Easiest way to check the lamps is to use a test meter for continuity across the two pins.
It is possible that the lamp holder contacts are getting dirty with corrosion where the pins make contact (or not) with the holder.
Another option is that the lamps may be of lesser quality so the pin thickness or spacing could be out.
Mike
BTW, which lamps are you using? There are reports of 300w or 150w lamps being used in error.
|