[R-390] Receiver Overload?

Sun, 10 Mar 2002 15:16:33 -0500

I am ignorant of many of the differences between the 390 and the 390A, so if
this is a well known fact, duh..sorry to waste your time...........
My 390A measures about 1.5 meg from the antenna trimmer shaft to ground,
with the IF module unplugged (P112). A quick look at the schematic shows
that there's a 1.5 meg resistor (R234) connected between the trimmer rotor
and ground. The rotor also goes to the AGC line via a 270K resistor (R201).
With P112 plugged in, the resistance at the rotor drops to about 470K. I
have grease and stuff on the helical trimmer gears, and don't seem to have
any problems with AGC weirdness.

Pete.

----- Original Message -----
From: "James Miller" <[email protected]>
To: "Bill Smith" <[email protected]>
Cc: <[email protected]>
Sent: Friday, March 08, 2002 11:38 AM
Subject: Re: [R-390] Receiver Overload?

> This happened to mine once when I had inadvertently contaminated the
> insulated gear shaft on the antenna trimmer control.  The antenna trim
> capacitor (inside the can) rides on the AGC line at the first RF
> amplifier.  If the capacitor has contaminants on it (such as from the
> wrong kind of spray or moisture) or if oils or cleaner spray has gotten
> absorbed by the insulated gear shaft at the top of the radio, then it
> will drag the AGC down (at the RF amp).  This could be your problem.
> You should never lubricate the antenna trim geating or the insulated
> shaft.  If it has gotten oil on it, a good non conductive oil/moisture
> displacement spray will be needed (Like "Big Bath").  Even using the
> wrong kind of contact cleaner spray or Deoxit to clean inside the
> trimmer can has caused weird things on mine (with th AGC).  It is a very
> high impedance circuit so it doesnt take much.
>
> Bill Smith wrote:
> >
> > I wonder what experts think...
> >
> > I experience RF overload on the BCST band.  I think I mentioned same
when I
> > first obtained the receiver.  After detecting leakage from E207 on
several
> > bands (measured high resistance to ground when it should have measured
> > open), I replaced mica capacitors in Z202 and others and the problem
seemed
> > to go away.
> >
> > But now it is back.  Symptoms are (1) intermod on BCST band.  I wish to
> > listen to a Los Angeles station from the SFO Bay Area, but with the
antenna
> > hooked directly through a small balun to the balanced input, the station
is
> > obliterated.  If I feed the antenna through a lossy antenna tuner and
balun,
> > signal is heard fine.  Local stations will pin the Carrier Level meter.
Not
> > hard, but meter will extend beyond 100.
> >
> > I have measured many resistors in the RF section by testing between the
B+
> > and tube pins, all seem close to values listed in the schematic.  Same
with
> > screen and cathode resistors.
> >
> > I get 9-10 volts from the agc terminals with strong signals, so there is
> > apparently plenty of AGC voltage.  I attached a RF choke to the tip of
the
> > VTVM DC probe and verified the same AGC voltage is present at E206 and
E207.
> >
> > Does this sound normal, particularly the 100+ reading from the Carrier
Level
> > meter?  Otherwise, all seems normal (I have removed the IF module and
> > verified all seems well in that section).
> >
> > Any suggestions are most welcome.  I will next attach a strong signal
> > generator to the unit and get better measurements.
> >
> > 73 de Bill, AB6MT
> > [email protected]
> >
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