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

Posted by Bill G. on 04/14/2009 09:55

:I have an older Zenith 7H820 with similar symptoms. Unlike the discriminator can shown for the newer T825, the schematic for the 7H820 shows two extra capacitors inside the can that are not across the coils, vice the one 15 pf capacitor between pins 4 and 5 shown here. Does anyone know the value of these capacitors in the older sets? Also, are the capacitors across the coils the same as discussed in this thread?
:::Hi Bill,
:::Thanks for the hints! I checked the voltages as you suggested. Pin 1 of the 12AU6 is -4.82 volts but pins 1 & 2 on the 19T8 are hovering around + .04v. Also, pin 3 which is supposed to be -.5 v is +13v. Could this mean problems with the caps in the discriminator can?
:::Thanks, 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
Hi Al,
The 8H832 uses the same IF cans as your radio. I have rebuilt a discriminator can assuming the same capacitors were used. I haven't had a chance to try it out yet.
While rebuilding it, I found that the BER105-ND insulators from Digi-Key worked well. The new capacitors can be easily mounted on the tabs near the coils.

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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