Hi, Skibsy the vast majority of service, parts, and owners manuals on the internet are "FREE" to download and all service manuals contain wiring diagrams in the back pages. The rest usually charge a modest fee around $10 sometimes more and there is a handful of obscure, rare, obsolete, and very old models that are no longer or never were available, then there is eBay where you may be able to find what you need at a very high price because of its age and rarity. Most of the "FREE" manuals will cover your exact make, model, and year otherwise one will be provided that comes as close as possible to your bike and will have most of the same info that an exact manual would have.
To download your manual please click on the blue links below. Good luck and have a wonderful day.
Yamaha DT100A 1974 Service Manual
OEM parts for Yamaha
Yamaha DT 175 1975
Yamaha DT Wikipedia