This is now very old forum but the outcome is not clear.
IMO the problem is not a as you say a 'DC problem' as the noise does not persist all time the amplifier is on.
I know that these amplifiers have a soft start curcuit.
There are 2 relays (2) which switch the high voltage after 60 seconds or so.
May it be a relay having its contacts stucked?
Why not have the relays checked by a technician - how they behave at switching the amplifier on?
Wish you luck
IMO the problem is not a as you say a 'DC problem' as the noise does not persist all time the amplifier is on.
I know that these amplifiers have a soft start curcuit.
There are 2 relays (2) which switch the high voltage after 60 seconds or so.
May it be a relay having its contacts stucked?
Why not have the relays checked by a technician - how they behave at switching the amplifier on?
Wish you luck