Also confirmed on 743B, center is negative, I repeat center is negative.
Why Fluke is not specifying this!? It is not marked on the unit (even voltage is not given), not mentioned in the manual apart of only to use BE9005 Battery Eliminator, and its datasheet also does not show or mention about it

Initially I tried with center positive with 7.5V and it did no work, but also did not get damaged. The reason I tried it with center positive is because after some search with no result I saw a third-party adapter in eBay that was advertised as center positive and apparently an alternative for Fluke's BE9005 Battery Eliminator
