On Thu, 2 Oct 2014 17:52:25 -0700 (PDT), Phil Allison
wrote:

wrote:

I wonder if it would pay to use a conformal coating if you happen to know the customer is going to put the unit into the same environment again.


** One maker of Mosfet power amplifier( Perreaux of NZ) had a major issue with PCB dust/fluff contamination - it made the amps crackle loudly or even blow up spectacularly.

Two mistakes;

1. There were signal tracks that ran parallel along the main PCB, spaced by about 0.4mm that differed by 240 volts DC.

2. The PCB was bare - ie not coated in the usual green resist.

After some years of service, "tracking" began across that 0.4mm gap - after cleaning, one could see the area was darkened in many places.

The usual fix was to thoroughly clean the PCB with alcohol, scratch away any darkened material and then coat the PCB with a clear lacquer sold for the purpose.

With really bad examples, I peeled off the offending tracks and fitted plastic coated wires in place.



.... Phil






Another fun problem that they had with a mid 80s model amp was an
intermittent popping that might show up once a week or so. This was
caused by a small signal diode that when heated or cooled didn't
exhibit the symptom. I spent weeks on the first one before resolving
this issue.