[R-390] Receiver Overload?

[Bill Smith]

----- Original Message -----
From: "matt parkinson" <mparkinson1@socal.rr.com>
To: "Bill Smith" <billsmith@ispwest.com>
Sent: Thursday, March 07, 2002 11:30 PM
Subject: Re: [R-390] Receiver Overload?


> Bill it still sounds like you are fighting a AGC problem.

I wonder.  I can turn the RF gain down, and recently adjusted the IF gain
control.  The intermod doesn't seem to be related strongly to either.  In
other words, the intermod is still there even if the RF gain is turned all
the way down, and the IF gain didn't make all that much difference, though
at higher levels the intermod worsened.  I am listening around 1070 khz.  I
don't know yet if the problem is aggravated around that frequency, but I
suspect it is.  By the way, I tried tuning at 1070 and also +0.070 (or
close) and experience the same problem, so it doesn't seem to be a problem
with the 1MHz RF position.

> I would do 2 things first take a look at the 2 mf cap which is the big one
on
> the IF deck and then make sure there is nothing wrong with the antenna
trim on
> the RF deck that will cause havoc with the AGC .

The 2 mf cap was replaced with a polyester mylar some time ago.  The
original caps were both leaky.  I was able to open the case of one of the
caps and fit the new 2mfd inside.

> That antenna trim has a lot to do with the AGC circuit and that will cause
your
> signals coming in to over load.
> I have serviced and refurbished many R 390a and I am currently working on
a 67
> EAC unit as we speak.
> It would be nice if you had spare decks that way you could isolate the
problem
> quickly .
> Let me know and the group know if you find the problem.

Not so far.  I inspected the fibre shaft, but it didn't look contaminated,
and the section where it fits through the chassis was clean.  The insulating
washer against the chassis looked ok also.

I then used a VTVM to measure the resistance of the AVC circuit.  Couldn't
find any leakage whatsoever (tested using a 100MOhm indication).  Also
rechecked the IF module but found nothing wrong.

Thanks for the suggestions!

73 de Bill, AB6MT
billsmith@ispwest.com

> Matt Parkinson
> KE6UOS
>
> 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
> > billsmith@ispwest.com
> >
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