Many of the original H28.5s came with Datamarine depth sounders and knotmeters. Here is one resource for their repair should you need it:

CONTACT DMI MARINE, INC
18 COMMERCE PARK RD
POCASSET, MA 02559
TEL:508-564-7491
FAX: 508-564-7497
THEY SPECIALIZE IN REPAIR OF DATAMARINE INSTRUMENTS.
http://www.dmimarine.com/