I just bought an older magna 6400 cable converter box, but the parental controls seem to be locked and I don't have any idea what the code is. Is there any way to unlock parental controls without knowing the code?

If its a newer box (less than 5 years) usually the only way to reset the parental controls was through the provisioning service the cable company had. If its a non digital box and just has a v-chip, you may just be able to remove the chip and boot the cable box. If you're really unlucky the parental control is a code written in a flash chip that will need to be erased.

it's from the early 80's, so it definitely isn't digital, I'll have to look to see if there are any chips like that to remove, though.

A lot of boxes have a default code that will unlock it regardless of what the code was originally set to. Others have a button sequence (hold button x for so many seconds, hold button x while powering the unit on, etc) that resets the unit to default. This is usually listed somewhere in the manual if you can find it.

Alternatively you may be able to reset it to it's default state by zeroing the NVRAM (by disconnecting the battery), but this will also zero the serial number and channel map "disconnecting" the box. There are various ways to reprogram the box after this, but I don't know if cable hacking is verboten on this board so I'll leave it at that.