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Troubleshooting Procedures
6–26
1780R-Series Service Manual
D. U460 pin 3
E. U460 pin 4
23. Connect the test oscilloscope probe to the emitter of Q250. With the variac
set to 110 V, temporarily short the +5 V supply to ground, and check that the
voltage on Q250 does not rise above 1.7 V. Power supply should operate in
short bursts and then shut down.
CAUTION. Power supply must be connected to the external resistive load.
Integrated circuits powered from the +5 V supply will be damaged by this test.