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

[Jacques Fortin]

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
______________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:R-390 at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html