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"Selected" components


Colin Herbert
 

Does 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.


Dave Wise
 

I agree with your assessment, Colin.

Dave Wise

This was an easy case. Other times it's hard to even come up with a hypothesis.
________________________________________
From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Colin Herbert via groups.io <colingherbert=blueyonder.co.uk@groups.io>
Sent: Wednesday, July 22, 2020 6:47 AM
To: TekScopes@groups.io
Subject: [TekScopes] "Selected" components

Does 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.


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.


Tony G4NGV <tonyvirago2@...>
 

I have seen the same. A specific oddball value R, in series with a pot!

On Wed, 22 Jul 2020, 14:48 Colin Herbert via groups.io, <colingherbert=
blueyonder.co.uk@groups.io> wrote:

Does 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.




Harvey White
 

Looking at any design with component tolerances, they can average out, or pile up one way or the other.  The idea behind the 500 ohm pot is to give a certain range of adjustment, say, ideally with the desired adjustment about halfway.  Selecting the value of the series resistor compensates for component tolerances in this build, and can center that range.

Harvey

On 7/22/2020 9:47 AM, Colin Herbert via groups.io wrote:
Does 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.



scm@...
 

Perhaps the resistor wasn't selected for value but for temperature coefficient. This might make some sense since it is a precision resistor with provisions made for fine adjustment during calibration. I haven't analyzed the circuit but there could be other selection criteria, as well (for example, noise or voltage coefficient, which might be critical in some circuits)


snapdiode
 

Well we have the list of checked tubes, I wonder if we have other lists?

Tubes are here

http://w140.com/tekwiki/images/1/14/157-_checked_and_tested_tubes_specs_and_procedure.pdf