[R-390] 3rd Mixer NOISE in R-390A.

[Jim Miller]

Some ideas::

(1) Bad resistor or capacitor around mixer circuit, or elsewhere in RF deck.
I have found the usual suspects to be screen resistors and capacitors,
cathode resistors, plate resistors...anything that handles "power".  They
will tend to change behavior as they warm up.  Your Rx seems unusually hot
(sensitive).  That could be a clue.  The gain of 3rd mixer or another stage
is abnormally high maybe?  Is AGC working properly in the RF deck?  Bad AGC
resstor or cap maybe?

(2) Loose screw or hardware, usually the screws that hold the tube sockets
or ground lugs.  They loosen with repeated temperature cycling.  Tighten
them all.

(3) Noisy or corroded switch contacts on the RF deck switch wafers.
Carefully clean with deoxit but don't get any on the wafer material
itself...it can act as a "load" in the high impedance circuits.  In fact,
AGC behavior can be reduced if you're too liberal with cleaners or
lubricants in and around switch wafers and the Ant. Trim gears.

Hope that helps.  73 N4BE




----- Original Message -----
From: "Phil Atchley" <k06bb@elite.net>
To: <r-390@mailman.qth.net>
Sent: Friday, July 11, 2003 3:08 PM
Subject: [R-390] 3rd Mixer NOISE in R-390A.


Hi.
I was going to align this '67 EAC before putting it in one of the listening
positions.  However, another problem has reared it's ugly head.  I mentioned
this briefly the other day but "thought" the problem went away.

When first turned on and it's "come to life" it is quite noisy, especially
of there are no signals to cover it up.  Any decent signal will cover it but
that's not good enough for me.  After the set has run awhile it seems to
abate.

I've traced it to the 3rd Mixer as pulling that tube OR disconnecting the
PTO (and shorting the jack to complete the cathode circuit) clears it up.
IF I pull the 2nd mixer it has no effect on noise level so it's between the
two units or in the 3rd mixer itself (noisy resistor or capacitor).  Since I
had to spend significant time and work in the PTO I thought that might be
the problem.  However, listening to it on an external receiver shows no
noise on the Oscillator.

Anyway, it looks like the RF deck is going to have to come out of this
receiver again.

WHAT I KNOW IT'S NOT!
1.  NOT a tube, all have been checked AND subbed.
2.  NOT the PTO, at least it 'seems' quiet and stable.
3.  Not the power supply filters, they've all been replaced.

QUESTION.  Has anyone else had problems with noise in this particular
circuit and if so what did you find bad?

And lastly.  This receiver seems to be exceptionally sensitive, even for an
R-390A.  Running the front end "noise test" (using line meter and peaking
the Antenna trimmer) it seems to be better than 10 dB.  This AFTER the
warm-up noise cycle has abated.

73 de Phil, KO6BB

****************************************************
*  DXing THE WORLD WITH THE FOLLOWING EQUIPMENT.   *
*  RECEIVERS:                                      *
*    '67 EAC R-390/URR Surplus Receiver.           *
*    YAESU VR-5000 Wideband,  REALISTIC DX-392.    *
*    Home built Longwave "Hybrid" CW receiver.     *
*    Others which pass thru or are waiting repair. *
*  ANTENNAS:                                       *
*    Coil Loaded 18.5 Meter Long Sloper.           *
*    Roof mounted Hustler 6BTV 6 band Vertical.    *
*    Scantenna Scanner antenna at 7 Meters high.   *
*  LOCATION: Merced, Central California.           *
****************************************************




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