Rider 15-1 Western Air Patrol Model 76, R-9 volume control (I'm assuming is 500k,just reading 500 in book)& R-21 not sure what type of control, but both have control of volume with dial knobs,there's also a tone control R-11 checks out ok, like to find out what type of control R-21 is & is 500k correct for R-9. Alignment is correct the best I can tell.All tube test good.

:Rider 15-1 Western Air Patrol Model 76, R-9 volume control (I'm assuming is 500k,just reading 500 in book)& R-21 not sure what type of control, but both have control of volume with dial knobs,there's also a tone control R-11 checks out ok, like to find out what type of control R-21 is & is 500k correct for R-9. Alignment is correct the best I can tell.All tube test good.
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According to the schematic, R21 is a sensitivity control. Schematic is hard to read and I can't tell where R21 is; but usually sensitivity controls are in the RF amp stage.

500 ohms appears to be the correct value for the R9 volume pot. Somewhat unusually, it is fed through a cap from the demodulator so there is no reason why it would need to be a large value (if the cap weren't present, the pot would need to be rather large to minimize DC loading).

You can localize the "low volume" problem by injecting an audio tone at the volume pot tap. If you have adequate volume at the speaker, you have a problem in the front end of the radio.