Nutone 3003 getting no power

My 3003 is tied into my doorbell chime box. The power appears to come into the chime box 1st. How, with my electric two prong power tester (with small light bulb) do I check where the power comes in to the door chime box, and where I am no longer getting any power? Chimebox, opened has lots of wires and card board covering some thing. Circuit breaker is OK. Any help at all, thanking y'all in advance. little cheryl

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