Fluke 5720A 2-67. ±250V and ±500V Supplies, 2-68. +PA and -PA Supplies, ∙ PA CT is the center tap

Theory of Operation 2

Analog Section Detailed Circuit Description

2-67. ±250V and ±500V Supplies

Input signals PA CT, PA HAC1, PA HAC2, PA LAC1, and PA LAC2 come from the main transformer.

∙PA CT is the center tap.

∙PA HAC1 and PA HAC2 are high voltage taps with approximately 400V rms and are fused by F201 and F204 respectively.

∙PA LAC1 and PA LAC2 are lower voltage taps with approximately 200V rms and are fused by F202 and F203 respectively.

∙PA LAC1 and PA LAC2 and bridge rectifier CR222 generate the unregulated ±250V supplies.

∙PA HAC1 and PA HAC2 and diodes CR217, CR218, CR220, CR221, CR223, CR224, CR227, and CR228 are in a bridge rectifier configuration to generate the unregulated ±500V supplies.

When the PA supply outputs ±185V, current is drawn only from the lower voltage taps LAC1 and LAC2.

2-68. +PA and -PA Supplies

Unregulated voltage for +PA is selected between the +250V and +500V unregulated supplies by control line +HI/LO V and diodes CR201, CR203, CR208, and CR210.

Transistor Q204 is a current source that biases VR212 and VR216.

Supply +PA is +185V when the +HI/LO V signal is above 3V. (Transistors Q205 and Q208 are turned on.) Transistor Q205 shorts VR216, while Q208 places a short across VR214, VR215, and VR219. A voltage of +185V appears at the gate of Q202 because of VR212. Approximately 140V appears at the gate of Q201. This 140V and about 250V at the anode of CR208 reverse biases CR201 and CR210, thereby shutting off Q201. Current flows out of the 250V unregulated supply through CR203 and Q202. Regulated supply voltage +PA is determined by the gate voltage of Q202, which is about 190V when +HI/LO V is above 3V.

When +HI/LO V is close to 0V, both Q208 and Q205 are turned off, and over 400V appears at the gate of Q201. CR203 and CR208 are reverse biased, and the supply current flows from the +500V unregulated supply through Q201, CR201, and Q202. The gate voltage of Q202 is approximately 370V, nearly the same as +PA.

The -PA side works exactly like the +PA side except -PA is switched between -185V and -365V by control line -HI/LO V.

The ±PA supplies can be replaced by the ±44SR unregulated supplies by moving switch S201 switched towards the top edge of the board. This feature provides the means to troubleshoot the Power Amplifier using reduced voltages.

NOTE

Make sure S201 is returned to the ±PA position before resuming normal operation.

2-69. ±PA Supply Current Limit

The current limit for both the +PA and -PA supplies is set to either about 90 mA (K201 de-energized) or 150 mA (K201 energized) by K201. Control line +LO/HI I controls relay K201. The high-current mode (150 mA) is used during operation in the 1100V dc range and the 2.2A range.

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