Re: 475A Chop Blanking and a Theory Question

Tom Lee

Hi Dave,

RE: 465, 475 and 485

The 485 actually predates the other two, and is a very different animal. There was very little cross-pollination between the 485 team and the folks who did the other two. There are several nice features of the 485 that I wish were more commonly offered (e.g., a built-in fast pulse gen, and two levels of input protection that make it hard to blow up the front end). Every time the red light goes on, I know that I owe John Addis another beer, 'cuz he's just saved me from a blown scope for the nth time.

The 465 and 475 had some personnel in common, and a lot of informal collaboration. Those two scopes are similar enough that a good understanding of either of them will take you pretty far in understanding the other. Lots of stuff in common, too, making organ transplants feasible in more than a couple of cases.

-- Tom

Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070

On 12/18/2020 16:09, Dave Peterson via wrote:
There are too many dimensions to all of this!
A) Ha-ha - ya got me! You don't have to get the first edition. It's just the sight of the red cover conjures night sweats.
B) I was looking at the Bodnar FPG. For the $$ I just have too many fundamental things to get first. Like appropriate 10x probes. Send me off into a whole other tread: vertical system calibration and circuit function. I know what I need to do, but I need two proper 10x probes to measure the preamp output. And just to have an appropriate high impedance probe just for poking around. Alligator clips and banana plugs are not appropriate tools for circuit analysis. I could spend several oscilloscopes worth of $$ on just basic bench equipment. I'm trying to prioritize and pace myself. I honestly am considering building my own FPG. Reading Leo's origin thread on EEVblog gives me ideas. Might be fun.
C) Bandwidth, math, transistors, amplifiers and filters: I want to help, but you're quite capable on your own. I also remember wanting to really grok xtor theory, and after getting into it in school I recall the mental rungs on the ladder of understanding. Again, layers on layers: discrete component topologies as applied in 1970 are not synchronous with deep sub-micron CMOS circuits. I'm not as fluent in Tektronix topologies, but I also do recognize a lot of basic BJT configurations. But then there's Tektronix's weird schematics - relative to my experience. The experienced guys on this forum will give better answers, but I'm re-learning myself and remember "the mysterious black box" that was a transistor. The BJT wiki has some really good descriptions and pictures that jive with those mental rungs. Anyway, there's BJT and FET physics, bias topologies, small signal models and analysis, frequency domain analysis, transfer functions, feedback, op-amps, ... There are a lot of facets to the things being done in these scopes. Compartmentalization and experience. Layers and layers.
D) Sorry, after playing electronic tech for 4 years in the Army: 99% of if is mechanical stupidity. But divide and conquer is the methodology, and I'd say you've got it pretty much down. Not as sexy as it seems from the outside. BTW, I just killed and resurrected my cheap Chinese function generator. I just stopped working a couple hours ago. I walked away in a bit of disgust, but after 5 minuets I went back and unscrewed the cover. After poking around online I got the courage to turn it back on and start checking some basic things - power etc. When I noticed a connector partially in it's socket. That's all it took. None of the voltage probing had anything to do with it. Just that it brought my focus to the innards of the box helping me spot the loose connector. Back in business.
E) I'm just beginning to dip my toes into the Trigger and Sweep circuit descriptions and calibration procedures. One thing I'm observing is a difference between my two scopes' triggering of chop signal observation. I want to figure out why they're behaving differently. The "working" scope has a very stable trigger, the "parts" scope is being a bit finicky. Then it occurred to me it would be helpful to see the chop signal without the blanking. Hmm. Wonder how I could do that?! Thanks to some guy on the TekScopes group I know just the transistor to pull to make that happen. Interesting.
I recall my struggle to understand BJT function, and I sense you have a mix of understanding and uncertainty. I'm re-examining the fundamentals I've gotten away from since being a CMOS jockey. If there are facets of xtor theory and operation that are frustrating you, and you can articulate them, I enjoy helping. I might say stupid things along the way, but I enjoy blundering my way into understanding. Let me know if there's something tripping you up.
F) Here's another one: 465, 475, 485. These are all concurrent designs from Tektronix. Yet they're so different in so many ways, while still being the same in a lot of ways. Did they have some mix of independent, common, and cooperative departments working on these? E.g. Your 475 ALT/CHOP circuit is so different than the 465. Why? Doesn't seem that significant a circuit to have such different designs while being produced at the same time? What's the R&D rationale behind that? Similar things with the 465 and 485 - though I haven't had the chance to delve too deep into it. Just rhetorical thoughts to share. Yet another thread/dimension. I am having fun tho!
Hope you're having fun. I got to have the day off to play today, and hope you're getting to enjoy some time off too. Thanks for sharing your thoughts and experiences. It's helping me as well.

On Friday, December 18, 2020, 03:11:18 PM PST, Jeff Dutky <> wrote:
Because everything looks like a bandwidth measurement when all you have is a fast pulse generator I had been measuring the rise time of every scope I have to see if their bandwidth matched their specs, and before I fixed the CHOP blanking problem the bandwidth of the 475A was measuring as something like 175 MHz (rise time of about 2 ns), which seemed very wrong. After fixing the CHOP blanking problem, however, I went back and measured the rise time again, and this time I got a rise time of between 1.5 ns and 1.3 ns, which gives the bandwidth as 233-269 MHz, which seems about right for a 250 MHz scope.

I am at a loss to explain how fixing the CHOP feature could affect the bandwidth of the scope (especially since I didn't have CHOP engaged when I was measuring the rise time), and it's entirely possible that some amount of operator error may be involved, but, as I also measured the rise time of my father's 475 when I got the low bandwidth measurement for the 475A, and found the 475 to be spot on at 200 MHz, I suspect I made both measurements correctly (my notes, sadly, do not tell me enough about what how I set up the scopes to be sure).

I suppose that it's possible that another NAND gate in U340, which contributes to another part of the vertical amplifier system, also had a dirty pin that was causing a bad connection, but I don't see anything like that in the schematic. I'm much more inclined to believe that my boneheadedness (like leaving the 100MHz bandwidth limit pulled out), rather than a dirty pin on U340 was the culprit, but I'd be happy to be proven wrong.

-- Jeff Dutky

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