Power Supply - Printronix p300 Maintenance Manual

2–8. Power Supply
–continued
c. +36 V Supply, P600 Only (Figure 2–40)
A full wave rectifier consisting of half of the +70 Volt bridge and two SCRs rectifies the voltage from the
+36 V winding. Each SCR controls the duration of current flow during one half cycle. A feed back circuit
maintains a relatively constant pulsating DC voltage at the SCR cathodes.
The pulsating DC voltage passes through a choke on the power supply chassis to filter R1/C1. Filtered +36
Vdc powers the hammer bank and the hammer driver PCBA. Filtered +36 Vdc is fused, regulated, and
connected to the circuit breaker to trip the circuit breaker when the +36 Vdc voltage becomes excessive.
Regulated +36 Vdc is also connected to a switching circuit that produces switched +36 Vdc voltage, VSW,
for the ribbon drive assembly.
The comparator amplifier samples the regulated voltage and compares it with the 1.0 Volt VREF level.
The SCR in the optical coupler ensures that the rectifier SCRs are turned off only at the zero crossover of
the AC waveform.
d. +70 V Supply, P600 and P600XQ Only (Figure 2–40)
The +70 Vdc supply powers the paper feed drive circuits. The supply is derived from the 36 Vac. A rectifier
produces a pulsating +70 Vdc which is filtered.
e. Reference Voltage Circuit, P600 and P600XQ Only (Figure 2–41)
Filtered +9.5 Vdc from the transformer secondary is attached to the reference voltage circuit and used as a
positive voltage reference source (+9.5 V) for the fault detection circuitry. The +9.5 V is fused and applied
to the +2.6 V regulator. The +2.6 V regulator provides the bias for the +5 V regulator and is the input to the
1.0 V VREF amplifier. The reference level is precisely set by factory selected resistor.
f. +5 V Supply, P600 and P600XQ Only (Figure 2–42)
The regulated +5 Vdc supply provides power to all TTL circuits in the printer. It also generates the hammer
bank master clear signal, HMC, at initial power on to prevent spurious hammer release.
g. Fault Detection, P600 and P600XQ Only
Four fault sources are monitored: +5 Vdc, –36 Vdc, +9 Vdc, and the paper out switch. Any time a voltage
supply is out of tolerance or the paper out switch is actuated, the corresponding LED is lit.
h. Paper Feed Disable, P600 and P600XQ Only (Figure 2–36)
The paper feed disable circuits are controlled by the paper drive enable signal, NPDE. When NPDE is
active, the paper feed disable signal NPFD is inactive and the paper feed pair signal, PF1/PF2, reach the
paper feed drivers. The VSW logic switch is enabled, and VSW powers the ribbon assembly thereby acti-
vating the ribbon transport. When NDPE is inactive, NPFD becomes active, PF1/PF2 are kept from the
paper feed driver and VSW is switched off, halting the ribbon transport.
P300/P600 Principles of Operation 2–61