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Theory of Operation
Analog Section Detailed Circuit Description
2
2-37
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.