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ARC REF 150 Buzzing noise

I have an ARC REF 150 amp with over 4000 hours on it, and recently it has developed a buzzing noise.  This appears after about 30 minutes of use, and seems to get steadily louder.  I thought it might be a tube.   I had KT150 power tubes in it and I swapped those out  for KT120's and re-biased.  No change.  I then swapped out the 6H30 input tubes and re-biased again.  Buzzing is still there.  It is very difficult to locate the sound on the boards.    I have also had several blown fuses, this has been in the midst of a move and may or may not be reIated to the buzzing.  I have to assume this needs a repair, but I am wondering if anyone has a guess or even better, experience with the problem.  I am in the Denver area and if anyone knows of a local repair opportunity, I would appreciate knowing that too.  I do not look forward to the expense and delay of sending it back to ARC repair.  Thanks.  Scott
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scottwsmith OP
60 posts
 
Should have mentioned that the amplifier is plugged into a Bryston BIT-20 power unit, an isolation transformer.  I am in a new house, but only for a few weeks.  I am unsure if I heard the buzzing in the old place.  Perhaps so.  I will try plugging the amp directly into the wall and see if that fixes the problem.  I would have to believe that a failure of one of the three transformers would be fairly unusual?  Thanks for the replies, I will update the post as soon as I have more results.
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scottwsmith OP
60 posts
 
The speakers are 89 dB.  Up to a few months ago, I didn't hear buzzing at all.  I did remove the amp from the Bryston BIT-20 and the buzzing didn't go away.  I will try turning off other circuits in house next.

It takes an hour or so for the buzzing to appear, as if it is usage related or heat related.
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scottwsmith OP
60 posts
 
Moved the power cord to another circuit, quite close to the power panel.  The amp was quiet for about an hour, which got my hopes up, but then started the nasty buzzing again.  It MIGHT be one of the three transformers, but the exact source is hard to locate.  It is obnoxious enough to be a problem, for sure.  Some of you have mentioned DC content in the power, but wouldn't the Bryston BP-20, which is a large isolation transformer (I think) eliminate that if it were present?  BTW, my most recent tests have not had the BP-20 in the power feed.  The Bryston does not seem to be the problem, as the amp acts the same with or without it.

I am going to call ARC service today, will report back on what I hear from them.
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scottwsmith OP
60 posts
 
Update after last post, there is no time dependency.  No matter when I turn on the unit, it will start to buzz after about an hour.  Anyone know what those transformers cost?
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