Eeyore wrote:

Ron wrote:
N_Cook wrote:
Ron wrote in message

It probly started off as a hairline crack around one of the soldered
joins, caused by repeated connecting and disconnecting. It started
'fizzling', no one noticed (or cared) until the amp failed to work by
which time there was a socking great hole in the board and a nasty
smell. It`a amazing how long a lot of musicians will put up with faulty
gear, prefering to ignore problems till the item stops working.

Doesn't really need a post mortem.


I agree that is the normal etiology and pathology, carrying on the
medico-speak. But this was a socking great burnt patch and billowing smoke
out of the 1/4 inch socket holes, that developed after sound check and
before use proper. Agreed there could have been long term, unnoticed, minor
fizzing but I've never seen this sort of damage in this sort of mains power
line circumstance.

I`m surprised, it`s a pretty common failure mode.
Bear in mind that the point of arcing usually comes before the
equipments internal mains fuse, but after the fuse in the plug top,
which is more often than not rated at 13amps.
Once the board starts to carbonise quite a large current can flow, and
the current is only flowing between two points it was intented to flow
anyway - at least initially. It doesnt take long for it to burn a hole
in a board.

I'd agree with your analysis. I have seen both hairline cracks such as you
mention and 'cold joints' caused by failure to use an appropriate soldering
temperature, or soldering time when soldering components with large thermal mass
such an IEC connector PCB pin ( pressure will keep it working for a while but
it'll finally go high resistance ). Even seen that on TO-220 devices.

p.s. ( The TO-220 device acting as a heatsink to the solder via its leads, hence
preventing the alloying connection needed ). I've literally seen TO-220 devices
'fall out' of a PCB after a year or two leaving 3 nice rectangular holes in what
look to be perfect solder joints.

p.p.s The hairline crack around soldered joints is especially common where the
drilled hole size is not ideal ( oversize ) for the lead diameter. I have seen
manufacturers specify hole sizes very poorly with the inevitable outcome. Hence my
PCB footprints often use my own estimation of the appropriate drill size rather than
the data book one. Plenty of other people do the same.

Graham