[Hallicrafters] S-27 Help Needed (Again)

[Richard Knoppow]

----- Original Message ----- 
From: "Joe Connor" <joeconnor53 at yahoo.com>
To: <hallicrafters at mailman.qth.net>
Sent: Thursday, May 09, 2013 9:29 AM
Subject: [Hallicrafters] S-27 Help Needed (Again)


I need your help again because the intermittent problem with 
my S-27 has come back.

The receiver plays well. After being on for about an hour, 
it sometimes goes dead. With a signal injected on the plate 
of the 1st RF (956), I get reception. With a signal injected 
on the grid of the 956, however, I get nothing. After the 
set cools off for an hour or so, reception is normal.

I've cleaned the pins on the 956, as well as the socket, and 
substituted several NOS 956s. This does not bring reception 
back. Voltages and resistances on the 956 socket are normal.

I have a couple of suspects: (1) C-4, a .002 cathode bypass 
cap on the 956; and (2) C-5, a 330 pf screen bypass on the 
956.

Do you think leakage in either of those two caps could cause 
my symptoms? Is it possible that one of these caps develops 
leakage only when the set has been on for a while? Can I use 
a regular film cap for the cathode bypass?

As always, thank you for your guidance.

Joe Connor
______________________________________________________________

     I don't think its capacitor leakage.  A leaky cap in 
the cathode would cause the voltage there to change. Same 
with the screen. Its puzzling that the voltages and 
resistances are all correct. Are you able to measure both 
hot and cold to see if there is a difference?   I assume the 
filament is working.  If it does this on all bands there is 
just not much in common. Even though the socket voltages and 
resistances are correct I would check for a cold solder 
joint somewhere.  You may get some results by poking at the 
wiring with an insulated stick.
     Double check the measurements.
     C-4 is a mica cap. I would replace it with another mica 
cap rather than a film cap.

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