[R-390] Popping noise problem

[James Miller]

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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