I inherited a Snap On Blue Point YA166 battery charger from a garage tenant. (65/75 amp charge 480 amp start assist). It's a large charger on wheels. It's not working. Negative clamp has a light which is supposed to glow when it's attached to a battery even with the charger off. It doesn't glow. There is no current coming from the charging cables when it's turned on. It does sound like it's working. The fan runs and the 'charger' changes tone when going from high to low.
Anyone have any idea what might be wrong with it and whether it's worth taking it apart to try and diagnose it?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13