09-26-2013, 01:19 PM
Had a similar parasitic problem with a radio using a 6SA7 with separate oscillator excitation. Sensitivity would drop suddenly until you unplugged the tube and replaced it, at which point it would work properly again.
Finally connected a spectrum analyzer and found the 6SA7 oscillating at 200 MHz !!. Cured the problem by placing a ferrite bead on the grid lead right at the socket.
I dont know if it is the mixer or osc that has the parastic in your case, but you might try beads on the grids of each. If you don't have any ferrite beads, you could try a 10 ohm resistor in series instead.
As to the tracking problem, is it the antenna stage or the osc that is causing the problem? Does the osc track the dial calibrations, or does it deviate from the indicated RX freq at the high end of the scale?
Do you have a link for the schematic of the radio?
Finally connected a spectrum analyzer and found the 6SA7 oscillating at 200 MHz !!. Cured the problem by placing a ferrite bead on the grid lead right at the socket.
I dont know if it is the mixer or osc that has the parastic in your case, but you might try beads on the grids of each. If you don't have any ferrite beads, you could try a 10 ohm resistor in series instead.
As to the tracking problem, is it the antenna stage or the osc that is causing the problem? Does the osc track the dial calibrations, or does it deviate from the indicated RX freq at the high end of the scale?
Do you have a link for the schematic of the radio?
