[R-390] RF IMD Problem SOLVED!

Tue, 3 Sep 2002 14:05:00 -0700

My guess is the "bad" tube is oscillating.  Might be interesting to put it
back in and test with a GDO, a field strength meter or another receiver.

Good hint, though, as I still have a lurking IM problem with my receiver, or
did at last check.

73 de Bill, AB6MT
[email protected]

----- Original Message -----
From: "Philip B Atchley" <[email protected]>
To: <[email protected]>
Sent: Tuesday, September 03, 2002 12:54 PM
Subject: [R-390] RF IMD Problem SOLVED!

> 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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