10-12-2020, 06:53 AM
This is the Rediffusion HF modulator which I repurposed for Channel 1 TV. One each for sound and vision: http://www.borinsky.plus.com/simple_channel.html It drives a 75R load with no problems. I can't remember the exact design of the transformer but I'm sure it wasn't critical. I've used similar circuits in PAL/NTSC coders.
You definitely don't want a big resistor in series with the centre tap. The outputs of a 1496 are essentially current sources. You're trying to develop a voltage across the load, not across a collector resistor. Actually a resistor to the CT isn't so bad, in theory there's no signal voltage across it. All it does is make the bias conditions more critical since it's effectively lowering the supply voltage. Remember that the current source at the bottom, signal input at the long tail pair, carrier input at the quad and output at the collectors mustn't clash, even at maximum swings.
You definitely don't want a big resistor in series with the centre tap. The outputs of a 1496 are essentially current sources. You're trying to develop a voltage across the load, not across a collector resistor. Actually a resistor to the CT isn't so bad, in theory there's no signal voltage across it. All it does is make the bias conditions more critical since it's effectively lowering the supply voltage. Remember that the current source at the bottom, signal input at the long tail pair, carrier input at the quad and output at the collectors mustn't clash, even at maximum swings.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
