Prevent oscilloscope scrolling
There are several possible problems here:


You haven't set the trigger properly. Make sure that the trigger mode selector is set to "HF", the source selector is set to an input that is active, and the trigger level is set appropriately.
You may be expecting this to be a storage oscilloscope. It is not. It can only display a periodic signal which is currently being supplied to the instrument.
The oscilloscope is broken. If so, throw it away -- this was an inexpensive instrument when it was made in the 1970s, and is unlikely to be worth repairing.