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Dodd 120 mechanical buzz?

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Hello All,

I just bought a pair of the Dodd Audio 120 monoblocks and when I go from Mute to Operate on the left channel amp there is a mechanical buzz (not through the speakers) coming from the amp that seems to be to the left of center mid-amp.

I haven't been able to get in touch with Gary and know he's been ill so I don't want to bug him. Does anyone know what the problem would likely be? Is there a choke or transformer midway back inside the chassis or could it be the output transformer? If it's just a matter of tightening a few nuts I can probably do that myself. If I need to replace a transformer (before it fails) I'd want to know about it. But so far I've not had the bottoms off to look.

In fact, if anyone could send me a pic of the inside of the amp I'd appreciate it.

Any and all ideas are welcome!

Follow Ups:

RE: Dodd 120 mechanical buzz? - davis419b@verizon.net 15:25:53 01/22/15 (0)
I just had a power switch replaced in one of my 120's yesterday by Gary. I ask him about the same problem with my amps. I told him that when I turn the left amp off the hum goes away. He said it was a ground loop issue.
RE: Dodd 120 mechanical buzz? - wheezer 13:06:52 07/08/14 (1)
Check/tighten up the lams of both the OPT and PT.

If the high speed diodes are "Fred" type, I'd look into
UF or Schottkys.
- RE: Dodd 120 mechanical buzz? - Frank A 20:25:09 07/09/14 (0)
  Wheezer it's interesting that you mentioned the hexfred diodes as being a potential source of the buzz. The amp does use hexfreds and the way they are mounted seems kind of flimsy to me.
  
  Thanks for the suggestions.
RE: Dodd 120 mechanical buzz? - Ralph 08:08:44 07/01/14 (1)
One possibility is a bad rectifier. If the amp employs a diode bridge at the input of the high voltage power supply, the failure of one rectifier (opening up and no longer conducting) would cause the transformer to rattle.

It is also possible that there is an induced swept resonance caused by the inductance of the power transformer and the capacitive aspects of the rectifiers. This is a common problem and can vary somewhat from amp to amp so it might not show up all the time. Easy to fix though.

I doubt that the transformer is on its way out. But you should get it sorted out as other parts may be involved that could damage the transformer. If this problem has been going on a while I would be less concerned about that; but if one amp can be quiet the other can too.
- RE: Dodd 120 mechanical buzz? - Frank A 08:42:50 07/01/14 (0)
  Hi Ralph,
  
  Thank you for responding. It is interesting that it could be a rectifier problem because the guy who sold them to me said he had just replaced the rectifier with "high-speed" diodes. Perhaps one of those is bad. I'll need to look into that.
  
  Thanks again!

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