video tx

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deepak007

Joined: 20 Sep 2007
Posts: 89

26 Jul 2008 20:50 video tx

hello

i have assembled a video transmitter circuit. its not working. the base voltage of Q4 is zero. please see if the circuit is correct. i don't see how the base receives it's voltage, because there is no connection from the positive supply to the base. or does it turn on by the ac signal voltage? also verify the layout.i am a beginner to CAD. i have used long trace to connect L2 to base of Q1. does it have any effect on the inductor.please help.

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vfone

Joined: 10 Oct 2001
Posts: 2467
Helped: 343

27 Jul 2008 15:38 Re: video tx

This TV transmitter circuit should work fine, I remember I’ve built something similar years ago.
The DC voltage on base of Q4 should be zero, because this works in Class C and the transistor is DC biased by the incoming RF signal.
Check if Q3 is oscillating, looking for signal in UHF TV band. The DC supply in the collector of Q4 is AM modulated by the video signal (Q1, Q2).
R4 (15k) needs some adjustments to get the right black level.
There is a need of a decoupling cap (100pF) on the emitter of Q2, that I don’t see on your schemataic.