01-05-2018, 08:55 PM
Possibly a better explanation:
Let's compare the 100-T/Coronet C-808 tuner schematic with the Fisher 202-T tuner schematic.
In the 202-T, which a Fisher expert on Audiokarma successfully converted to FM Multiplex, the signal for the FM Magic Eye is taken from the 4th IF which has its own set of diodes internally, connected to a network consisting of a 15K resistor, 5000 pF capacitor, and 8 uF electrolytic. The signal from this is split between the EM84 Magic Eye grid and the FM muting circuitry. So adding a Wide-Band ratio detector transformer to the 202-T would not affect the operation of the FM EM84 Magic Eye tube.
[Image: http://www.philcoradio.com/images/phorum...T_part.jpg]
Partial schematic from a Fisher 202-T, Sams Photofact version.
In the 100-T, the signal for the FM Magic Eye is taken through R41 from the junction of R39, R40 and the negative lead of C4 (as shown in the partial Sams schematic below). Note that in the original Fisher schematic, the part numbers are R54, R50, R53 & C66, respectively. Now with a Wide-Band ratio detector transformer installed and C4 (C66) reversed, the signal polarity going to the EM84 grid would be reversed! So, in my mind anyway, to make the Wide-Band transformer conversion work in the 100-T, one would have to reverse the polarity of the Wide-Band ratio detector transformer's internal diodes. Click HERE to see how I eventually solved the problem.
[Image: http://www.philcoradio.com/images/phorum...T_part.jpg]
Partial schematic from a Fisher 100-T, Sams Photofact version. Note that the part numbers are different from the Fisher factory schematic.
Anyway, that's how it seems to me.
Let's compare the 100-T/Coronet C-808 tuner schematic with the Fisher 202-T tuner schematic.
In the 202-T, which a Fisher expert on Audiokarma successfully converted to FM Multiplex, the signal for the FM Magic Eye is taken from the 4th IF which has its own set of diodes internally, connected to a network consisting of a 15K resistor, 5000 pF capacitor, and 8 uF electrolytic. The signal from this is split between the EM84 Magic Eye grid and the FM muting circuitry. So adding a Wide-Band ratio detector transformer to the 202-T would not affect the operation of the FM EM84 Magic Eye tube.
[Image: http://www.philcoradio.com/images/phorum...T_part.jpg]
Partial schematic from a Fisher 202-T, Sams Photofact version.
In the 100-T, the signal for the FM Magic Eye is taken through R41 from the junction of R39, R40 and the negative lead of C4 (as shown in the partial Sams schematic below). Note that in the original Fisher schematic, the part numbers are R54, R50, R53 & C66, respectively. Now with a Wide-Band ratio detector transformer installed and C4 (C66) reversed, the signal polarity going to the EM84 grid would be reversed! So, in my mind anyway, to make the Wide-Band transformer conversion work in the 100-T, one would have to reverse the polarity of the Wide-Band ratio detector transformer's internal diodes. Click HERE to see how I eventually solved the problem.
[Image: http://www.philcoradio.com/images/phorum...T_part.jpg]
Partial schematic from a Fisher 100-T, Sams Photofact version. Note that the part numbers are different from the Fisher factory schematic.
Anyway, that's how it seems to me.
--
Ron Ramirez
Ferdinand IN
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