Fluke 5720A service manual PA Operation 220V AC Range, High Voltage Assembly Support Mode

Theory of Operation 2

Analog Section Detailed Circuit Description

2-149. PA Operation: 220V AC Range

During ac operation, Power Amplifier gain is -10 as determined by the 4.99 kΩ input resistor R17, and 49.9 kΩ feedback resistors (R11 + R12 + R13). Control line SW3 turns Q50 on, which references the +input of the precision op amp in the input stage to OS COM. The Oscillator assembly (A13) is set to the 22V range and its output OSC OUT HI is connected to the input resistor R17 by relay K10A. The Power Amplifier output is connected to the feedback resistors R11-R13 by relay K12B.

The Power Amplifier output is attenuated by a precise 1/100 by 220V range ac attenuator. The attenuated signal is connected to OSC SENSE HI, where it is sent to the Oscillator Control assembly (A12). The Oscillator Control assembly regulates the Oscillator Output so that an exact calibrated ac signal appears at OSC SENSE HI. Since the 220V range ac attenuator is completely characterized (as explained below), the exact desired signal appears at PA SENSE AC and hence at the appropriate sense point at the output.

The 220V range ac attenuator circuit contains op amp U4, a 400 kΩ/4 kΩ resistor network Z1, and transistor Q54. PA SENSE AC, which is connected to PA OUT HI at the load, is connected to the 400 kΩ input resistor (pin 1) of Z1 by relay K16. The 400 kΩ/4 kΩ node (pin 3) of Z1 is connected to the inverting input of U4.

During ac operation, control line C0* is inverted by U8, which turns on Q58 to connect the non-inverting input to OSC RCOM. Transistor Q54 supplies current gain for the output of U4 to drive the capacitance of the OSC SENSE HI line. This voltage is connected to the 4 kΩ feedback resistor (pin 4) of Z1. The output is connected to OSC SENSE HI by relays K10B and K11. The dc feedback 500 kΩ/25 kΩ resistor network and the sense-current cancellation circuitry are disconnected by energizing K15.

The sense signal, PA SENSE AC, and the output signal, PA OUT HI, are routed to the High Voltage Control assembly (A14), where relays K10, K13, and K3 connect them to HV SENSE and HV OUT. HV SENSE and HV OUT are connected to the binding posts by the motherboard relays in the same manner as in the 1100V high voltage mode. Refer to the High Voltage assembly theory of operation for more information.

2-150. High Voltage Assembly Support Mode

2-151. High Voltage AC 1100V Range

During high voltage ac operation, the output of the Power Amplifier is routed to the input of a step up/down transformer on the High Voltage Control assembly (A14). Overall gain from the Oscillator Output to the High Voltage Output is -100 as determined by the 4.99 kΩ input resistor R17, and 500 kΩ feedback resistor (R3 + R4 + R81 + R82 + R83). Relay K10A connects the ac signal OSC OUT HI from the Oscillator Output assembly to input resistor R17. Relay K12 remains open as shown on the schematic, while K15 is energized to remove feedback used in 220V-dc operation.

Output from the High Voltage Control assembly, called HVAC on the schematic, is connected to the feedback resistors. Relay K17B is energized to close the feedback loop. Control line SW3 is high, which turns Q50 on and turns Q51 off. Because sensing back to the Oscillator Control assembly is done by the High Voltage/High Current assembly, the output of the 220V range ac attenuator is disabled from OSC SENSE HI by relay K11.

2-152. High Voltage DC 1100V Range and Current 2.2A Range

Operation of the Power Amplifier is the same for the 1100V dc and the 2.2A current functions. In these functions, the output of the Power Amplifier is routed to a step up/down transformer on the High Voltage Control assembly (A14). The Power Amplifier

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