[R-390] Motorboating on R-390A

[James Miller]

Seems like I saw something like this in mine once, it appeared to be related
to AVC level.  A number of the tubes in the RF-IF chain are powered from a
180V (?) line which is unregulated.  As AVC action causes these tubes to
draw less or more plate current, there is a changing load placed on that B+
line.  The same B+ line also powers the mixers and maybe even the Xtal
oscillator (I'm working from memory here).  As that line fluctuates, the
oscillators could also.  However, in my case it turned out to be a bad mixer
tube loading down the oscillator that drives it... I think the 2nd mixer is
controlled by AVC also.  As it's operating point changed with AVC, it would
load down the output of the oscillator that was driving it causing the
oscillator to shift.  Replaced the mixer tube and it got better,  JM

----- Original Message ----- 
From: <R390rcvr at aol.com>
To: <r-390 at mailman.qth.net>
Sent: Saturday, January 22, 2005 11:34 AM
Subject: [R-390] Motorboating on R-390A


> Good day all:
>
> Nice weather here in Michigan, about 12" of snow, and soon to be 30 mph
> winds!
>
> I just put a Motorola R-390A up on my bench for the first time. The first
> sympton that popped up was   motorboating on all bands.   It occurs every
70-75
> KC, like clockwork. In some cases, it is louder with the BFO on. It
disappears
> with the antenna disconnected. The motorboating is 5-7KC wide, with a
slight
> peak in strength centrally. I have subbed every tube except the
rectifiers,
> 3TF7 and OA2 without any effect. If there is a very strong signal, it
isn't very
> noticeable, but in quieter areas of the band, its is quite loud. I haven't
> found any switch settings that effect it at all, except, not present in
Calibrate
> setting.
>
> I would appreciate any thoughts you might have.
>
> Thanks. I will be out shoveling for a while!
>
> Randy Stout
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