[R-390] Popping noise problem

[James Miller]

My posts don't seem to be making it through, so I will try again:

James Miller wrote:
> 
> A follow up...
> On my 390a if I recall correctly the popping would NOT occur as much
> when the RF gain was cranked down.  It seemed to occur more when the RF
> gain was at maximum and there was a strong signal, which meant the diode
> load voltage was at it's highest magnitude...hence the coax insulation
> would tend to break down (causing a pop) when the RF gain was full up
> and a strong signal (AM or carrier) was present.  Claudio mentioned that
> he hears it when the RF gain is turned down (full CCW)...so maybe
> (hopefully) his problem is not coax related.  Just a thought.
> 
> Someone emailed me asking for me to discuss the procedure I used... So
> here it is...If it has been determined to be the coax that is bad, you
> can either attempt to thread new ones through the wiring harness or
> leave the bad coax as is in the harness and route the replacements
> separately, perhaps using tie wraps to secure them to the outside of the
> harness.  I chose to remove the old coax and thread replacements through
> the harness.  Believe it or not, I succeeded in doing this while leaving
> the lacing reasonably intact.  The replacement coax should be the very
> small stiff variety with smooth plastic cover (RG-174 I believe).  The
> stiffness and smooth casing helps in threading it through the harness.
> I used the old coax as a "puller."  Cut the old coax from the diode load
> terminals, and solder its braid to the braid of the replacement.  Keep
> the junction as compact and smooth as possible so that it will pass
> through the harness easily.  And you may need to use a lot of silicone
> spray to lubricate the harness and the replacement coax as you thread it
> through.  You will need to loosen or temporarily remove any cable
> clamps, and some, but not all of the lacing and insulating sleeves may
> need to be removed to facilitate movement.   Needle nose pliers, good
> eyesight and a lot of patience pushing and pulling are also needed.
> Alternatively you can remove all lacing, replace the offending coax, and
> then relace the harness with new lacing or tie wraps.  After getting the
> new coaxes into the harness and the ends positioned where they need to
> be, you will need to make the connections.  This is easy at the diode
> load terminal points and the limiter switch at the front panel.  But
> there are connections at the IF module connector that must be done
> also.  You will need to remove the cover from the IF module connector
> and replace the connections there.  In my case, I chose to replace all
> coaxes related  to the IF module connection to the diode load terminals
> and from there to the front panel switch.  My receiver also had the
> diode load test point on the front panel so this had to be accommodated
> as well.  All in all, about  a 2 day job, 2 hours per day for me.  Just
> be sure it's not a bad capacitor etc. first, to avoid this cable repair
> if you can.
> 
> Good luck
> Jim N4BE
> 
> Claudio Spiritelli wrote:
> >
> > Hello to all,
> >
> > I just discover a problem with my 390A. In order to ear the problem I must
> > do the following:
> >
> > RF gain .......... completely CCW
> > Local AF gain......completely CW
> > Limiter ...........OFF
> >
> > In this situation there is a "creaking" and "popping" random noise in the
> > speaker.
> > During normal operation is very hard to ear and only in presence of weak
> > signal.
> >
> > I tried to localize the source of the noise by "sectioning" the receiver.
> > The AF section does not produce the noise, as well as the RF section.
> > The source appears to be on IF section. I did section more deeply and I
> > removed the "Diode Load" jumper and the noise is still there.
> > This make me to believe that the source is between the "Diode Load jumper"
> > and the first AF amplifier, witch leaves only the "Limiter".
> > Analyzing the noise with a "scope", I see the noise as a very short but high
> > spike on the "signal line" in the "limiter" section ( actually when the
> > Diode load jumper is in place, the noise propagate backward to the detector
> > as well).
> > On the B+ line , I see low frequency variation ( with the scope at is
> > maximum sensibility of 1mV/div) witch I think is the "filtering" trying to
> > bypass the spikes, so I am assuming that the noise is not coming from there.
> >
> > When I turn the limiter on, the noise disappear on the audio as well as I do
> > not see it with the scope and the B+ line does not have any low frequency
> > variation. That made me to believe that the capacitor C536 was the cause.
> > Wrong.
> >
> > There are not to many other components in that section that can cause such a
> > noise and after I replaced the tube ( 5814A), I did run out of options.
> > One more piece of information, it appears that the noise goes away after the
> > receiver is on for more the 1 hour.
> >
> > Unless my analysis is wrong and the noise has different source, I need some
> > help from the experts.
> >
> > Thanks for all
> >
> > Claudio Spiritelli
> >
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