I have LunaPro (old one that runs on mercury cells) and it consistently meters 0.7-1EV over Sekonic L508 I tested it with (at least in the range 0-15EV). How can it be adjusted(meter seems to be zeroed properly)? How much would cost to send it over to Gossen for calibration (if at all Gossen accepts old meters)? Any other ideas?
If it has a grey case,Gossen do not service these;No spares available. What cells are you using, 1.5 volt or 1.35 volt> For consistent errors in old meters, I just compensate by adjusting the ASA value and do regular cross checking with my other meters. However if you want to recalibrate yourself then go to Rick Oleson's site and look under tech notes for the Gossen meter 'words and music'.
rick_oleson.tripod.com
You can get very good results using a sunny cloudless day as a standard light source.