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PostPosted: Thu Oct 08, 2015 1:47 pm 
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Location: Albany, NY
We've now seen two separate incidents here in the Amp forum where TMB18 amp builders reported problems with rectifier flashing/arcing and blown fuses when the amp is switched out of standby and high DC voltage is applied to the amp circuit. While such a problem could be caused by several things, both incidents were traced to the same build defect, so I'm posting this "sticky" so that other TMB18 or Brit 45 builders can be on the lookout for--and hopefully avoid--this problem.

In both of these recent cases, the cause was traced to a high DC voltage short to ground between one of the positive terminals of the large "can" dual capacitor and the edge of the mounting hole through the metal chassis. Though the hole is sized so that both of the positive terminal tabs (one yellow, one red) are well away from the sides of the mounting hole when the can is properly centered, the issue arises when the ring clamp on the top of the chassis is tightened and pulls the can off-center in the hole. With a bit of rotating/repositioning of the can in the clamp, the tabs can be readily positioned such that no contact with the chassis occurs. TMB18 & Brit 45 builders should be on the lookout for this potential problem, since both amps use this through-chassis mounting method for the can cap adjacent to the mains socket.

BYOC is looking into enlarging the can cap mounting hole to provide more clearance for the tabs, but it's still a good idea to keep an eye out for this potential shorting issue. See marked-up photo below showing the contact between one of the positive terminal tabs of the can cap and the edge of the chassis mounting hole:


Attachments:
can_cap_short.jpg
can_cap_short.jpg [ 51.62 KiB | Viewed 14033 times ]

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PostPosted: Mon Nov 16, 2015 11:13 pm 
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Joined: Sun Aug 30, 2015 11:35 pm
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It took quite some amount of doing to make sure there wasn't a short with that part. Glad the hole will be enlarged in the future. I think that is a wise design choice.


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