The AF GAIN potentiometer on my Collins KWM-2 had failed, giving me a choice of 2 volume levels – Barely audible and EAR BLAST. I put off replacing it because the pot is blocked in by a bracket that carries the bandswitch shaft. Messing with that looks like a sure path to disaster. Also, I could not find instructions or hints anywhere on the web and a post to the CCA mailing list generated the internet equivalent of crickets.

My Plan

  1. Remove PA TUNING & LOADING shafts.

The shafts
The top shaft is the coaxial PA TUNE/LOADING control and the white shaft below drives the band wafer switch inside of the PA cage.

Shafts inside the cage
View from inside the cage. The outer shaft drives the pulley that moves the loading capacitor. Below you can barely see the band wafer switch.

I hope that by loosening the setscrews on these collars that I can withdraw the shafts without having the loading pulley fall free (but I’m not counting on it)

  1. Move the bandswitch shaft out of the way.

It looks as if removing the shaft would cause the bandswitch rotor to fall out of the switch. I don’t want that to happen, so my plan is to remove the circlip from the fiberglass shaft. I hope that once that’s out of the way I’ll be able to push the shaft into the PA cage. I’m not sure if I’ll be able to leave the bandswitch pulley on the shaft but I’ll try.

Circlip
This circlip appears to be the only thing preventing me from pushing the fiberglass bandswitch shaft back into the PA cage. If you want to remove this clip you need retaining ring pliers with angled prongs. Read the warning below!

  1. Remove the 2 screws that attach the shaft bracket to the chassis. With that out of the way I should have enough room to remove and replace the potentiometer. See photo above.

The bad bracket
This bracket has two functions. It retains the bandswitch pulley and shaft and it blocks your access to the AF pot.

Plan Executed

The plan worked. I now have a working AF control. Much nicer than the two AF GAIN choices I had before. Silent and Blast.

Here are my notes after performing the work:

  1. Make notes of PA and loading capacitor engagement vs the panel control pointers.

  2. The two setscrews outside of the cage free the tube shaft that controls loading. The first setscrew inside of the cage, just behind the pulley, releases the inner PA tuning shaft.

  3. When you remove the PA shaft, there is nothing keeping the loading pulley from dropping, so hold the pulley against the cage wall while you withdraw the shaft and have a temporary stub shaft in your other (3rd) hand that you can slide in to keep the pulley in place while you do the rest of the work. Photo below.

Keep tyhe pulley from falling out
This is not where the replacement pot goes! (JK) Use a temporary shaft to keep the pulley from falling on the floor. A 1/4inch drill bit would also work here.

  1. The circlip on the fiberglass bandswitch shaft is not a spring steel circlip. It might be brass, but whatever it is it deforms very easily. Use just enough pressure with the clip pliers to get the clip out of it’s groove. Then you can push the shaft back into the cage and the pulley that’s blocking the AF control is out of the way.

  2. Now you can actually remove and replace the potentiometer. You have an opportunity to clean and regrease the pulley axle when you reassemble everything.

The bad bracket
This bracket has two functions. It retains the bandswitch pulley and shaft and it blocks your access to the AF pot.

I hope this helps the next KWM-2 owner who needs to replace their AF GAIN pot. When I pried open my original one I found that the carbon track had delaminated from the plastic.