that's hard
to find and difficult to apply correctly.  I gave mine away for parts.

Stick with solid state scopes.

And if you buy a used scope, make sure you have full deflection of the
beam, IE make sure the traces travel the entire range of the scope tube,
from left to right.

Ham Radio Festivals (Hamfests) are a very good place to find used
scopes, these take place all over the country.  Don't ask me for a
schedule or online info, 'cause I don't have that answer.  Ask your
local electronics store.

There's a magazine called Nuts-n-Volts with a LOT of ads from people and
surplus companies that sell used/refurbished scopes.  This is a good
place to look.  It's printed on paper the size of Computer Shopper. 
Check you bookstore or electronics store.  You might find info on the
nearest Hamfest in here.

FWIW, I landed a deal from a surplus place on a Tektronix 7834 Storage
scope 500Mhz mainframe, with (2) 7A26 vertical plugins, 7D15 Universal
Counter, 7B53A Horizontal plugins, 7D01 Logic Analyzer (with probes),
DF1 Display Formatter, and probes/adapters.  A very powerful diagnostic
tool.

Price: $400.

Catch #1: I had to sort through his pile of plugins to find working
ones.  When you buy surplus, there is no guarantee that these things
will work, the responsibility is yours.

Catch #2: the 7B9x horizontal plugins are actually the correct series
for this mainframe; the 7B5x series will work, but a mismatched delay
line means that on very fast sweeps you won't see the trigger edge.  I
took the thorns with the roses.

I did my homework & I knew what I was buying.  This series of scope was
over 20 years old but is very well built.

Sellers of refurbished scopes *do* guarantee 100% functionality of their
scopes, obviously for more $$$.

BTW, this *was* a clean deal.  Why so cheap?  All that equipment was
dumped by major corporations to purchase newer stuff, this happens on a
regular basis and has provided a good market for refurbished bench
instruments.

MC