Re: Tek 494AP VR board mystery


Sergey Kubushyn
 

On Thu, 15 Jul 2021, John Miles wrote:

I'm almost 100% sure it is just a snafu that happens all the time and was
quite widespread back then when CAD didn't exist yet but leave 0.00001%
doubt of missing something.
Agreed, that is very weird. Same with those resistors... usually you try to
minimize the number of BOM line items by consolidating resistors, but they
seem to have done the opposite here.

The LC BPF at the output of the A69 post-VR amp is also funky. On the
schematic, the inductors are 0.652 uH, 0.568 uH, 0.652 uH, 0.94 uH, 1.047
uH, 1.047 uH, and 0.94 uH, but in the parts list they are more
reasonable-looking values. Meanwhile, the capacitors are bog-standard 220
pF and 470 pF parts.

The filter could be explained by the engineering department getting a little
too carried away with their new computer, or maybe they just measured the
parts they used and transcribed the values to the BOM without rounding.
Likely true for the "1.047 uH" toroids, but it doesn't explain the others,
and nothing explains CR3068 as far as I can tell. I wonder if it's an IP
protection feature. If Hickok or Lavoie or somebody like that were to clone
the instrument, things would get awkward if they had to explain what that
diode was for in court. Similar to the practice of adding fictitious towns
to a map to guard against commercial copying.
I doubt that IP protection guess. Back then they didn't have CADs and those
designs were not just computer files. It was a bunch of paper drawings,
mylar tape PCB designs and so on. Not even starting on somebody willing to
make verbatim copies, it was totally impossible even if somebody decided to
do that. There was no way to just COPY something back then. Yes, one could
take the Tektronix schematics (that were not secret and were included in
service manuals) and make a copy of their instrument. You still had to go
through entire cycle of at least re-drawing their schematics, then do
mechanical design, PCB layout (with Mylar tape or something like that) and
much more. There is no way one could do this using just dumb monkeys -- it
would require engineers. And such things like that diode or multiplying BOM
lines is almost impossible to miss -- it would've been noticed and fixed in
the process. That is not even starting on their proprietary chips and other
stuff that should've been somehow re-implemented.

It is not like you can just blindly copy [stolen] design files and do
something with your own label as it is possible today. It was way more labor
intensive back then and it required brains. Brains were an absolute
necessity back then. It is a handicap nowadays but back then things were
totally different...

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