[R-390] Cathode Resistors in 1st and 2nd Oscillators in R-390/URR

[Barry Scott]

Not all the resistors are that out of spec.  Most are running within
5%.  I just happened to find these two in the crystal oscillator deck
that are excessively out of tolerance and will be replacing them.  I
_might_ replace the 68k and 82k resistors in the oscillator deck as
well; however, the 82k is quite buried but, perhaps, not impossible
with quiggs.

Thanks,
Barry - N4BUQ

On Sun, Nov 10, 2024 at 9:04 PM 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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