02-26-2013, 11:10 PM
After looking and scratch my head it looks like a quadrature detector. The cathode,control grid, and second grid act as an oscillator at the IF frequency 9.1 mc. L300 needs the be resonant at 9.1 to cause the oscillator to operate. I suspect that when you wired in your replacement it caused the FM1000 to conduct but not oscillate.
Terry
Terry