[R-390] More "bad" resistor comments and questions (a bit long)

[Tim Shoppa]

"Barry" <n4buq at knology.net> wrote:
> I'm in the process of replacing those 27k screen resistors.  I clipped one
> end of each one from the "Switched RF/IF B+ Line" side and measured the
> resistor completely out of circuit.  I'm seeing about 49k now whereas I was
> seeing about 39k before.  I was measuring from Pin 2 of the main IF chassis
> plug to the #6 pin of V502 and V503 so I should have had nothing but those
> 27k resistors in the line I was measuring so I can't account for the
> difference between the "then" and "now" readings.

These resistors when they "go bad" are often no longer purely ohmic.
(Giving you different resistances when measured with different meters,
or even in the other polarity.)

They may be incredibly sensitive to humidity and phase of moon too,
and just the heat of being unsoldered or the change in lead strain
from being clipped might change their values like you saw.

> Something else that's kind of strange.  There are 82k resistors from pin 6
> to ground.  With the other resistors now clipped (isolating the #6 pins from
> the rest of the IF module), I'm seeing about 76k for the values of these
> resistors.  The only other component in this equation are the 5000pf disc
> ceramic bypass caps also from pin 6 to ground.  Is it possible these bypass
> caps are showing some DC resistance (i.e. very leaky) and that's causing me
> to see some parallel resistance across those 82k resistors?  If so, then
> this may account for why the 27k resistors have nearly doubled in value over
> the years.  If those 5000pf bypass caps are that leaky, then they would
> cause excessive current to be drawn through those 27k resistors constantly.
> Does this sound reasonable?

82K to 76K is under 10%. Don't sweat it. I agree that this is opposite
the direction that carbon comps usually age.

Seems unlikely that a whole bunch of disk caps would go leaky in that
way. In my experience disk caps are more likely to fail open. (Or
in transmitter power stages simply burn up - most of my transmitters
have suffered some failure in the final compartment that consisted
of sparks flames and smoke!)

If you really have these pins open-circuit now, you might want to
take a megger and check out socket resistance, especially if it looks
like the socket insulation may be decaying.

Tim.