[R-390] Popping noise problem

[AI2Q Alex]

Hello Claudio:

Thanks for your posting. Recently, I had a 1958-vintage Collins KWM-1
transceiver on my bench. A block-style mica capacitor in the set's product
detector was operational, but noisy as all get-out.

In the KWM-1 circuit, in Standby mode, audio from the set's product detector
was fed into the set's AF amplifier string, along with sidetone when on CW,
and I could hear that puppy crackling away like mad in the loudspeaker when
I was transmitting, and of course it was "in the background" on receive,
although well masked by incoming signals.

I zeroed in on it by sectionalizing, as you're doing, using a signal tracer
and scope.

It was a satisfying feeling to clip it out and see all the noise go away.
Finding it was relatively easy; replacing it was tricky (very tight
quarters), but now all's well in KWM-1 land.

Good luck. You'll find the bad part--and then you'll be very happy!

== AI2Q, Alex in Kennebunk, Maine  .-.-.





-----Original Message-----
From: r-390-admin@mailman.qth.net [mailto:r-390-admin@mailman.qth.net]On
Behalf Of Claudio Spiritelli
Sent: Sunday, January 27, 2002 4:50 AM
To: R-390
Subject: [R-390] Popping noise problem


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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