Head light - 1981 Sportfire

[Fire22]

Hi, when I bought my 81 this summer, the headlight wasn't working but the other lights worked. I bought a new head light (Philips H5006C1 Halogen Sealed Beam) and plugged it in and it worked. I just had my first ride a couple weeks ago and after a couple miles I noticed the low beam was not working, switched to high beam and it worked. Today, another short ride and again after a few miles the high beam burnt out. All other lights work (tail, brake, tach, speedo).
Any ideas or suggestions would be appreciated, either I bought the wrong light or I have an electrical problem?

Thanks

[smitfire]

Your voltage regulator is out. I have them

[Matt]

You also might have the headlight wire rubbed through somewhere creating a short.

[Fire22]

thanks guys, I will check the wire for shorts, and put a meter on the headlight wire and see how much voltage I have at around 6000 rpm. I have a 79 Trailfire parts sled so if its the regulator I can see if its still with the parts sled and swap it in.

[Fire22]

Does anyone know if I can check the voltage regulator on the bench? I pulled the one off the parts Trailfire and it measure 136 ohms from the yellow wire to the black. The one on the Sportfire (suspected culprit) measures 146 ohms.
I have not yet tried measuring voltage with the motor running, weekend job.