[R-390] Popping noise problem

[James Miller]

I had the same problem with my 390a.  Someone on the list suggested that
the miniature coax cables were experiencing a breakdown of the center
insulation due to aging.  After some experimentation, I discovered that
was indeed the problem.  After replacing the coax cables from the IF
module to the diode load connector, and from there to the Limiter
control, my problem was fixed.  Apparently the little coax cables begin
to break down with age.  Unfortunately this can be a tedious job!  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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