[R-390] Receiver Overload?

Fri, 08 Mar 2002 18:03:49 -0700

Me thinks you have a problem with the Balanced antenna input transformer...
Grease, dirt, a cold solder joint... or something in the capacitor... I've had
that happen before... try the "C" input and see if it is still present... the
transformers are very easy to remove if you need to check the guts... 73 de
Jordan...

Bill Smith wrote:

> ----- Original Message -----
> From: "matt parkinson" <[email protected]>
> To: "Bill Smith" <[email protected]>
> 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
> [email protected]
>
> > 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
> > > [email protected]
> > >
> > > _______________________________________________
> > > R-390 mailing list
> > > [email protected]
> > > http://mailman.qth.net/mailman/listinfo/r-390
> >
>
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