Hello,
I have similar problem. Contacted Rohde Schwarz, they said that must contact SKY Messtechnik which are in charge for HAMEG etc. Finalu succeed to get service manul for HAMEG HM-1508-2. Unfortunately no circuit diagrams. Asked SKY Messtechnik but they refused to provide and offered to repair instead. Anyway, if you need the Service Manual, please send a PM