what would cause a ac servo to fry

[SALS450SLS]

1980 450sl , new ac servo replaced ac control and blower motor , only defrost works blows hot no control hot or cold , replaced ac amp behind glove box, ac had thumb wheel control as norm, all speeds worked fan in hi wasnt as strong as in def mode. worked 5 min. then back to same as before only hot defrost , removed ac amp and looked inside foud transistors were burned , somewon said it could be a bad hot water motor near servo ,
any help would be welcome sal sl