[R-390] 3rd Mixer noise, STINKIN' COSMOS PTO 8^)

[Phil Atchley]

Hi. =20

Well, I located the source of the warm-up noise in my '67 EAC.  When it =
was
first turned on it would snap, crackle and pop and just generally be =
noisy.
I'd traced it to the 3rd mixer stage.  So, I pulled the RF deck and =
replaced
all the resistors/capacitors in the 2nd and 3rd mixers as well as a =
"bulged"
2.2K resistor in the plate coil circuit of the RF amplifier.  That =
resistor
had measured 5.5K 8^(  I had checked the PTO by listening to its signal =
on
another receiver.  It appeared to be nice and quiet with no drift or =
noise.

After re-assembling the unit (I'm getting to be a whiz at the RF deck
removal/installation) I again checked it with no apparent change in =
status.
Still noisy at turn-on.  On a whim I disconnected the receivers PTO and
patched in the "parts" PTO, not bothering to mount it in the frame.  =
This
PTO while operational is really only good for parts as I've swapped out =
the
spring loaded linearizing disk, the heater wires are disconnected and =
the
output coil slug damaged.   After tuning the PTO to roughly the same
frequency as the receiver was tuned to (as evidenced by the local =
station on
1480KHz) I listened carefully. =20

EUREKA!!  Just the normal white noise or hiss of a normally functioning
R-390A.  To be sure I patched in the original, SNAP, CRACKLE POP!  On a
hunch I pulled the 6BA6 tube out of the parts unit and put it in the
original PTO.  EUREKA AGAIN!  No more snap crackle pop!

Now, you can ask, why didn't I test the tube earlier?  I DID, before =
doing
an alignment of the PTO and it tested good.  Checking it now shows a =
shorted
tube with VERY low transconductance!

THE SICKENING THING IS I AM PROBABLY NOW GOING TO HAVE TO GO THROUGH A
SEVERAL HOUR PROCEDURE TO SET ALL THOSE 48 LINEARITY SCREWS again. . .   =
 =20

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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