So I finished the retrofit to the 504 devere head for a cold light -and have the metrolux hooked up - though I'd screwed up some by getting the polarity of the probe backwards - so I had to resolder it. I only discovered this because I tried to recalibrate the unit but came up with a snake eyes on the illumination value. At any rate. That's all fixed - but now I can't seem to recal the unit. I cannot imagine that they (the mfr) used PROMs (write once read many) instead of EPROMs (eraseable). That wouldn't make any sense. Is there a way to reset the unit to default values?