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 Post subject: Philco 39-6 Problem
PostPosted: Feb Mon 16, 2004 2:49 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 828
Location: Memphis, MI, USA
Hi everyone,<P>I am working on a Philco 39-6 (riders 10-1). Recapped the chassis and measeured all resistors.<P>The radio howls and receives no stations until you touch the grid of the 78 tube and then all stations come in clear as a bell and I can tune across the dial. Transformer between the Det-Osc is good. <P>What am I missing?<P>I am open to all ideas.<P>Thanks in advance.<BR>Tom Guest<P>------------------<BR>


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 Post subject: Philco 39-6 Problem
PostPosted: Feb Mon 16, 2004 3:46 am 
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Location: Oak Creek, WI USA
Does the shield have continuity to ground? I've seen radios do all sorts of strange things when it's not. <P>Just a suggestion<BR>Regards,<P>------------------<BR>Paul


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 Post subject: Philco 39-6 Problem
PostPosted: Feb Mon 16, 2004 5:02 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Memphis, MI, USA
Hi Paul,<P>If you means tube shield this chassis doesn't have any or show any signs it ever did. Should there have been some on this set?<P>Thanks<BR>Tom Guest<BR><P>------------------<BR>


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 Post subject: Philco 39-6 Problem
PostPosted: Feb Mon 16, 2004 5:58 am 
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I have never seen a prewar radio that has no shielding on the IF tubes, because without them the radio does exactly what you describe. So I'd say the short answer would be yes, there should have been one.<P>I've never seen a 39-6. Somewhere there should be some way for a shield on that tube to contact the chassis, but I don't know what. I have a radio that uses a Goat shield that has metal spikes screwed to the chassis that contact the shield; otherwise the tube socket is surrounded by a metal collar onto which the shield fits. Or something along those lines. Either way, try shielding the tube and ensuring the shield has continuity to the chassis, and see if that fixes your problem. Good luck.......<P>Regards,<P>------------------<BR>Paul


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 Post subject: Philco 39-6 Problem
PostPosted: Feb Mon 16, 2004 6:39 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: NC
Tom,<P>You may have a problem with the AVC in this set. Note that the grid of the 6A7 and the grid of the 78 are both connected and also connect to the 51K resistor that connects to the diodes of the 75 (2nd detector) tube. <P>I suspect that placing your finger on the grid of the 78 tube is lowering the bias on this grid....much as the AVC circuit should do when there are strong signals or interference.<P>You may want to measure the bias voltage on the grid of the 78 tube. It should be just slightly positive with no signal, and go more negative with a strong station.<P>------------------<BR>Ken


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 Post subject: Philco 39-6 Problem
PostPosted: Feb Mon 16, 2004 6:39 pm 
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Location: Evanston, IL
Tom,<P>Sounds very much like the IF stage is going into oscillation. This can be due to a missing tube shield, tube shield not gounded to shield base around the IF tube socket. <P>Rusted/loose tube socket/shield base rivets making poor or no chassis contact.<P>Grid and plate wiring and/or components aound the IF tube socket causing feedback. This is called "lead dress."<P>Bad IF tube. <P><P>------------------<BR>Chuck Schwark<BR>The Philco Repair Bench at<BR> <BR> <A HREF="http://www.philcorepairbench.com" TARGET=_blank>http://www.philcorepairbench.com</A>


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 Post subject: Philco 39-6 Problem
PostPosted: Feb Mon 16, 2004 9:29 pm 
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Location: Memphis, MI, USA
Hello,<P>Thanks for all of the replies. I will see if adding a shield helps. The chassis has no provisions for a shield ground. Just plain tube bases. Most Philcos have the square tube shields that require a holder on the chassis. This one has none.<P>I will let you know tonight after I try the shield.<P>Tom Guest<P><P>------------------<BR>


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 Post subject: Philco 39-6 Problem
PostPosted: Feb Mon 16, 2004 10:16 pm 
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Location: Evanston, IL
Tom,<P>After looking at the schematic, it indeed, does not show the IF tube being shielded. Could then be lead dress or the tube itself.<P>Also look for an open First IF transformer secondary winding.<P><P>------------------<BR>Chuck Schwark<BR>The Philco Repair Bench at<BR> <BR> <A HREF="http://www.philcorepairbench.com" TARGET=_blank>http://www.philcorepairbench.com</A>


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