The orthophonic credenza was the ultimate hi fidelity of acoustic
recording technology.  The "head" you refer to is called a reproducer
(no piano in sight).  The original reproducer was probably made of pot
metal which often erupts into powder eventually, so gluing back is not
uncommon.  If you are lucky some of them were brass.  Some models used
an electric motor.  Some used springs.  If the unit is the wind up
version, then you must dismantle the motor and clean it all with
carburator cleaner or other degreasing cleaner.  I use my ultrasonic
cleaner with clock cleaning solution.  The spring MUST be removed and
cleaned very well and repacked with grease or you risk breaking it
from sticking together in the old muck that is in there. The wound
spring will stick to itself and as it plays, will release explosively
which will break the spring before too many times of doing that.

The reproducer for the credenza will be entirely encased in a metal case
with slots around the radius on one side.  The diaphragm inside, you
will notice is made of specially pressed aluminum.  The shape of these
Art Deco type of designs pressed into the aluminum are very important
to the credenza's ability to reproduce hi fidelity without amplifying
the snap, crackle and pop of the old Victrolas you usually hear.  The
tone arm goes down into the credenza case and into a folded horn which
also helps depress crackle noise and amplify what is recorded on the
record.  The horn has two openings, a large one for bass and a smaller
one for midrange and highs.

I cannot tell you about parts but just do not do any business with the
folks in Davenport Center as they are too crazy to deal with.

D. L. Bullock Piano World St. Louis