[R-390] Popping noise problem

[Claudio Spiritelli]

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