[Heathkit] problem with agc on sb-301

[yash at aol.com]

I just finished rebuilding a SB-301, works great with one problem. The agc does not work well at all. It is functioning ,but at a very small amount. The s meter is vary lazy and stingy. I went thru the circuit replaced the 2 diodes ,all the caps and resisters  ,no change . tube exchange also did not improve the agc. some times a signal will be received that is like a 20/over9 ,loud and distorted,tune off the station and then go back and its level is near normal and not distorted. switching the agc to off ,does not increase the volume of the received station at all,just stops the action of the s meter. I have run out of things to look for ,any help on this would sure be a big plus.
thanks
dale wt4t