N_Cook wrote:

Obviously I've come acros melting/burning, starting from bad connection
arcing , but not this amount of damage, so am requesting other opinions.

A superfluity of mains fuses all ok, on 240V UK mains, 5 amp in plug, 4 amp
chassis mounted and internal 5 amp all in series. Mains transformer seems
right sort of primary and secondary resistance. All other internal fuses ok
and no other visually obvious problems.
Burning right thru the pcb for 1/2 inch around the L pin of the IEC, melting
of the plastic of the IEC above the burning and even the linecord plug
surrounding the L pin melting where heat conducted through the pin
presumably. Remainder of IEC receptacle distorted from heat but holding
together.
The N pin soldering at the pcb is bad, but has continuity, I assume
pre-existing rather than from heat damage as 3/4 inch from the main L
arcing, so temp would not have reached solder melt point on the N pin. I
assume the L pin solder was worse and initial cause of arcing. Amp was just
idling , with no sound throughput, is that why so much damage as only tens
of mA passing in the arcs. ? If 0.5 amp or more, then the damage would have
been more extreme , but shorter duration, before total break in pcb track or
solder joint meant a fuse action in effect, before greater heat damage could
take effect. The remnant pcb charring is not obviously conductive (30 M ohm
DVM)

Incidently the earth pin soldering although smoke stained looks fine,
coincidence? or passing of electricity have an initiation effect on good
solder to become bad or does it require porous solder or something
inherently bad with it for a heating effect to come into play, long before
any full arcing?

Has someone spilled liquid into it?

If you measured the resistance of the charring with low voltage, you
might have had a false high reading; with 240v you could find that it
flashes over or sputters badly.

What would the normal running current and the maximum fault current from
a shorted transformer secondary have been? A bad joint of 5k-ohms in a
circuit carrying 20 mA will develop 2 watts, which is enough to char a
PCB.


I have come across some connector pairs purporting to be IEC-compliant,
where the pins and the socket connections hardly touched. I submitted
them to Trading Standards and was told that both halves did comply with
the specification - so you might have come across a similar problem to
that; and the fault may not be in the soldering.



--
~ Adrian Tuddenham ~
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