[R-390] More AC Filter FFT

Perry Sandeen via R-390 r-390 at mailman.qth.net
Mon May 12 22:46:58 EDT 2014


List,
 
I apologize in advance for this long post.
 
 
<Wrote: IMHO, the original filter does not
leak, it has enough capacitive reactance to ground to trip a GFCI.
 
True
 
<Wrote: The problem of shocking the operator
goes away if the radio is properly grounded. If it is properly grounded, there
is no need for a GFCI.
 
Not quite.  Stuff happens.
 
I spent the last 25 years of my working years as a
Bio-medical Electronics Technician and am certified as such.  Electrical safety was the top of our list.
 
Let me explain a bit.  The heart is most sensitive to having its
signals disrupted at 59 Hz.  Above and
below that frequency, it is far less sensitive to disruption. The following is
simplified a bit but is accurate.
 
The lowest amount of current that they found that
would upset the hearts electrical signals was around 200 micro-amps applied
directly to the heart.
 
This led to the following requirements:
 
Ground cord resistance to chassis had to be less
than 500 milli-ohms.
 
Maximum ungrounded leakage with both normal and
reversed polarities of the AC line to the instrument had to be less than 500
micro-amps.
 
Most electrical instruments used had a leakage in
the 100 to 200 micro-amp range and a ground cord to chassis resistance around
100 milliohms.
 
Electrode leakage current (patient monitors) had
to be less than 10 micro-amps with 110 VAC applied.
 
These requirements basically meant that if you
even had a two fault failure, the patient’s heart would not go int0
fibrillation.  That is a condition where
the heart beats out of synchronization and blood is not pumped to the
body.  This will spoil your day permanently
after 5 minutes.
 
With the normal ungrounded leakage out of the
original filter, and one moist hand on the chassis and the other moist hand on
a good ground, the 5 milli-amp leakage is enough to kill you by putting your
heart into fibrillation.  That is why
GFCI’s have a max tripping current of 5 milli-amps.
 
Now Bubba Perrier and others have been *bit* by a
hot *A* or SP 600 chassis.  And probably
more times that we want to admit. 
 
Usually we get an *OWIE* followed by words not
heard in church. Especially if we then drop it on our body parts.
 
The difference between an *OWIE* and getting
killed depends basically on two conditions.  One is the skin moisture.  The
second is the entrance and the exit point of the body.  The shortest path is hand to hand as it
provides the closest path of least resistance to the heart.
 
Now I have seen and have bought, numerous
receivers where users solved this problem by adding a there wire cord grounded
to the chassis. It is quick and El Cheepo. At the time it was being implemented
there wasn’t the availability of line filters we have now.  It took the PC market and the noise from switching
regulators to spur the market to what we have today. 
 
Yes it works well until there is a defect in the
ground wire or the GFCI issue arises.
 
Now the correct solution these days is to replace the
original with a low leakage computer type of filter.
 
With a little notching in four places in the
original round mounting circle, the Corcom 3EEA1 filter is an EXACT match for
the screws of the original filter.  It is
safe and usually costs less than five dollars.
 
When you replace the filter there is another
issue.  In the original wiring
configuration, one input wire went to the power switch and the other to the
input fuse.  It is simple to reconfigure
the as there is plenty of room due to the new filter being smaller.  That leaves plenty of room to add a MOV to
the incoming line and replace the bathtub C103, 50uF/50 volt, with a new
electrolytic as this is a filter capacitor for the RF gain control.
 
At one time we thought those bright nickel-plated
tube shields were the best things for tubes. Then the IREC black aluminum
bodied tube shields with beryllium copper heat wicks were introduced and we
found that they were far superior to what we had before and we changed over to
them.
 
In the same manner new line filters have been
manufactured that have better RF suppression characteristics and solved the
high leakage/GFCI tripping problems of the original AC input filter ande are
easily retrofitted to our receivers. So for me it is a better part replacement,
not a modification.
 
Avoiding the chance of an *OWIE* or worse is my
motivation for putting them in my receivers. As Tisha pointed out it is our
choice to make.
 
Regards,
 
Perrier


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