HP 5890 SERIES II Service Manual page 11

Power On Sequence
As long as correct Line Voltage is provided to the instrument, the following events should occur when
the instrument' s Line Power Switch is placed in its ON position. Main Transformer (T1) primary winding
is energized (fused by F2 on AC Board or F1 & F2 if split phase). (Refer to Section 9 of the IPB portion
of this document for main PCB information.)
The three secondary windings (3 VAC, 40 VAC, and 120 VAC) of T1 are energized. All are fused on the
Main Board. These voltages arrive on the main board through connector J10. (Refer to appendix A of
this document for all connector information.)
The 40 VAC is rectified and divided to produce +24 VDC and +15 VDC.
From these DC supplies, the +10 VDC, the +5 VDC, and the Master Oscillator (for clocks) begin to
function.
As these other supplies begin, the (POWER ON PULSE) and the CPU portion of the circuitry begin to
initialize, first themselves, and then through the Data Bus and other portions of the instrument.
One of the initializations that occurs through the Data Bus is in the Triac Control section to produce the
control signal for K3 (zone power relay) on the Main Board and the signal for K1 ( oven contactor) on
the AC Board.
K3 relay allows distribution of the fused 120 VAC secondary from the Main Transformer.
K1 power contactor on the AC Board allows line power distribution to the Oven Fan Motor and to the
Oven Heater Triac circuit. Note that the oven fan will run as long as power is allowed (under software
control) through K1. However, the oven heater is NOT energized until oven heat is requested.
SVC 1- 4
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