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Antique Radio Technical Forum
For Johnny on his Philco 41-285

Posted by Doug H on 04/19/2009 15:04

Johnny, from what I remember of your post this morning, you injected a 455 khz signal modulated with a 400 hz audio tone signal into three different grids. When injected into the grid of the XXL 1st Det. this did not produce an audio tone in the speaker nor did it produce an audio sound when injected into the 1st IF 7B7 Grid. You did, however get an audio sound from injection into the 2nd IF 7B7 grid. There are only a few components between the last two injection points, the 1st IF tube and its associated wiring circuits, and the second IF transformer. I would first inject the modulated signal into the plate circuit of the 1st 7B7 tube, be sure to using a blocking capacitor to keep the DC voltage from the signal generator. If you get the audio sound, then the 1st IF tube is bad, or the grid bias components are causing the loss of signal. If no sound is heard, then most likely the 2nd IF transformer (item 34 on schematic) is failing to process the signal. In this case, check the primary and secondary windings each for continuity. Also the trimmer cap (item 34A) may be bad or turned so far out that it can not pass the signal. If you have continuity in the both primary and secondary windings, then I would try adjusting the 34A trimmer to see if the signal will pass through at some point.

I hope this helps lead you to a solutioin, since it sure sounds like the audio circuits are functioning now that you hear the 400 hz tone from the speaker when the signal is injected into the second IF tube (7B7) grid.

Doug H



For Johnny on his Philco 41-285 
Doug H 04/19/2009 15:04 
Johnny 04/19/2009 17:23 
Doug H 04/19/2009 19:17 
Johnny 04/19/2009 20:12 

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