[R-390] RF IMD Problem SOLVED!

Tue, 3 Sep 2002 19:54:18 +0000

Hello all.
Well, today I decided that I was going to "solve" the IMD problem that I
was experiencing, come H#!! or high water (and that's not swearing)! 

So, I pulled it out of the listening position and put it back on the
repair table, no easy thing since I "nearly" pulled my back out on Dons
unit yesterday.  

First thing I noticed is that I only had the IMD problem when using the
sloper 60 foot long sloper antenna (with 2 loading coils), not on the
vertical.  IF a receiver has ANY IMD problem at all it will show up on
this antenna as it is resonant at 1550 and I have two strong local
stations on 1480 & 1580kHz, so that didn't surprise me any.

First thing I did was check the RF gain control action.  While it lowered
the gain of the receiver it had no affect on the IMD at all, things were
still a jumble at the high end of the BCB.  So, I figured, AHA!  It is
being generated BEFORE the first RF amplifier tube.  I disassembled the
relay, cleaned it and it's connectors with De-Oxit D5,  No affect. 
Cleaned the tube socket pins of the RF amplifier and first Mixer, no
affect.  ALL wafer switches, coil socket pins and tube sockets had been
previously cleaned and Caiged while the RF deck was out.  

NEXT, out of desperation I changed the RF amplifier, 6DC6 tube with one
borrowed from Dons Receiver.  EUREKA!!! IMD totally gone AND the S meter
moved from 80dB to fully pegged on my local station (as other R-390A's
have done in the past)           

What I DON'T understand is this.  These two tubes checked IDENTICALLY in
the B& K emission tester, both checked IDENTICALLY for gain in my
receiver at 21MHz.  Neither test shorted or gassy!  RF GAIN CONTROL had
no affect on the IMD.  What really messed up my thinking was that since
the RF gain control didn't affect the IMD the RF amplifier must NOT be
the culprit.  

YOUR THOUGHTS ???

So anyway, while I have it out on the table I'm going to go ahead and do
the critical "Nit Picking" alignment that I usually don't do until a set
has been in operation a week or more.  That'll save pulling it out again.

73 de Phil  KO6BB
Loving home provided for wayward Boatanchor Receivers
[email protected]
Merced, Central California
37.18N  120.29W  CM97sh

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