Well, I've presented quantitative calculations explaining why capacitance matters in this circuit, and thus why series combinations are helpful. In rebuttal, you provide "but they might still have made a mistake." That's moving the goal posts, Chuck.
I am happy to concede (for the nth time) that Tek's engineers were not flawless. But the series resistors in the 475A are that way because of capacitance, not incompetence.
--Tom
--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
toggle quoted message
Show quoted text
On 11/25/2020 18:27, Chuck Harris wrote:
Just because they documented their fix for a mistake
doesn't mean it isn't a mistake.
You can gain more wattage in a pair of resistors than
you can in an individual resistor in the same board space...
If, you can use a little extra altitude to hold the pair
of resistors.
I have seen way too many brown burned tektronix boards
to ever believe they didn't make mistakes with heat.
-Chuck Harris
Jeff Dutky wrote:
Chuck Harris wrote:
So, I would guess that a smaller resistor was specified when the
board was designed, and it was found to get too hot
but this resistor pair is present in the schematics, not just for the 475A, but also in the early 475 service manual schematics. I know that's not really a refutation of your point, but it sure looks like they meant to do this from really early on.
The on-end resistors are clearly visible in the PCB images. I haven't opened up my oldest 475 to check the physical board, but I was planning to do that, and will report back what I find.
Also, I know that schematics do not necessarily precede the physical objects they represent, so they may not accurately reflect original intent.
I once worked at an engineering company where, as we were packing a large machine to be shipped to the client the lead engineer was taking each part and comparing it to the existing drawings, in order to find parts that had been modified (or completely fabricated) during testing and development. When he would find a part that didn't have a drawing he would quickly gin one up in AutoCAD before the part was packed and shipped. My impression, at the time, was that this was part of our contractual obligations to the client, but it occurs to me now that it may have been entirely internal; so that our people would be able to correctly reassemble the machine at the client site based on the engineering drawings.
