[R-390] FL-101 replacement Corcom IEC power Filters

[Tisha Hayes]

At one time I did compliance engineering and type acceptance testing for
radio, telcomms and computing equipment. The first product I ever tested
for our company completely failed in every category of Part 15 and Part 68.
It was just a neat little switching power supply with a supply fuse and a
power switch. Since it was post-engineering and "only" had to be modified
slightly to make the FCC happy I had to resort to engineering out my own
solutions.

Under Part 68 testing we also had to comply with the rather stringent
requirements of not splattering noise back across the telephone network.
They referred to noise as longitudinal or metallic noise. Some of it had to
do with the ideal balanced circuit of the telephone audio pair.

Part 15 had more to do with how much noise our little switching power
supply and oscillators were spewing out into the general environment
through the line cord or radiated from the case.

Parts of this testing were done in a giant wooden barn way out in the
country with no nearby transmitters. They had a reference antenna attached
to an arm that could sweep from vertical to horizontal, while also rotating
the antenna to change polarization and the DUT (device under test) was
mounted on a giant wooden turnstile that was rotated with ropes and pulleys.

It took all day, just for us to go through the testing with the spectrum
analyzer to find every form of emission coming out of our little modem/
data transfer switch. We failed the test several times and it was back to
the drawing board to figure out what was next to isolate the source of
interference that we were generating. Needless to say it was a painful but
long remembered exercise in product design.

I learned to get the best line filter possible for the device. Yes, I
engineered a few of my own but in almost every case it was cheaper and
easier to go with a Corcom (on our stuff, with the IEC plug) to clean up
what trash that switching power supply was putting out there. Sometimes it
was as simple as changing how the wires were routed in the device, how big
the ground plane there was on the circuit board, was there a star washer on
the ground wire to the chassis...

I also learned that the entire idea of "type acceptance based upon similar
products" is a complete load of B.S. That is the current FCC approach; "if
something has four legs and eats grass it must be a horse". So if you test
one horse you have tested them all. The reality is far different, even
innocuous changes in a device can have profound impacts upon what it does
to the EMI/RFI environment.

Our receivers are less critical but even in the 1960's someone recognized
that due to a minor flaw in manufacturing the radio made measurable noise
from the IF. Hence they added a mod to put finger-stock in certain parts of
the chassis. I guess they did not want an aircraft carrier catching a
nuclear version of a H.A.R.M. missile because some radio op was catching
the west coast show of Johnny Dollar on one of the spare '390's.

As had been mentioned, the radio has plenty of places where stray RF is
going to get inside of the radio. Much of this can be cured if owners would
put the utah plate back on, the top and bottom covers and the tube shields.
We know the caps in the line filters are getting leaky and now are tripping
GFCI outlets or putting a pleasant AC voltage on the chassis. Rather than
just bypassing the filter module I prefer to put a $40 filter on my $800
radio.

-- 
Ms. Tisha Hayes. AA4HA

*""In this denial of the right to participate in government, not merely the
degradation of woman and the perpetuation of a great injustice happens, but
the maiming and repudiation of one-half of the moral and intellectual power
of the government of the world." -- Fredrick Douglass"*