I have a 1983 XL185S that developed a problem in the lighting circuit for the headlight.  The headlight has worked normally for years.  A couple of weeks ago, both filaments burned out.  I ordered a replacement headlight, but decided to rig an old XR headlight to use until the new bulb arrived.  I put in a new 12V H3 halogen bulb, and wired the headlight into the XL low beam circuit.  When I cranked the bike, the bulb seemed normal brightness; when I revved, it was extremely bright.  The bulb failed in only a few minutes.  I put a digital multimeter on that circuit, and read about 14 VAC at idle, increasing with revs.  I saw about 38 VAC at what was probably about 5000 rpm.  The 1983 XL185S has a 6V system, with one coil in the alternator dedicated to the headlight only; no rectifier or regulator in that circuit.  Any ideas what might have caused the alternator output to increase?  Any idea how to troubleshoot or diagnose the alternator?

Bill Evans
XL185S
XR200R