Re: SUCCESS! The "sick" 475A is now the "fixed" 475A!



Well, my neural net seems to be quite open to calibration, but my eyes are another story

I have actually been running tests (quantitatively unsuccessful, but qualitatively enlightening) on the visual bandwidth of my scopes (feeding in a Z-axis signal and seeing how many discernible dots I can fit across the screen), and this has really driven home what the actual precision of the scope display is (it's not more than about 9 bits in either direction). I recall my father saying something similar about the 4014 terminals that he used: the resolution of the drawing commands was 4k x 4k, but the actual screen resolution was much lower (I don't recall the exact numbers, but i now expect that they couldn't resolve more than 512 dots across the small [12 inch?] screens, and probably not more than 1k across the large [20 inch?] CAD displays we saw once at a Goddard open house). By my calculation the 400 and 2200 series scopes aren't actually displaying more than 640x400 pixels, and only 320x200 resolvable, distinct lines. This severely limits the precision of any measurements you make by eye on screen despite the apparent smoothness of the waveforms.

I'm still a little mystified by exactly what concrete tasks I might need a volts or watts per frequency measurement for, but I'm happy to wait for enlightenment while I play with a new toy. I'm a very hands-on learner, even when I'm rigorously going through theory need to have concrete examples to test myself against and to illustrate academically presented topics.

I see that there were two SAs in the 400 series, one much more "analog" (and of much older manufacture) than the other, and at least two more in the 2700 series (they look like siblings to the 2200 and 2400 series scopes). They seem pretty rare on eBay, and are asking much higher prices than the comparable era scopes. At some point I will need to read through the manuals for the 491, just to see how the spectrum analysis was done in the days before microprocessors.

Everything I have learned since taking up this hobby has fed a keen awareness that, even when they are in perfect working order, your instruments are lying to you (if only by omission).

-- Jeff Dutky

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