# Strange burn mark under filter cap on Vox Night train 15 watt amp



## Church-Audio (Sep 27, 2014)

Just wanted to give a heads up found a really strange problem on this amp. Customer had many strange issues with the amp taken to several techs.. Took out the filter caps to measure them out of circuit. And they all tested fine except for one. Replaced it but noticed a carbonized burn mark on the pcb board under the cap. The 1k Node resistor connecting to that cap was also burnt out. So the amp was dead. I figured that this must have been a "mistake" trace from the factory. But I cant imagine anyone inspecting the boards not noticing this issue. In any event if this is a robot assembled amp and it very well might be at least for component stuffing it might be there are others out there with the same series of issues. I ending up milling a line between the carbon to cut the resistance between the two pads now the amp seems fine. And is stable, Here is the serial number 003553 seems like an early one. 

if anyone has one on the bench that has "ghost" issues. This might save you some trouble finding the issue. From the solder side the burn mark was invisible ! and it was only seen once the cap was removed from the pcb. Never seen anything like this before.


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## nonreverb (Sep 19, 2006)

Looks like an arc line to me. All it takes is a little contamination on the board and *poof*.
I just had a Mesa MKIIC that somehow got some coke or other syrupy substance on the power board between the ground rail and B+. B+ found a path to ground via that spot and arced a perfect little path of carbon across it.


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## Church-Audio (Sep 27, 2014)

Could have got wet maybe at some point in time. I dont know never seen this under a cap before. The cap was not totally sealed with glue to the board so its possible.


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## nonreverb (Sep 19, 2006)

Maybe snuck past QC.


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## jb welder (Sep 14, 2010)

I've never seen an arc across a cap before either. You said one cap was bad, this one? Possible anything spewed out of it and that is what arced? Otherwise, maybe some flux?
It's hard to believe an arc could jump such a big gap without there being some contamination or residue there to help.
Or maybe, as it was on the component side, a bead or thread of solder got through and was sitting under the cap till it vaporized?


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## Church-Audio (Sep 27, 2014)

My best guess it was a crappy circuit board issue the factory that made the boards did poor qc and there was a small bit of a trace there and it got blown out at the factory. Or it got wet I don't know like I said I too have never seen this.


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