Hello John
If you have a read of the posts in the following link, I think you will find that the focus problem with your 1604 is likely to be due to the smoothing capacitors in the power supply.
https://www.vintage-radio.net/forum/...ght=Gould+1604
You can use the “Search Facility” on the forum to find lots of information on the Gould 1604 and 1624 oscilloscopes.
Regards Stan.