Hi, I have just bought a Yamaha Dt 175, it is a 1973 model. My bike won't start and isn't producing a spark to the plug. I have seen the bike running perfectly  however it has been stood over the winter prior to me buying it and not been used or started! I have charged the battery fully and the ignition light is on, the horn works but no spark to ignite! If possible could you please point me in the right direction of were to start and look to identify the issue. I am very much the novice so any help is greatly appreciated.

 from near Salisbury, UK, Sedgey.

Where are you based - UK, US or Down Under?

The DT175 wasn't introduced until 1974, before that the bikes were designated CT1, CT2 and CT3 - check out the VIN table in this site's Tech Library to confirm what you have. This point might seem a bit anal, but it will ensure that, if you need any parts, the correct ones are bought. 

Going back to your problem, I would clean the faces of the points.

It would be great to see photos of your bike.

Welcome from NZ downunder. I second all that GREATUNCLEAL says above.
Just to add [unless i missed it] if yours is a CT3 they have CT1 on the engine & frame prefix right from "70 to "73 inclusive.
Not sure what happens when they become DT's in "74 but maybe just a number prefix.
Anyway--if ya lucky just give the points a scratch through the flywheel window emery paper or with maybe a points file [do they still exist lol].
Or i've seen some sandpaper nail file sticks around that are stiff enough to do the job if there's room to do it through the flywheel window [or trim them narrower].
But after that squirt some flush some through the open points just to flush any grit out.
Maybe not the best option but going for the easy one if you don't have the correct flywheel puller.
Yamaha flywheel pullers for this are cheap & everywhere on ebay & fit not only yamaha.
I have no luck linking fleabay so hope the number works by entering it.

eBay item number:182851872499

Yep that works by entering the number only on here--worked for me anyway. Just a sample puller but what you need 27mm left hand thread.
Don't know why my writing style changes up top for no reason--annoying.
www.ebay.com/sch/ebayadvsearch

Greetings from Tennessee 

Thankyou for your reply and advice GR8UNCLEAL and RT325. I am in the UK, up in the northeast. I have checked the tech library and it would appear it is a CT3. I have cleaned the points best I can through the flywheel and also the spark plug today but still no spark. The lad I bought the bike from did have the flywheel off and changed the points so I'm unsure if something has happened during the process? I will get a flywheel puller ordered and dig a little deeper into this. I am quite the novice when it comes to engines so I really appreciate any advice given. The bike is at my pals home and I am getting it to my address tomorrow so hopefully get some photo's across soon.

Howay man, howay! 

Technically I am half Geordie, on my mum's (mam's!) side, and must have fifty or so cousins up there.

Repro points can be shite! 

He has probably put the insulating washers on the wires from source coil, and condensor on in the wrong place, and source coil output is going straight to earth.

Was just going to write what YAMFAN said above, yep be sure the wire is on top of insulating washer or will be grounded.
Also be sure to tighten the flywheel fully or damage the crank key.
If ya luck the flywheel might come off easily if previous owner has been there & not tightened it up much.
Also hope whatever puller he used didn't damage the windings.
& there's a little circle piece that sits in to keep the bolt from grounding sideway but you'd be unlucky-unlikely to find that missing.
What i'd do [carefully] surmising it might be a 'not tight' fit is get a lever behind the flywheel & back the nut out flush then give it a TAP.  Don't tell anyone i said that!! haha.          
While waiting for your new puller that is.
& when puller arrives be sure to remove both spring & flat washers as puller is a fine thread & needs full engagement to be safe. Back the center puller bolt right out so you don't get a false impression of when the puller body is right home.
Just saying--as have seen all sorts of damage over the years caused by carelessness.
 

Thanks so much to GR8UNCLEAL, Yamfan & RT325. Really appreciate your advice, will give it a go.