[R-390] Re: Them sneaky resistors

Mon, 07 Jul 2003 15:08:03 -0700

Hi Bill, Phil, all,

  Wow - same problem here. Just yesterday I had to replace a couple of resistors in the 
RF Amp of a National NC-98. One was a 470k control grid resistor and the other a 47k 
screen grid resistor. They were as open as a blown fuse. I examined them under a 10x 
magnifying lens and could see no signs of over-heating or any other kind of stress. 
Down with carbon comps <g>...

73 Ken kb7rgg

> Hi Phil
>
> I doubt you missed your failed resistor.  I recall recapping and checking
> resistors in a HRO-7R.  The set played fine, but died about two weeks later.
> I was rather surprised, but opened it to find several of the 470K resistors
> had opened up.  I replaced them and thought my troubles were over.  They
> were, for another two weeks or so.  To make a long story short, I replaced
> all the 470K resistors in that set.
>
> National seemed to use a supplier who provided particularly poor resistors
> (at least from a longevity standpoint).  I have found more poor resistors in
> those receivers than any of the other popular manufactuers.
>
> Interestingly, there was no obvious current or temperature change that could
> have affected the faulty resistors.  Most were in grid circuits, used to
> decouple the AVC line, for example.  Perhaps the shock of the temperature
> change when unsoldering associated bad capacitors started some sort of
> process.
>
> At any rate, it doesn't seem that unexpected that running your set will
> affect some of the components and some of them can take a while before they
> obviously fail.
>
> 73 de Bill, AB6MT
> [email protected]
>
>
> ----- Original Message -----
> From: Phil Atchley
> To: [email protected]
> Sent: Sunday, July 06, 2003 6:58 PM
> Subject: [R-390] Them sneaky resistors
> 
> 
> Hi.
> Since I've been waiting for the PTO and the capacitors to arrive so I can
> finish the overhaul of this R-390A receiver I've had time to spend a little
> extra time checking and re-checking components in such places as the IF
> module, audio module etc.  So far everything has checked in spec resistance
> wise.  Except that is for one resistor in the screen circuit of V508.  It
> should be 47K and was 104K.  Now, I went through that amplifier very
> methodically from one end to the other THREE times before finding it on the
> third trip through the module.
> 
> Now, either a gremlin crawled into that resistor between the second and the
> third check OR I somehow overlooked it on the first two passes, and I went
> through it with a fine toothed comb!  Since V508 is the AGC amplifier this
> could cause weak AGC action when I finally get the set finished.
>
> Moral of the story?  You can't check those old resistors enough times 8^)
>
> 73 de Phil, KO6BB