[R-390] Putt, Putt problem.

[Jim Temple]

Hello to the list,

OK, I feel that I should review the symptoms and my attempts to correct the
problem, so that everyone is still up to speed on the problem, as well as to
correct my misstatements.

First, I substituted a known good IF deck and the symptoms remained the
same, regardless of the IF gain setting, RF gain setting, and AGC or MGC
setting.

Second, I rotated the IF gain control (on IF deck) stop to stop during all
phases of the below symptom review, with no change in symptoms.

Third, I substituted ALL the tubes with different, tested tubes.  Are they
good tubes??  Can't say, except that the odds go down that the tube is the
problem.  In addition I cleaned and DeOxit'd the tube pins and sockets.

Fourth, the symptoms are on all bands, except 03 and below.

Fifth, the problem will peak with rotation of the antenna trim
control...band 03 and below will peak on internal noise.  (This may or may
not be significant, because of the gain characteristics of the various
bands).

Symptoms....

1.  In AGC position.
    a.  RF gain at 5.  AGC voltage 0.  Little rushing sound.  Receiver
apparently stable.
    b.  As RF gain is increased, background/rushing noise slowly increases
to what seems normal and strong.  As expected.
    c.  As RF gain approaches 9, the AGC voltage abruptly increases to about
11/2 volts, and the putt, putt/screeching  sound appears.  Increasing the RF
gain control beyond 9 will increase the AGC voltage to about 8-9 volts.  The
putt, putt/screeching sound remains as long as there is AGC voltage.
    d.  The carrier level remains essentially 0, until the putt, putt
begins, then the carrier level will increase as the RF gain control is
rotated to the stop.

2.  In MGC position.
    a.  The symptoms are identical to above, except the receiver blocks,
instead of putt, putting.  The carrier level will abruptly JUMP to full
scale.  As I understand it, the carrier level will increase with overload in
MGC mode.

3.  It is clear that the problem occurs at the same position of the RF gain
control, whether the function switch is AGC or MGC.  The only difference is
the receiver putt, putts in AGC and blocks in MGC.

It seems that the problem is an AGC problem, except that the symptoms will
not disappear with substitution of a known good IF deck.  The symptoms
remain with substitution of a known good IF deck.

Substitution of the RF DECK TUBES produces no change.  Individually removing
and replacing the mixer tubes and the rf tube removes the symptoms, but each
tube individually removes the putt, putt.  So no help here in isolating the
stage.

I have just received a suggestion to look at the 6DC6 stage, specifically
C227 (the metal cap on top of the RF deck).  I will let you all know how it
goes.

Thanks, guys, for all the super suggestions.  I'm still hard at work.  So
long for now.

Jim