[R-390] Receiver Overload?

Fri, 08 Mar 2002 11:38:23 -0500

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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