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Antique Radio Technical Forum
Zenith T825: No FM, just static

Posted by Mark Johansson on 02/20/2008 22:38

Hi Bill,

Well, That fixed it! I decided to go ahead and rebuild the can so I stopped on my way home at the local electronics surplus place & got all 4 caps for a grand total of 87 cents. Pulled the can & disassembled it, removed the mica disc & used a dremel to trim the pads so they didn't touch any more, put it all back together & soldered on the new caps. Cover on, back in the radio & now it works, no more static & tunes in lots of stations! Now it's time for a recap & alignment. There seems to be some distortion in the audio on FM that's not there in AM but hopefully an alignment will help that.

Thanks so much, I couldn't have fixed it without your help, Mark
::::
:Hi Mark,
: This is classic Silver Mica Disease. You may be able to avoid rebuiding the discriminator, though.
: The leakage seems to be between pins 3 and 5 of L15. The capacitor going between pins 4 and 5 is a 15 pF capacitor, eventhopugh it is not shown on the schematic.
: Try this. Disconnect all leads to pin 4 of the discriminator and rebuild the circuit with a new 15 pF capacitor under the tube. Your voltages may come into line and FM may come back.
: Even if you wind up having to rebuild the discriminator, you will need to do the 15 pF replacement out of the can.
: By the way. The capacitor on the primary of the discriminator is 24 pF the two on the secondary are 68 pF.
:
:Best Regards,
:
:Bill Grimm



Zenith T825: No FM, just static 
Mark Johansson 02/13/2008 17:12 
{:o)<= 02/14/2008 16:12 
Edd 02/14/2008 16:55 
Thomas Dermody 02/14/2008 17:37 
Bill G. 02/14/2008 17:55 
mark Johansson 02/15/2008 12:47 
Bill G. 02/15/2008 13:43 
Mark Johansson 02/19/2008 22:28 
Bill G. 02/20/2008 08:33 
Mark Johansson 02/20/2008 22:38 
Bill G. 02/22/2008 15:34 
Thomas Dermody 02/22/2008 17:42 
Al R. 04/14/2009 09:18 
Bill G. 04/14/2009 09:55 
Al 04/14/2009 12:37 

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