30-11-2019, 11:41 PM
Back in the 60s we had some sets in for repair with intermittent faults on FM. In the days when all that was available was the Light, Home and Third, the fare was, shall we say, not to our taste!
Radio Caroline on FM would have been much better! Early attempts to build an FM modulator based on FM tuner oscillator designs worked up to a point but stability was a problem.
A WW design for an FM stereo test generator eventually provided a solution. A sample, built on a home produced PCB was so stable that the bare board worked fine just laying on the bench and radiated well without an aerial!
I found a scribbled copy of the circuit a few years ago and made a neat copy. Here it is:
The trimmer can be replaced with a fixed cap ~10pF. Tuning was done by adjusting the air spaced coil - but there were lots of open spaces in Band II in those days! I'm not sure if the original included a trimmer across the Varicap/3p3 combination but fine tuning could be introduced by varying the voltage at the top end of the 100k that biases the varicap into a linear part of its characteristic.
It is definitely a 'quick'n'dirty' way of adding an FM modulator to the stereo encoder.
Radio Caroline on FM would have been much better! Early attempts to build an FM modulator based on FM tuner oscillator designs worked up to a point but stability was a problem.
A WW design for an FM stereo test generator eventually provided a solution. A sample, built on a home produced PCB was so stable that the bare board worked fine just laying on the bench and radiated well without an aerial!
I found a scribbled copy of the circuit a few years ago and made a neat copy. Here it is:
The trimmer can be replaced with a fixed cap ~10pF. Tuning was done by adjusting the air spaced coil - but there were lots of open spaces in Band II in those days! I'm not sure if the original included a trimmer across the Varicap/3p3 combination but fine tuning could be introduced by varying the voltage at the top end of the 100k that biases the varicap into a linear part of its characteristic.
It is definitely a 'quick'n'dirty' way of adding an FM modulator to the stereo encoder.
