Power Supply - Tektronix Phaser 340 Service Manual

5
Theory of Operation

Power supply

5-14 Phaser 340 Color Printer
In general, the power supply has two main, yet interrelated sections: the AC
section and the DC section. In the AC section, AC power is routed to 10
opto-isolated triacs which, under main board logic control, supply AC power to
the eleven heaters in the printer.
Four fuses provide current protection to the triacs. Fuse F2 through F5 protect
the power supply from, most often, a shorted triac. If a F2 through F5 fuse
blows, it is best to replace the power supply rather than the fuse. Otherwise,
with the fuse replaced, but the triac shorted, AC power may be applied to the
heater without the printer even being turned on. Each time the main board
turns on a triac to activate a heater, it is turned on for only a fraction of a second.
The main board must constantly readdress each heater it wants to remain on. By
this means, if the print engine firmware should fail, the heaters automatically
shut off. The printer is also protected by five thermal fuses located inside: the
drum heater, the printhead's two ink reservoirs, the paper pre-heater and the
cap/wipe/purge assembly. A thermal fuse opens in the unlikely event of a
"runaway" heater following a firmware failure. If a thermal fuse opens, its unit
must be replaced.
The DC power supply generates +5 volts for the printer's logic, + 40 volts for the
motors, +54 volts and -40 volts for the printhead drivers, and +12 and -12 volts
for serial communication. A fuse provides protection for the switching power
supply in the DC section.
Warning Do not touch the power supply; AC line voltages are present. The
power-switch does not interrupt AC power to the power supply, it
only signals the supply and the printer logic to begin a shutdown
sequence. Even with the power switch in the OFF position, AC
line voltages are present on the power supply, heaters and heater
wiring.
There are no field adjustments necessary on the power supply.