Westinghouse H886K21

[Electronic M]

If the noise goes away completely with the IF tubes pulled then the problem is either up stream of the 1st IF tube you can pull to kill the noise or it is in the AGC.

If the noise goes away with the 1st IF tube pulled, then odds are it is in the tuner, or it is local interference. If you have a signal source that operates on other channels than the one your currently using it may be wise to try different channels. There is a large gap (FM band + other stuff) between channels 2-6 and channels 7-13. So switching from low band VHF to high band or vice-versa may be a wise test (to get away from wide band noise). Also if a local DTV channel is using the same RF carrier freq as your signal source, you will likely get noise from that DTV carrier leaking into your set.

One way you can eliminate AGC noise is to connect the AGC buss to an external adjustable DC supply 0-25V and use that supply to clamp the AGC line to a proper fixed voltage.