C466 Has been replaced,
Have you
tested it?
...other than that I'm not sure how to test those components.
It's possible the inductor turns are open or one diode u/s
Remove the modules from circuit to prevent spurious results.
Do a diode check between 6 & 4 and then 6 & 3 on the ratio detector note their polarity is reversed relative to each other.
On the second module, do resistance checks between the pins and refer to the values on the diagram.
The small value capacitors in these units are quite reliable normally, and from your original description of the fault, alignment is not a problem.
I have occasionally heard of one of the diodes going bad, this then effectively makes the receiver demodulate the signal as amplitude modulation, giving your symptoms.
Further, try injecting a swept signal at the IF and check the discriminator slope with your scope. The full manual has details of this and is available on line, Electrotanya site has a free preliminary version of this.
can someone explain how they work?
Yes, they can, (Google is your friend).
Regards
X