OK, found the manual. I used to have this scanned on pdf but cannot find it. Must have been lost in last computer crash a few years back.
There are two settings, by dip switch only. They are credit only and credit selectable. Yours must be set on credit selectable now. Just push the cashout button BEFORE you begin playing. It should light up (if bulb works) and stay lit. Then as you play the credits will accumulate to upper limit, again set by dip switch. If you push cashout again it will drop all credits.
On the 8304 board the dip switches for above are on Dip Switch #2. I'll take a phone pic of the settings and post them.
Jim