|Author:||FredB (registered user: 2418 posts )|
|Date:||Mon, May 14th, 2012 @ 14:34 ( . )|
+1 These are my thoughts too. It seems to me to be a grounding problem. If you touch the chassis and the amp quiets down, I would expect an opening in the grounding circuit some where between the input jack sleeve and earth ground.
Checking for continuity between the input ground and the ground pin on the power cord is a good place to start. I would also check the wall socket into which you are plugging the amp's power cord. Often wall sockets can be wired for the wrong polarity or with a missing ground connection.
Checking that and failing to find a problem, I would use a continuity tester to check the grounding circuits in the amp.
On 05/11/2012 @ 10:56, mhelyar86 wrote :
Grabbing at straws a bit here, but i still think the problem may be in the grounding. measure the resistance from various grounded points such as the input jack sleeve and the negative side of the filter caps to the earth pin on the power cable, right where it plugs into the wall. (obviously unplug first). Should be close to zero and similar for all points as they're continuous. Sometimes resistance will tell you more than continuity on a multimeter.
: your scope probably has its ground clip earthed through it to the Mains protective earth (some don't), so I'd say if connecting that to the chassis improves things, it must be providing a better earth than your ground scheme has without it. What grounding scheme are u using? Is the scope plugged into a close power socket to the amp? I tend to plug them both into a single good quality multi plug surge protecter to minimise any ground loops. if your house wiring is old, it may be worth trying it in a different socket or a different house.
: The AC on your OT primary sounds like just power supply ripple, which should be sawtooth shape on the scope and is normal.
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