B&K Precision 1253 - Manual - Manual (Page 27)

Short-term high-voltage stability problems may be evident on the inner horizontal staircase of the PLUGE pattern.
Look for trapezoidal distortion of the lighter rectangles.
The Needle Pulse pattern is a more sensitive indicator of short-term stability problems. The upper half of the pattern
is black with narrow white vertical lines on both sides. The bottom half of the pattern is reversed. As a CRT beam
moves from the top half of the frame to the 100 IRE bottom half of the frame, a large increase in CRT beam current
is suddenly required. The thin black lines in the bottom half of the frame will bend outward if the high-voltage drops
and in most cases will return to their normal position before the bottom of the frame as the high-voltage supply
recovers. The amount of line deflection and the length of time it takes the line to return to normal are measures of
the high-voltage performance. In extreme cases the line may wiggle in an "S" pattern as the supply attempts to
recover. In some products, with excellent high voltage supplies, the line may show minimal or no short-term stability
effects.
The Overscan Bounce pattern can be used to observe long-term high-voltage stability. The Overscan Bounce
pattern changes repeatedly from a low to high APL. As the picture APL changes the picture size may momentarily
expand and contract. The amount of change can be measured by observing the percentage shift in overscan as the
high-voltage supply settles. Each of the lines on the borders of the Overscan Bounce pattern represents a 2.5%
change in size.
If high-voltage stability problems appear excessive it may be desirable to reduce the Contrast control setting and
settle for a somewhat less bright picture.