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Re: First post - Hello and a question

Chuck Harris
 

Kuba,

It is simple, very few newbies can afford to buy top dollar
calibrated equipment.

I have had the luxury of living near a surplus dealer that literally
has gone through tens of thousands of tektronix, HP and other scopes.
Of the Tek scopes, virtually all work, or are only slightly broken.
Typically the failures are mechanical (someone broke all the switches
on purpose), or the odd failed tantalum.

Every 7000 series scope that I have pulled out of his pile has worked
near perfectly after I have fixed the bad tantalums. Most were in perfect
calibration. As a caveat, I never pulled out any that had been smashed,
or were incomplete. There were so many, why bother with the trash?

So, I can without reservation recommend to a newbie that 7603 he finds
on Ebay that shows a clear waveform on the screen. Particularly if it
comes from a seller that has a reputation for selling checked out stuff.

Use an unchecked scope? Nope, you will have to look at some basic
signals. The calibrator will easily tell you if the amplifier is behaving
linearly. How? Simple, step through the calibrator output values, and
watch where the square wave's tops and bottoms hit on the graticule lines.
Set a 1cm square wave, and use the vertical controls to move it from the
bottom of the screen to the top, noting the size of the square wave.
Trying different V/cm values on the same voltage square wave, and note
the size changes. It's not complicated, and it is not at all hard.

If you do these simple things, your scope will be good enough for audio
work, and most other work.

-Chuck Harris

Kuba Ober wrote:

I was talking about "aligning" audio circuits, e.g. adjusting operating
points of various stages, checking response, etc. Not about any sort of
RF work.

If I wanted to see a nonlinearity of a stage in an audio power amp, for
example, it'd be nice to believe that the scope's vertical system is
linear enough, etc. Same goes for step response: hard to do with a scope
that may well distort even a perfect square wave.
Anyone who is at all capable of making those kind of measurements
would surely measure the input square wave first. If it looks like
a square wave, then the scope is good enough for the task.
The input will look like a square wave even with a badly nonlinear vertical channel. The output "square" wave will then typically be much slower than the input one you fed to your system. With nonlinear vertical, the slower transitions on the output square wave will look distorted, and you may end up chasing ghosts, especially if the audio amp alignment procedure mentions e.g. "adjust Rxxx for output transitions to be smooth".
I just don't believe in using unchecked instruments, and a reasonable way to check a 7603 with plugins is to use the classic calibration trio in a TM503, plus a mainframe standardizer.
How on earth can anyone recommend using an unchecked, unknown scope to a newb is beyond me. Newbs tend to misunderstand limitations of instruments they use, so they are very likely to just blindly trust the trace, even if someone experienced would check things twice first. You know, things like using too much or too little of vertical deflection, not centering the signal and hunting differing rising/falling edge aberrations, and so on. It's just very easy to hit those on an uncalibrated scope methinks.
Cheers, Kuba
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