Tektronix 575 122C trimmer capacitors


Keith
 

Yet another odd question for my circa 1962 575 Curve Tracer. I could use some forum wisdom on this one.

Mine is a 122 equipped machine, but there are a few oddities in how my 122 features are implemented, at least as compared to the 1964 era instructions posted at Tek Wiki.

Most obvious is the absence on my machine of C239. This is supposed to be a variable capacitor 80-480 pF, located on the ceramic strips near where the AC power comes in - down by the fan wiring, power input, fuse holder area. (Tek part number 281-0044-00 is shown on page 5 of the Mod 122C Modifications PDF as posted.) In these instructions, this part has an asterisk, which says "Effective SN 101-4769 only", which I take to mean "you will have to add it to these S/N machines". But mine is S/N 57xx and it doesn't have this part either. So I'm confused...

Should I just put one in there and not worry about it? I do see some issues on the display with what could be that AC noise coupling though the secondary of the transformer - something Tek warns about. I also don't get a real clean line in some of the positions in the CAL procedure, especially the part when trimming C706 A - D. When I set up for that, I see a sort of Mobius loop of finer loops, as if it is some HF AC riding on top of the signal.

But I've checked my hot and neutral wiring, as well as the pin 16 and pin 14 stuff on the primary as well. All that looks correct. The only thing I see missing is this trimmer.

Thoughts? Cautions? Advice? I do have a reasonably correct form factor trimmer that is close (100pF - 500pF) Should I just slap it in there and not worry about it?

Thanks for anyone's assistance with this issue.


 

Keith,

The notation "effective XXX-YYY" means that only instruments in that range of serial numbers had the component, not that they lacked it and had to have it added. Instruments outside that range, like yours, lack the component and don't need it.

-- Jeff Dutky


Keith
 

Hi Jeff,

O.K., so I get what you're saying, but I am still struggling to make this make sense. Beyond the obvious answer for "...what was C239?" (duh...a trimmer capacitor) The deeper question is this. Why was C239 considered important enough to put on all the instruments from 101-4769, and then unimportant enough to be mysteriously removed without apparent further notice?

Where I look, C239 was NOT a part of the regular build documentation as published in 1961. At least, I can't find it in there. Source for this observation is Version 1 of the 575 Manual parts manifest and schematics, capacitor list, Sec 8-1, pub. 1961. This manifest of electronic parts makes no mention of C239. In addition, adjusting C239 was not mentioned in the 1959 calibration document either. However strangely, C239 definitely IS called out, both as being present and requiring adjustment in the "Company Confidential" document called "575 Mod 122C Sup" from 1967! I find that reference on page 5, top of page, pub. 1967. In that document it explicitly states that C239 was present "...from S/N 101 thru 4,7xx."

So, is it possible that C239 was somehow only mentioned in manuals published before 1961, and then just totally deleted?

The bigger question is, what did C239 do? Why did they put C239 in units 101 through 4769 and then stop? And, is it possible that Tek sold over 4600 of these C239 equipped units between 1957 and about 1959? Wow! That's a lot of curve tracers. Of course, it was the beginning of the space race and all.

Ah, the mysteries of Tektronix!


 

Keith,

I obviously don’t know the answer to your question, but I can speculate with the best of them: first, Tek often used SN 101 as the first instrument in the model line, so what you are seeing is not a part that was added after the fact, but a part that was there from the first instrument (i.e. the first instrument would be SN B000101, not SN B000001). In a case where the start SN was higher, however, I can imagine that they found a problem with the design after shipping some number of instruments and made an engineering change to correct for it. The subsequent change removing the trimmer could be because they replaced other components with higher precision/accuracy parts, so that the trimmer was no longer needed.

— Jeff Dutky


Dave Wise
 

They probably fixed it some other way in late instruments. Tek was constantly improving their designs, and sometimes this meant backing out an old fix in favor of a new one. You can buy scans of various instruments’ Mod Summaries from the VintageTek Museum. They’re interesting reading.

Dave Wise

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Keith via groups.io
Sent: Thursday, August 19, 2021 3:57 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Tektronix 575 122C trimmer capacitors

Hi Jeff,

O.K., so I get what you're saying, but I am still struggling to make this make sense. Beyond the obvious answer for "...what was C239?" (duh...a trimmer capacitor) The deeper question is this. Why was C239 considered important enough to put on all the instruments from 101-4769, and then unimportant enough to be mysteriously removed without apparent further notice?

Where I look, C239 was NOT a part of the regular build documentation as published in 1961. At least, I can't find it in there. Source for this observation is Version 1 of the 575 Manual parts manifest and schematics, capacitor list, Sec 8-1, pub. 1961. This manifest of electronic parts makes no mention of C239. In addition, adjusting C239 was not mentioned in the 1959 calibration document either. However strangely, C239 definitely IS called out, both as being present and requiring adjustment in the "Company Confidential" document called "575 Mod 122C Sup" from 1967! I find that reference on page 5, top of page, pub. 1967. In that document it explicitly states that C239 was present "...from S/N 101 thru 4,7xx."

So, is it possible that C239 was somehow only mentioned in manuals published before 1961, and then just totally deleted?

The bigger question is, what did C239 do? Why did they put C239 in units 101 through 4769 and then stop? And, is it possible that Tek sold over 4600 of these C239 equipped units between 1957 and about 1959? Wow! That's a lot of curve tracers. Of course, it was the beginning of the space race and all.

Ah, the mysteries of Tektronix!


Keith
 

Yeah, reckon so.

The plot has thickened a little bit as I've searched even more documentation on this pesky little trimmer capacitor.

Here's the latest: Specifically, in the 1964 published version of the field version of the Mod122C, the mysterious C239 capacitor is specifically mentioned. In fact, the part is specifically INCLUDED in the field kit for Mod122C. Even more, there is a specific instruction about how and where to add the part, including a drawing of where it goes!

See for yourself: https://w140.com/tektronix_575_mod_122c_modifications.pdf

pdf pages 5, 16, 17, 21 manifest, mentions, drawings, cal procedure for the use of the part.

This seems to mean at least one thing with a fair degree of certainty. On Scopes built before 1964 and not already Mod122C, Tek was expecting at least some of the field modified 575 to be missing this part. Thus, the kit made provision to add the part. That's why it is part of the kit as issued in 1964. Remember, by 1964 the 575 was well past the "#101-4779" notation.

Bottom line: In 1964, Tek thought C239 should be there on 122ModC versions.

So,

a.) C239 - it appears it was NOT present on some (or all?) 575 versions built before 1964.
b.) It appears that by 1964 Tek specifically ADDED it for 122ModC field modifications on such scopes.
c.) One can only presume that, in Tek's opinion, it is a necessary part for 575 122ModC, (or at least the field version of the mod)

So then why is it MISSING from my circa 1962 575 Mod122C curve tracer?

The only conclusion I can draw is that it was an improvement made after 1962 and not a deletion. Otherwise, I can't come up with a reasonable way to explain why on my '62 model, and on others I see posted on TekWiki, it is absent. But in 1964 field modification kits, C239 was clearly specified and supplied.

Great! now I'm torn...
Maybe I'll just put one in and see if it creates a time-vortex or black hole or something. :-)

Keith
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