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SECTION 7
SERVICE INSTRUCTIONS
3.
Remove the thumb screws (2 Fig. 11) and
brackets (1 Fig. 11) that secure sieve beds
to unit vertical divider.
4.
Remove the hose clamps and tubing from
fittings at top of sieve beds.
5.
Remove hose clamps and reinforced tub-
ing from fittings at bottom of sieve beds.
6.
Install
new
sieve
beds
in reverse
order.
Make sure the end of bed with nine digit
serial number and label is at the top of
unit.
NOTE:
Ensure that the sealing caps are
removed from the new
sieve beds just
prior to connecting hoses.
7.
Leak test hose connections with a certi-
fied leak detection compound
such
as
"Snoop" or "Search".
8.
Close cabinet covers and secure with cabi-
net fasteners.
Four-Way Valve
The pneumatically operated
four-way valve
(14 Fig. 11) alternately distributes pressure
supplied by the compressor to the two mole-
cular sieve beds approximately every 11 sec-
onds. While one bed is being pressurized the
other bed
is being exhausted
through the
four-way
valve.
Exhaust
gases
passing
through the four-way are directed through
the manifold to the purge valve so they can
be released from the unit.
The
four-way
valve,
while
operating
pneumatically,
uses an external
pilot valve
that allows pressure to shift the spool inside
the four-way. This pilot valve receives a signal
(approximately
12-15
VDC)
from
the
MC
board
causing it to open
and
allow
pilot
pressure to activate the four-way. When the
four-way is activated the right sieve bed will
pressurize.
Once the bed
fills the oxygen
accumulator tank, the cycle is changed
by
removing the signal to the pilot valve. This
will cause the four-way to deactivate and
pressurize the left sieve bed.
NOTE: The four-way valve will not shift until
the oxygen accumulator tank is filled. This
will take
approximately
11
seconds
de-
pending on altitude and liter flow.
If the four-way
does not shift, the alarm
system may be activated and the short and no
cycle lights on the MC board will illuminate.
See Section
5, Alarm
System.
If the alarm
10/87
16
system is not activated, short cycle times and
low oxygen concentrations will result because
the same bed will continue to pressurize.
There are several reasons for the four-way to
malfunction. The cause of failure must be
determined
before
the
proper
corrective
action can be taken.
Some reasons for four-way failure are:
(1) Continuous or no voltage to pilot valve on
four-way.
(2) Little or no pilot pressure.
(3) Foreign matter inside valve preventing
spool from shifting.
(4) Defective or damaged four-way.
(5) O-rings
on sleeve
may
be worn
pre-
venting the valve from shifting properly.
To test four-way valve:
1.
Connect pressure gauges furnished in the
DeVO Service Kit to the test points (Fig.
22) located on the manifold.
2.
Turn
unit on
and
observe
the
green
indicator light (8 Fig. 12) labeled four-way
on the MC
board.
When
this light is
illuminated the four-way should be acti-
vated and the right bed should be pres-
surizing.
If the
left
bed
pressurizes
instead, this indicates that the four-way
did not properly shift.
If the right bed
does
pressurize
first,
observe whether pressure is shifted to the
left bed once the shift point pressure is
reached or if the right bed proceeds to
pressurize again. In either case, if either
bed
continues
to pressurize
without
shifting, further testing is needed.
Refer
to the normal
cycling sequence
on
page
24.
3.
Place the test leads of a volt meter across
the terminals (See test position C, Figure
16) on top of the pilot valve. Approxi-
mately
12-15 VDC
should
be measured
while the green four-way indicator light is
illuminated (voltage will be supplied for
approximately
11 seconds or until the
oxygen accumulator tank is filled). If this
reading is not obtained, check for loose or
broken wires from the four-way valve to
the MC board. If no loose or broken wires
are found, replace the MC board.
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