Service manual

RC2000 Service Manual Chapter 2 Theory of Operation 15
Research Concepts, Inc; 10679 Widmer; Lenexa, Kansas; USA 66215 WWW.RESEARCHCONCEPTS.COM
Quantity Type Value Ref. Designators
1 TMOSN BS170 Q1
1 TRANZORB P6KE6.8 D12
1 VOLTREG LM140-5 VR2
1 ZORB P6KE6.8 D6
Total Parts: 147
2.3 Drive Board
The motor drive board, located in the center of the RC2000 chassis, contains the circuitry which converts
the AC power from the drive winding of the transformer to DC and, using solid-state devices, switches
this DC power across the motor drive lines. Other circuitry on the board monitors the current flow to the
motors and provides fail-safe drive power interruption.
2.3.1 Assembly Diagram
The drive board, shown in Figure 5 is 7 3/4 inches long, 4 inches wide, and 3 inches tall with the installed
components. On the left side of the board is a large aluminum heatsink on which is mounted eight
MOSFET devices. On the front of the board near the 3-terminal power inlet Molex connector, J1, are
two normally open drive relays. The back side of the board holds the 6-terminal drive output connector,
J2. Control of the various board functions is performed by the digital board through the 16-pin header,
J3.
2.3.2 Circuit Description
Figure 6 contains the schematic diagram for the drive board. AC power enters the drive board from the
secondary of the transformer through connector J1 after having been routed through the rear-panel-
mounted, 12 amp, circuit breaker. The power is converted to an unloaded 40 VDC through bridge
rectifier, BR-1 and capacitor, C2. Two relays, K1 and K2, provide a normally open connection to the
azimuth and elevation drive MOSFETs respectively. These 24 VDC relays are actuated through open
collector Darlington drivers U2A and U1A and voltage dropping resistors R12 and R13. A series-pass
voltage regulator, VR1, provides a 12-volt supply for the current sensing circuitry. The combination of
resistor R38 and diode D11 performs a "pre-regulation" function to limit power dissipation in VR1.
2.3.3 Current Sensing Circuitry
The azimuth drive current flows through a precision power resistor, R17. The voltage drop across the
resistor is scaled by the resistor network, R14-R16 and R18-R20. This voltage is presented to the low-
pass difference amplifier circuit consisting of OP-AMP U3B and surrounding circuitry. This provides a
single ended voltage which is proportional to average current flow with the characteristic of about 0.166
volts/amp. This sense voltage is then presented to a comparator circuit consisting of U3A and
surrounding circuit elements. Potentiometer P1 provides the comparison voltage which triggers the drive
fault signal, an active high TTL level on pin 8 of header J3 labeled AZ_FAULT. A precision voltage
reference, U3E, provides a stable 5.5-volt reference signal for P1. The standard factory adjustment for
P1 is for an azimuth test point voltage ( at AZ_TP) of 3.25 volts. An exact duplicate of this circuit
performs the current sensing function for the elevation axis.