Yep.
Will do, thanks. What does an intermittently-failing SPC diagnostic mean for actual measurements, and what do I do about it if it does fail occasionally? (Now that I've actually bought it, that is.)
Cheers
Phil Hobbs
Looking at my notes from when I was shopping (ended up with a TDS2014 refurb direct from Tek) the only ding I have on the 7xx series is that they only look at knob inputs between samples. The first time I used one I wondered why people left them set at 2.5k points instead of the max 15k points. Well, at 15k points you'll pull your hair out trying to dial in the settings. I also noted that the "shift" button for some functions was annoying and zooming after capture took too many steps.
-- Ben Jackson AD7GD http://www.ben.com/
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It's a compensation for the vertical accuracy of the scope, and should be run when the scope has warmed up for 20 minutes or more. From the service manual:
"Passing the signal path compensation confirms the signal path for all vertical scale settings for all channels. Passing the internal diagnostics ensures that the factory-set adjustment constants that control the bandwidth for each vertical scale setting have not changed. This internal routine is not an adjustment. It is a temperature compensation routine that compensates for the current operating temperature to optimize oscilloscope performance."
If it doesn't pass reliably, it means that the scope does not meet its vertical specification, especially when making lower voltage measurements. If it fails, there's not much you can do about it; I believe the 744A is out of support. I talked with a fellow from Norway Labs once who claimed that the usual culprit is the attenuators, but I had a TDS784A that was failing the test on CH2. I replaced it with a known-good unit, but it continued to fail. They claim that they can fix the acquisition boards, but it is not cheap! Tektronix never released schematics for the TDS5XX-7XX to the public, but I suspect that Norway Labs managed to get them somehow. There's another fellow named Hakan (sp?) who posts to the Tektronix Yahoo groups that claims to have the diagrams, but he doesn't seem to be willing to scan them.
Well, my TDS 744A arrived yesterday, and it's pretty nice. It passes SPC (though the log says it fails every few months due to clipping at
-5.119 divisions, which is well off-scale).
The display isn't quite as bright as it once was, but hey, all channels work, and it's a whole lot of scope for $849 including shipping. Score one for eBay, luck, and patience.
I also went ahead and ordered the 15-drawer 28x30x60 inch Vidmar cabinet, because with all my stuff in boxes I can't find a ruddy thing, and there are some PMT things I want to build right away. It'll be here in about 3 weeks, which will make both me and the second shift manager happier.
I got sniped out of a HP 3562A dynamic signal analyzer, but since my top bid was $750 I kind of expected that to happen. (My consulting practice is pretty new, and one doesn't want to get visions of sugar plums before Christmas Eve.) ;)
Tomorrow I'm driving up to Lawrence's to pick up the HP rack, so I'll have something to pile instruments in.
My basement is actually starting to look like a real laboratory. I'll post a photo or two in ABSE once it's in better shape. Now I just have to save up for an optical breadboard and a couple of lasers, and I can do real things again. It's really amazing--for $10k or so, when I'm done I'll be nearly as well set up as I was at IBM Watson Lab. (Not counting the picosecond tunable laser and that sort of stuff, of course.)
Cheers
Phil Hobbs
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