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System Service
WARNING
Death or serious injury may occur
if
the following precautions are
not observed.
While the input power is connected, use caution when working
inside the power supply. Many exposed conductors carry low DC
voltages which are capable of supplying heavy currents if
short-circuited, resulting in high heat and the possibility of painful
bums. Use caution when manipulating metal tools or probes. A
wrist watch, or a metal necklace, bracelet, or ring must not be
worn. Avoid dropping tools, screws, or other metal objects onto
conductors. Remove power and recover dropped objects at once;
if forgotten, damage could result later.
AC powerline voltage and 130 volts DC are exposed when covers
are removed. Exercise extreme caution when working in the power
supply with covers removed, and never work under this condition
unless another person
is
nearby and within sight. Also, remember
that the test equipment is floating with respect to earth ground, so
the cases can be at. the same line voltage as the point being
measured in the power supply. Thus test equipment must be
temporarily enclosed and marked dangerous to alert all personnel
of unsafe conditions.
If
feasible, before Performing any work inside the power supply,
unplug the AC power cable and wait 3 minutes for filter capacitors
to discharge. To prevent explosion resulting from internal heating,
always be sure to replace filter capacitors properly with respect to
polarity.
B-16.
HIGH-VOLTAGE POINTS. The highest AC voltage in the power supply is the AC line
voltage (250 volts rms, 350. volts peak). The highest DC voltage in the power supply is 130 volts.
The AC line voltage is exposed at the input circuits of the power supply and at filter choke Ll. The
130 volts DC is exposed at the preregulator filter circuit and the inverter assembly A7. Additionally,
AC voltages of 240 volts peak are exposed at transformers Tl and T2.
B-17.
TEST EQUIPMENT GROUND. If the test equipment has a metal case, the negative test lead
should not be connected to the case. Also, the negative lead should not be connected to digital
voltmeters that have floating (guarded) inputs, to multimeters, nor to power supply chassis. Instead,
the test equipment chassis should be connected to the computer system cabinet earth ground
through the test equipment power cable. For the oscilloscope, a three-prong to two-prong
conversion plug must be used on the power cable. Consequently, the oscilloscope case will "float.'
with respect to earth ground. All test equipment should be plugged into the AC power convenience
outlets provided in the computer system bay cabinets.
B-16
JAN 1977
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