UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - FM to AM converter

Phil G4SPZ · 1st Dec 2018, 9:53 am

Hi David,

The more I look at the circuit, I think the Q of the tuned circuit isn’t high enough. The 100pF main tuning capacitor should be more like 500pF to meet the old ‘1.5pF mer metre of wavelength’ rule of thumb, and those small encapsulated chokes have a lower Q than the older more traditional larger components would have. This is pointing to losses in the tank circuit which could be causing the problem with the wave shape. I agree, it should look more like that from the MiniMod in your last screenshot.

I can’t remember the component values for the MiniMod tank circuit but it would be worthwhile looking them up.

1st Dec 2018, 9:53 am	#52
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: FM to AM converter - variation on a theme! Hi David, The more I look at the circuit, I think the Q of the tuned circuit isn’t high enough. The 100pF main tuning capacitor should be more like 500pF to meet the old ‘1.5pF mer metre of wavelength’ rule of thumb, and those small encapsulated chokes have a lower Q than the older more traditional larger components would have. This is pointing to losses in the tank circuit which could be causing the problem with the wave shape. I agree, it should look more like that from the MiniMod in your last screenshot. I can’t remember the component values for the MiniMod tank circuit but it would be worthwhile looking them up. __________________ Phil Optimist [n]: One who is not in possession of the full facts