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Troubleshooting Procedures
1780R-Series Service Manual
6–29
4. Set the 1780R-Series instrument POWER switch to ON.
5. Set the 1780R-Series front-panel controls for best viewing of the color bar
vectors on the Vectorscope CRT and of the waveform displayed on the
Waveform CRT. Turn the SCALE control to obtain the highest level of
illumination for both CRT graticules.
6. Set the autotransformer to low line (90 V for 115 VAC operation; 180 V for
230 VAC operation).
7. Set the 1780R-Series POWER switch to OFF (STANDBY). Connect the
180 W, 10W, resistive load between the top end of A2L638 and chassis
ground. Miniature alligator clips may be used for connecting the load
between A2L638 and ground. (The +60 V supply is applied through A2L339
to the top end of A2L638; see Diagram <6>.)
8. Turn A1R466 fully clockwise (maximum resistance). Then, turn this
adjustment counterclockwise about one-third of its rotation range. Set the
POWER switch to ON.
9. ADJUST Current Limit Adj (A1R466) slowly counterclockwise until stable
operation is obtained. Typically, stable operation occurs near the center of
the adjustment range.
CAUTION. Avoid turning the adjustment fully counterclockwise (minimum
resistance); otherwise, the switching transistor, A1Q250, may become damaged.
10. Set the instrument POWER switch to OFF (STANDBY). Disconnect the
resistive load. Set the POWER switch to ON and check for stable operation
at low line. The instrument should operate normally.