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Heatercathodeshort · 28th Feb 2015, 1:32 pm

The small chokes in the video amplifier are correction chokes to peak the high frequency capabilities of the stage. The effect of shorting them out will probably result in about .5mc/s lack of definition.
They can be rewound by hand using very thin 47swg wire in a single layer. Don't worry about the exact number of turns, just fill the complete length of the ceramic/glass former.
With the video amplifier now active you can check back the IF stages. With full RF output on generator at vision IF frequency connected to pin 1 of V6 [VA] you should see well modulated bars on the screen. IFT 6 is peaked at 36.5mc/s so rock your E2 a little either side of this with the modulation on. If not, check voltage on pins 5 and 7 of Z77. A much weaker signal should be observed when generator is connected to pin 5 anode. If zero, check D77 vision detector and make absolutely sure L17 is continuous. We need go no further back at this point. V4 V5 and V6 stages can then be cleared of suspicion.. [My therapist informs me I am a hopeless case but don't blame yourself.] Regards, John.

28th Feb 2015, 1:32 pm	#94
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Marconiphone VRC74DA The small chokes in the video amplifier are correction chokes to peak the high frequency capabilities of the stage. The effect of shorting them out will probably result in about .5mc/s lack of definition. They can be rewound by hand using very thin 47swg wire in a single layer. Don't worry about the exact number of turns, just fill the complete length of the ceramic/glass former. With the video amplifier now active you can check back the IF stages. With full RF output on generator at vision IF frequency connected to pin 1 of V6 [VA] you should see well modulated bars on the screen. IFT 6 is peaked at 36.5mc/s so rock your E2 a little either side of this with the modulation on. If not, check voltage on pins 5 and 7 of Z77. A much weaker signal should be observed when generator is connected to pin 5 anode. If zero, check D77 vision detector and make absolutely sure L17 is continuous. We need go no further back at this point. V4 V5 and V6 stages can then be cleared of suspicion.. [My therapist informs me I am a hopeless case but don't blame yourself.] Regards, John.