A friend of mine is afraid to work on a three gun projector,
which coincidentally has the same anode voltage as the 7704,
7904, etc, 24 KV.
It all depends on the current. For an anode accerating system, there is
very little (or no) current, so the power supply only has small capacitors
in the voltage multiplier. There is maybe 10-20 milliJoules stored.
Remember that the human body has a capacitance of 160pF typically, and you
get a static charge walking across a rug or stroking a cat of about 10kV in
dry conditions. You can certainly *feel* that - it causes me to yelp every
time I get out my car in dry weather - closing the door is a real adventure.
As to the lethality, I don't know the formal details, but I gave myself a
fright with my 7704A once. In a moment of brain fade I unplugged the HT
from the CRT (with the scope off), and made the mistake of touching the
exposed pin on the connector where it exits from the supply. Youch! Of
course the capacitors hold the charge (duur), so you get a nice 21kV belt.
It is enough to make you not want to repeat the exercise, but I'm still here
to tell the tale.
The anode supply in older scopes is far more dangerous, because you have
maybe 500V after the rectifiers across around 150uF of smoothing capacitor.
That is 9 Joules, and more than enough to send you off to the pearly gates
if you grab it with a vengeance. Once the skin is punched through, the
body's resistance is only around 100 ohms - so you get 5A flowing from the
500V for 15ms (RC time constant). A rather scary site at
t.html says that 5A (but for 1 second) causes "Ventricular fibrillation (the
rhythmic pumping action of the heart ceases.) Muscular contraction and nerve
damage occur. Death is most likely."
I guess that the energy comes into this - so 5A for 15ms is the same as 75mA
for 1 second. They say about 75ms "Extreme pain, respiratory arrest, severe
muscular contractions. Individual cannot let go. Death is possible."
So watch that old tubed gear!
Craig
