Hi Fellas, i am getting close to finishing my restoration project of this 1974 DT 175 model A . the sealed beam headlight is not working as both filaments are bad ,is it possible to dismantle one of these units to replace the lamp or do we have to replace the whole thing ? any help would be great. thanks. Jez...

Jez, there have been a couple of us that have done just that. The results are the headlight isn’t focused, IF you are a night time rider. Your light is fried, so nothing to lose, try carefully disassembling it. H4 is the replacement bulb I used.
Others have bought a headlight that fits in the bezel, but uses different clips. Some searching on the forum should show you what the alternatives are…
ALSO before you start up your bike and check the headlights out, be sure your rectifier is in and working. If something is wonky, you’ll fry more bulbs.
Sneezles61

This might be worth a read, as Sneezles says depends if you want to ride at night.
CT1 Alternative Headlamp Unit from Yambits - Vintage Enduro Discussions (yamaha-enduros.com)

And fit a 6v voltage regulator, to save the same thing happening again.

thanks for the replies . is there a way of testing the rectifier before starting the bike or is it a case of trying it when its running ?

Gr8 uncleAl, that’s what I meant.. regulator… but rectifier as well…
Sneezles61

"is there a way of testing the rectifier before starting the bike or is it a case of trying it when its running ?"

Yes, if you have a meter that can test resistance. Checking procedure is in the service manual, which should be in the Tech Library (see top toolbar).  

"Gr8 uncleAl, that’s what I meant.. regulator… but rectifier as well…"

Loads of fleabay listings mix these up, together with cdi units and ballast resistors. If you have patience, you can snaffle a bargain as a result of this.