Hi Fellas, i have finished my latest project restoring this 1974 DT 175 model A. i have had to replace the headlight bulb three times in just a short period of time so i presume that there is an issue with voltage regulation ? i cant see anything on the system that looks like it is designed to control the voltage output, the lights get brighter the more revs ? has anyone had issues like this and rectified it ? any ideas would be great .thanks.....

Yes, a known problem on pre 1975/76 6v bikes.

Yamaha issued a Service Bulletin on the subject and this is in the Tech Library - I have attached a link below.

Vintage Yamaha Enduro Fans - Service Bulletin #M5-084 re Voltage Regulators (yamaha-enduros.com)

Another way of 'curing' the issue is to fit a 12v headlight bulb. However, I've no idea what quality of light you get when doing this! 

Also important to make sure you have a good battery installed and charging (rectifier not bad)

Check it's actually charging back to the battery which sort of regulates the system providing all bulbs are as original specs. A more modern AC single wire regulator is good though which will save your bulbs in a situation like you now have. lucky you have replaceable bulbs & not a sealed unit.

Hi All, thanks for all the useful replies.the bike has a brand new battery and all the bulbs are as the spec and i fitted a new rectifier during the restoration.i have just read the stuff on the regulator fitting as a amendment and its good to know that there was a problem and they have provided a solution . i have managed to find one and i will fit it and hopefully it should cure the issue.thanks for all the help. ill keep you informed .

I'll edit to say sorry to go on.
I wouldn't say they had a problem as such & the only problem was owners letting the battery run dry so there's no regulating going on with battery taking up the excess with lights on to keep it charged. Daytime charging is done from another wire from the lighting coil then from what I think or remember the night time charge then comes off the yellow wire that drives the lights. So regulator was to save bulbs mainly--I think!!
Double edit lol just be sure it's charging across the battery with a dc volt meter or as I used to do use an amp meter between negative battery lead & earth on the frame.
Ok, morning rant over for me & happy to be corrected.