Jim,
The DB100A or '+' was intially used with the 'Big Boy' starter set. It can be used as a booster with other sets. From the Digitrax 'Big Boy' manual:
Connect the two gray terminals, SYNC & GROUND, with a short length of wire, to select the booster mode when DB100 is initially powered up.
I have an old DB100a myself(picked it up from a broken up Big Boy set the dealer had for $40). I use it as a booster and it works great. Depending on the 'vintage' of DB100, it will give you between 4.5 and 5.0 additional amps.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin
I have posted this on digitrax as well I have a DB 100+ booster that says offline . The power and offline lights up when powered and there are no beeps ,nothing comes up on the dt 400 either any suggestions would be helpful thanks Jim