Hi Bruce! Merry Christmas to all!
Thanks for uploading the user manual - I've also been looking for the service manual too.
No schematic, but there is a repair guide for the squeal fault for the DH800 at the bottom of Paul Fox's DH page here, which may also have some other schematic info:
http://homepage.ntlworld.com/paul.fox4/casiofixes.htm
Are you just wanting the service manual for keepsake, or you have a problem with your horn?
Here is another place:
http://www.wdgreenhill.com/manu/casio_a.htm
I have bought Casio service manuals off this guy before, he has the DH800 listed among his manuals - it's not clear if it the service manual or the user manual. Email him and find out - he had some very hard to get service manuals that no one else had. They're paper copies though only, not pdf.
Cheers, Graham