mrwrench,
from your comments , I would suggest that you first deal with the 2 units with noise.
It would seem that it is very likely a volume control problem which might be alleviated with a proper electronic cleaner spayed into the actual control or worse case a replacement control.
If you can get these 2 units working then you can consider if you wish to repair the other units .
Regards,
John G. VO1 CAT