Re: "Selected" components


Ray
 

From experience outside of tek,It is common practice to through a battery of tests and sometimes decide a particular resistor with a decade box based on engineering criteria, then solder in a resistor with the value needed. We somtimes put in a higher value then soldered in a paralell resistor to get where we needed to be.  RaySent from my T-Mobile 4G LTE device------ Original message------From: Colin Herbert via groups.ioDate: Wed, Jul 22, 2020 08:48To: TekScopes@groups.io;Cc: Subject:[TekScopes] "Selected" componentsDoes anyone know what criteria were used by Tektronix to "select" components? This may be a far-ranging question, but the specific instance that I am looking at is in an FG504 function generator. In the triangle wave generator circuit and connecting to U375, resistor R358 is described on the schematic as "selected", and in the electrical parts list it is described as "RES., FXD, FILM: 3.74K OHM, 1%, 0.125W (NOMINAL VALUE, SELECTED)". Since this resistor is in series with a preset variable resistor of 500 Ohm, what would be the purpose of selection and what would the criteria be? Maybe ensuring that the preset adjusts to the middle of its adjustment, more-or-less? I can see that 500 Ohms is close to 1/9th of the total resistance of the series pair.

These series resistances connect pin #8 of U375 to the -15V rail and the preset adjusts the "DC level" (it isn't achieving that function very well at the moment; all waves are offset and distorted). There is also a 316 Ohm resistor connecting pin #8 to ground. Similarly, on pin #1 of U375 there is another 316 Ohm resistor (R352) going to ground and a 3.83 Ohm resistor (not marked as selected) connecting to the +15V rail.

It would be nice to solve this dilemma, because on my FG504 someone has substituted what seems to be a 3K Ohms preset for R358 which is adjusted to ~610 Ohms. That all seems a bit too low.

TIA, Colin.

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