[Boatanchors] RCA AR-88 problems

[Richard Knoppow]

----- Original Message ----- 
From: "David Stinson" <arc5 at ix.netcom.com>
To: "Boat Anchors List" <boatanchors at mailman.qth.net>
Sent: Tuesday, June 04, 2013 1:35 PM
Subject: Re: [Boatanchors] RCA AR-88 problems

    I may have solved this. I investigated two things: the
bypassing of the screen grid, and the grounding of the tube
can. I found that the instability was reduced by moving the
bypass capacitor.  I should first say that most of the
grounds in the AR-88 are made directly to tabs on the
chassis so do not rely on screws or terminals. Secondly,
there is a shield across the edge of the 2nd RF socket.  The
original bypass capacitor was wired so that it reached on
the side of the shield opposite to the compartment with the
tuned circuits in it to the side with the 1st RF stage and
was grounded to a tab on the shield. The tube socket has
four grounded terminals, shield, cathode, suppressor and one
side of the filament. All are grounded with very short
pieces of wire to a tab on the chassis adjoining the socket
and a second tab in the adjoining compartment (through a
hole). When I replaced the capacitor I wired it point to
point to between pin 6, the screen grid, and one of the
other pins thinking this would result in the best ground.  I
found that if I dressed the capacitor upward and grounded it
directly to the wall of the shield there was much less
tendency for the stage to oscillate.  I had cut the leads to
short to run the cap around to the other side and it seems
to work as it is.  This reduced but did not get rid of the
oscillation so I began to poke around some more.  The can of
a metal tube is supposed to act as a shield. It goes to
ground through Pin 1 in many metal tubes including this one.
In fact, the terminal was grounded to the chassis.  However,
it was obvious the tube can was not doing an adequate job of
shielding so I tried a direct connection from the socket
terminal to the wall of the shield just above it. This seems
to have tamed the oscillation.  I can't be absoulutely sure
yet and have been fooled before but I think maybe this
works. The grounding is evidently very critical. I may
experiment some more with this.
     I also had a problem with two of the tuned circuits in
this stage running out of range of the trimmer capacitor and
added a small cap to get it resonant.  My grounding fix (if
it is a fix) did not affect this so I must investigate it
separately.
    More when I have more, I hope this problem is gone.
     Thank you to all who helped.

--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com