HP Designjet H-Series and HP Scitex FB950 Printer - Troubleshooting vacuum issues

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5.2 Shared components
There are numerous components that are shared by the vacuum system and the pressure system,
and will carry one or the other depending on the moment in printer function. Most of these
components are located on the carriage.
5.2.1 Manifold and valves
The manifold assembly (see Figure 6) receives both the long tube for vacuum and the long tube for
pressure. The individual valves, one for each color, are switched by the printer to apply either
negative pressure (vacuum) or positive pressure depending on the moment. The normal state of the
valve is open to allow vacuum to enter the air tubes that descend from the valves to the printhead
reservoirs.
5.2.2 Air tubes to the printhead reservoirs
The dual air tubes (see Figure 3) connect the air valves to the printhead reservoirs. Depending on
the valve state either vacuum or positive pressure is allowed to pass through the tubes and act on
the ink in the reservoirs.
5.2.3 Printhead reservoirs
The sealed printhead reservoirs (See Figures 22 and 23) allow the vacuum to be exerted against
the ink to hold the ink in place, or become pressurized to force ink out through the printhead jets
during cleaning or maintenance.
5.2.4 U-tubes
On FB950 and H45100 and H45500, all with two printheads per color, an opaque U-tube at the top
of the reservoir connects the two printheads and allows air to move between them. A second U-tube
at the front of the reservoir allows ink to flow between the two printheads. While similar in
appearance, the U-tube for air and the U-tube for ink are different materials and are not
interchangeable. The air-tube material has an adverse reaction to the ink and will deform. Generally
these U-tubes do not fail, but occasionally it may be desirable to replace one due to suspected
leakage.