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