You can test for a bad cartridge by reversing the two RCA plugs on the amp that come from the pickup.
Another test is to plug in a source such as radio tuner or MP3 player into those two RCA plugs while the machine is playing.
Next test is to connect the "aux output" plugs (also RCA) on the amp to an external amp. If you now have sound from both jacks the problem is most likely in the power amplifier stage.
These Wurlitzer amps were very reliable, but it is 44 years old now.
You might try these groups to find someone who will rebuild the amp:
http://dir.groups.yahoo.com/group/wurlitzer/
http://www.phonoland.com/viewforum.php?f=3
Or here for a schematic is you are up to doing the troubleshoot/repair yourself:
http://www.verntisdale.com/Wurlitzerschem.htm
And:
http://thearcadeboneyard.com/filelisting61.php
Note that the machine needs to be properly lubed and a general manual is also a good idea.
Rob>NYC