The 10K makes the receiver "less sensitive" it punches down the receive when you have ambient noise that gets into the AM Detector - your C222 but there is another "quirk" you won't recognize unless you've had to fix a noisy receiver due to a vehicles noisy ignition.
This board is a decent performance board but they did some stuff that - well...doesn't make sense unless you are trying to understand the "ambient noise versus Birdie" the radio can generate.
On the bottom (Foil side) of the board - the BLUE arrow points to a 330pF cap that straddles the wire that sends power from the CB/PA switch on the front panel (a proximity to noise source location) - the way this works, when it's in PA mode, the radios IF is simply cut off by applying power to the Emitter leg, raising it's voltage , so the thing can not amplify any signal - quieting the receiver.
So what is the problem - 330pF doesn't cut it when it comes to noise sources getting "rejected" more like injection into a radio - maybe you don't have this problem, but others did.
The fix is pretty simple, remove the 330pF install a 0.01uF where the 330pF WAS, then change C219 from a 0.047 (473) to a 103 - ON TOP OF. R303 gets split in two - the R303 is a 1.5K so you'll need Two 680 ohm 1/4W resistors and one gets the pigtail from the CB/PA switch - free end inserted where the pigtail went, The resistor is IN series with the pigtail wire from the front panel CB/PA switch. The R303 location gets the other 680 ohm resistor and you solder the 0.01uF (103) Disc cap where the 330pF was.
Now, that 10K - was part of this carborundrum it's pretty aggressive, so you may want to now back off that value to, say - 15K, to allow the mod done at the last IF stage, which sends your AF Detector signal for the output and noise fix for CB/PA switch - which goes back to why I recommend that C222 changes to a 0.001uF (102) to reduce the dampening effects and losses from the weaker signals are getting sucked down into that 0.0022uF (222) cap instead of thru the ANL filter. Less capacitance helps the brightening of the audio, lessens the losses, and seems to offer a good trade off from the white noise hiss versus the quieter receive the radio originally had.
Interesting you had mentioned earlier about a Wide versus Narrow filter - you'd have to go to the 1st IF stage to fix that problem and that too is a 10K or 15K (should I?) issue. The trade off in making ANOTHER MOD to the IF strip is the changes to an otherwise nice balance the radio had to recover some
sensitivity that was lost in the original design for them to obtain the REJECTION (Read this as -
Selectivity ) as listed on the spec sheet.
Locate C207 - change that value, Lower or Higher to achieve the Narrow to WIDE response (Lower cap value - smaller less signal - Narrow response) The Resistor (R208) can also provide some control, but not as a value to obtain Narrow or Wide, only Signal Strength thru to be mixed in the IF of the next stage.- the cap handles the RF - the Resistor simply limits it's power or strength into the next stage.
So if you've read this far and still are with us, and haven't run out of the building screaming...Well, Welcome to Radio...