[R-390] When to replace carbon composition resistors (CCR)

[Jim Whartenby]

It is my understanding that after the CCR was manufactured, it went through a sorting for tolerance.  All resistors that were within 5% of the mean value got the gold stripe.  The silver stripe was given to all that were above +/- 5% but at or below +/- 10% of the mean value.  The remaining were given no stripe if they were within 20% of the mean value.  So there are no 10% resistors that are within 5% of the mean value unless they have drifted to this value over time and temperature.

Jacques makes a reasonable suggestion that resistors that are 20% or more above or below the mean value are suspect.  The accumulative effect of component tolerance, "the tyranny of numbers," as it was described in the mid 1950s, was the downfall of tube based electronics. The more complex the circuit, the more prone it was to a failure.  This led to the military specifications for all types of circuit components from vacuum tubes to resistors.  

I believe that all of these military component specifications have been rescinded such is the quality of modern components.  So if one is inclined to replace a CCR, the better resistor to use is the carbon film resistor.  They are more stable in value and less affected by time and temperature.

One last thing, the least reliable component is the vacuum tube followed by the capacitor.  I will always check tubes first, follower by a visual inspection.  Capacitors are eyeballed next followed by resistors.  Then it is time to troubleshoot.

Regards, Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Sunday, November 10, 2024 at 09:05:07 PM CST, Jacques Fortin <jacques.f at videotron.ca> wrote:   

 Hello Barry,

IMHO, if all the resistors in your RF deck are at ~20% higher than their
marked value, it can be well the cumulative tolerance of all those making
trouble more than only one individual value.
Sorry to deliver the same message again, but every part which is not 5%
close to the stocklist value should be changed.
The Solid Carbon resistors may also develop a funny behavior in the sense
that they measure a given value when they are cold which drifts to a higher
one when the current passes thru.
Nothing that was used in a R-390 at the build time was expected to be still
OK 70 years later...

73, Jacques, VE2JFE in Montreal

-----Message d'origine-----
De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
part de Barry Scott
Envoyé : 10 novembre 2024 20:56
À : R-390 Mailing List <r-390 at mailman.qth.net>
Objet : [R-390] Cathode Resistors in 1st and 2nd Oscillators in R-390/URR

I mentioned that the screen/plate resistors in my R-390/URR are a bit high.
Today, I removed the oscillator deck and measured those.  The 68k is about
71k so not too bad.  The 82k is at 87k so a bit worse but I'm still not sure
how much that affects the performance.

What I did find is the 150-ohm cathode resistors are both at 190 ohms.
I presume that may be the primary cause of the low output for both of the
oscillators.  The 2nd mixer has a bit lower voltage at the test point than
does the 1st mixer so that 82k may still need to be replaced although it is
still in tolerance.

Thanks,
Barry - N4BUQ
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