You're measuring it wrong. You don't measure the AC output of a transformer across ground. You have to measure it across it''s two poles.
In the case of a Space Invaders.....assuming your transformer is the same as mine....The top lugs are the outputs. If your label is still intact, it'll read......16.5, 9, 0, 9, 16.5, 0, and 12.
If you set the meter to AC volts and read across the two 16.5 lugs....you should see 33 volts.....arcoss the two 9 lugs, 18 volts. The 0 between the two 9s is the center tap for these windings. This means if you read from 0 to either of the 9s, you should see 9 volts AC. read 0 to either of the 16.5, you should see 16.5 volts. Now the last two lugs...0 and 12 are another winding. If you read across these lugs, you should get 12 volts. One note, these are unregulated volts....and generally run higher than their stated values. That's fine and normal.
Edward