[Hallicrafters] S-27 Help Needed

[Richard Knoppow]

----- Original Message ----- 
From: "Joe Connor" <joeconnor53 at yahoo.com>
To: <hallicrafters at mailman.qth.net>
Sent: Friday, May 03, 2013 1:14 PM
Subject: [Hallicrafters] S-27 Help Needed


I'm working a Hallicrafters S-27 and need help with a weird 
problem. This is an old rust bucket but I couldn't resist 
trying to fix it up. Even though it's filthy and a bit 
rusty, it has potential. With the Hallicrafters push-pull 
audio, it should sound really good.

Here's the schematic from Rider's: 
http://www.nostalgiaair.org/PagesByModel/897/M0008897.pdf

I'm got it so it plays well on Band 3, the FM broadcast 
band. However, after it's been playing for a while (20 
minutes or so), it conks out. I have audio but no reception. 
With signal injection, I get reception if I inject the 
signal at the plate of the 1st RF tube (956 acorn tube) but 
not at the grid of that tube. The voltages on the plate, 
screen and cathode of the 1st RF are reasonable. If I turn 
the set off and turn it back on an hour or so later, it 
plays without any problem. Thus, it sure sounds like the 
problem is heat-related.

My first thought is the 956 tube. However, because it's an 
acorn tube, I can't test it, but I can get a replacement if 
I need to. Does a heat-related problem with the 956 tube 
make sense? I don't have much experience with acorn tubes so 
I don't know if they are susceptible to that kind of issue. 
There aren't a lot of components before the 956 tube and the 
caps in that area are the usually reliable small 
value-micas, so I don't see any other prime suspects.

Do you have any other thoughts of what I should be looking 
at?

As always, thank you for your help.


Joe Connor

     If you get something with a signal injected at the 
plate but not the grid and the voltages are all there it 
sure sounds like the tube. I would try removing it and 
cleaning the socket and tube pins and see if that makes any 
difference.  Its not too usual for tubes to quit like that. 
Acorn tubes can be short lived but the usual failure is to 
get weak. You can try exchanging the 956 and 954 to see if 
it makes any difference. the 954 is a sharp cut-off pentode 
and the 956 is a remote cut off pentode but they will work 
well enough in the wrong sockets to use for trouble 
shooting.
    Many tube testers _will_ test acorn tubes, perhaps 
someone in your area has one and will either test the tube 
or loan you the tester.
    I have an S-36 which has been on the back burner for a 
couple of years with a cracked grear in the dial drive. I 
made the mistake of taking the drive apart and now have to 
try to re-assemble it.
    I think BTW, that the IF alignment procedure for both of 
these receivers is not quite right.  The reason is that the 
center frequency is not constant because of the way the 
mutual inductance is varied to get the wider bandwidth.  Let 
us know how this comes out.


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