Instruction manual

TV6C • 6
CIRCUIT DESCRIPTION:
Video signals from J1 are suitably terminated by the 75 ohm resistor R6 and
coupled through capacitor C1 to the clamp diode D1. The clamp forces the sync
pulses on the video to a fixed DC level which will reduce 'blooming' on some
video scenes. Video gain pot R3 permits you to adjust the amount of video
applied to the modulator section of the transmitter, its function is similar to the
contrast control on a TV receiver. The bias control R7 adjusts the 'black' level of
the transmitter. Ideally you want the transmitter to produce some level of signal
even when a totally dark screen is presented. This is to allow the TV receiver to
properly maintain sync and sound when no screen video is seen. This control is
adjusted in conjunction with R3 for best all-around performance.
Transistors Q1 and Q2 are the modulator devices. Television signals are
complex waveforms which are AM (Amplitude Modulation) modulated upon an
RF signal carrier. In this case we use Q1 and Q2 to control the supply voltage to
the power amp, thus varying the RF power output in step with the video signal.
The audio signal is FM (Frequency Modulation) modulated on a subcarrier
oscillator. To understand the purpose of the subcarrier oscillator, think of it as
another transmitter that operates on another frequency from the video
transmitter. In actual practice, the audio subcarrier is 4.5 Mhz higher than the
video carrier frequency. Transistor Q3 is a 4.5 Mhz oscillator which is FM
modulated by having its bias voltage slightly varied. Audio signals are coupled
into Q3's base through capacitor C2 and resistor R4, thus varying the bias
voltage at an audio rate causing frequency modulation. Coil L1 and its internal
capacitor form the tank circuit of a Hartley oscillator which is tuned to the
required 4.5 Mhz frequency. This FM modulated signal is then applied to the
modulator section through C5 and R9. Resistor R9 sets the proper amount of
subcarrier signal in relation to video signal.
The transmitter operating frequency is set by coil L4 which is part of a Colpitts
tank circuit along with capacitors C7 and C9. Transistor Q4 uses this tank circuit
as its feedback network and oscillates at whatever frequency you select.
RF output from the oscillator is amplified by transistor Q5 whose supply voltage
comes from the modulator section. This voltage is varying at a video and audio
subcarrier rate, thus modulating the RF power output. Matching to the antenna
and low pass filtering is provided by the two capacitors C12, C13 and inductor
L5.
Power is supplied by an external 12 -15 volt DC source. This voltage is regulated
at approximately 9.4 volts by the 8 volt regulator, VR1 and the series connected
diodes, D2 and D3 in VR1’s ground leg.