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Here's one way to troubleshoot a shadow mask that has come loose.

Note: The chassis must be bolted in before trying this.

- Place two identical cardboard boxes, 8-10 inches tall, on the floor in front of the set.

- Tilt the set forward, resting the top-left and top-right corners of the set onto the cardboard boxes.

- Turn the set on and check to see if the displayed image has changed.

- Turn the set off.

If the display changed, the shadow mask is the culprit. If you are not sure,

- Place two additional cardboard boxes under the bottom-left and bottom-right corner of the set. The front of the CRT should now be perpendicular to the floor.

- Turn the set on and check to see if the displayed image has changed.

- Turn the set off.

If the display changed, the shadow mask is the culprit. If you are not sure,

- Remove the two cardboard boxes located under the top-left and top-right corners of the set.

- Turn the set on and check to see if the displayed image has changed.

- Turn the set off.

If the display changed, the shadow mask is the culprit.

As you can imagine, this method, as well as others, is not foolproof.