The VCO is designed using a BFR91 transistor and BB149A varicap. Buffer stage transistor is a BFR91, then into a 6dB pi-pad. Output amp is an ERA5 then fed out to a F-socket. The UHF PLL used is a MB1501 driven by the protocol converter MC68HC908 QY4 chip. Modulator section has an input switch for video input A and B followed by a PAL pre-emphasis. NE592 8 pin sets the video gain and provides the 2 video outputs for positive and negative going video. MC68HC908 QT1 Microcontroller drives the 2 relays and provides a PWM output that is then filtered. After the filter there is a MPF102 FET that is across the gain pins of the NE592 amp. Outputs from the NE592 are connected via the second relay to the bias trimpot. Two BB149A varicaps are used for tuning and for modulation. The PLL DC control voltage is feed into a three-pole filter and then to a BC548 transistor. A LM2941 regulator is optional but can be added to provide switchable voltage to be fed up the coaxial cable to a preamplifier or LNB as an up converter.

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MK II FMTV Modulator mods

Parts: 10k x2 3.3k 47 390 20pF smd 100pF smd x2 0.1uF

Add a 3.3k resistor from pin 1 of the NE592 across to pin 5 as well as adding two 10k resistors from pins 4 and 5 to ground. Remove and link out the 20k trim-pot feeding the modulation varicap. Add a 390 ohm resistor in line with the 22uF capacitor feeding the base of the BFR91 oscillator transistor. Between the two BB149A varicaps change the 22pF smd capacitor to a 10pF. On the emitter of the BFR91 oscillator transistor change the 22 ohm resistor to a 47 ohm. Add a 0.1uF capacitor from supply to ground next to the coil feeding the collector of the BFR91 oscillator. Place next to both chokes after the PLL loop filter a 100pF smd capacitor.

Checking turning range of the VCO adjust CX1 to lower the operating frequency range. By changing the turn spacings on the coil feeding the collector of the BFR91 oscillator for upper frequency range. It is important that you don�t over couple CX2 between the oscillator and the buffer stages.