[Hallicrafters] S-27 Help Needed

[Joe Connor]

     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