I would talk to Michael at MAJ Electronic, who hold the obsolete HH parts inventory.
Be sure to read their homepage fully before you make contact, though. He is a nice enough chap and very helpful, but he doesn't suffer fools gladly. Schematics are free but they charge a £5.00 admin fee for sending and Service Manuals cost a bit more.
Anyhoo, by the fault you describe I would be very suspicious of the array of SAD1024A delay line chips on the top PCB. If any of these are stuffed (more than likely), then you are SOL as there are none left at MAJ - and good luck finding them anywhere else.