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


Dave Peterson
 

Jeff,
I bought a cheap function generator that I wouldn't take home to meet my parents. Much less brag about here.

However, it is a modern piece of technology that I think can outperform my old 465 well enough to tell it how to dance. Modern run of the mill digital electronics are running at several orders of magnitude higher frequencies than this old scope.

I'm currently using it to generate square waves to use in place of a time mark generator. From what I've seen so far from the calibration procedure, and I've done a little tweaking on my existing scope with an even more embarrassing source of square waves (a free function generator app on my cell phone), the primary purpose is to provide a time standard to adjust sweep timing and also geometry edges for rotation and "Geometry, R1442" vertical alignment with the graticule. So giving you clean square edges for aligning the X and Y axis of your trace. The 465 can just make out the vertical edges of around 10-20ns. Plenty good for 465 edges.

Other entries in the calibration procedure use the time mark generator to calibrate (check and adjust) the B-sweep. I'm not clear on the process yet. I'm sure there are others. What I've seen of time mark generator output look like pulses more than square waves. But the effect is the same - regular vertical edges. The cheap FG will do pulses too. Anything actually.

The point being, I think a decently competent square wave generator can suffice as a stand-in for the time mark generator. Standards for level and time are subject to suspicion. But again, my sneaking suspicion is that the cheap function generator is within the tolerances of what the 465 can measure. I continue to contemplate how to validate it's time and levels. But this is far better than nothing, for a lot less $$.

Calibrated? No. Tuned up? Hell yes!

Dave

On Monday, December 7, 2020, 04:39:39 PM PST, Jeff Dutky <jeff.dutky@gmail.com> wrote:

demianm_1 wrote:

for a scope a time mark generator and a leveled signal generator would cover most tasks
Yes, it's the leveled signal generator that seems like the most challenging thing to roll my own. I've read that the time mark generator can be substituted with a fast pulse generator (which I have) though I don't claim to understand how the time mark generator is used or how the pulse generator could stand in for it. At some point I will sit down, read the entire calibration process, and read the manuals for the relevant calibration equipment, just so I can make a somewhat more informed decision about this.

You can get a full set of 500 series plugins for calibrating a 475 on eBay for about $500, though there's no guarantee that they are complete or completely functional. I could pay for calibration for a couple scopes with that same money. I suppose I could buy the calibration equipment, do any repairs that appear necessary, and then pay to have THEM calibrated, and then I'd be set to do my own calibrations. I feel, however, that that would only be worthwhile if I were going to be doing this as something other than a hobby.

-- Jeff Dutky

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