[R-390] Them sneaky resistors

[Bill Smith]

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
billsmith@ispwest.com


----- Original Message -----
From: Phil Atchley
To: r-390@mailman.qth.net
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

****************************************************
*  DXing THE WORLD WITH THE FOLLOWING EQUIPMENT.   *
*  RECEIVERS:                                      *
*    '67 EAC R-390/URR Surplus Receiver with a     *
*            HAMMARLUND HC-10 Sideband adapter.    *
*    YAESU VR-5000 Wideband,  REALISTIC DX-392.    *
*    Home built Longwave "Hybrid" CW receiver.     *
*    Others which pass thru or are waiting repair. *
*  ANTENNAS:                                       *
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*    Roof mounted Hustler 6BTV 6 band Vertical.    *
*    Scantenna Scanner antenna at 7 Meters high.   *
*  LOCATION: Merced, Central California.           *
****************************************************




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