Air condition unit freezing up. Advice on repair

[Virginia Jones]

I just had a tenant tell me the ac unit is freezing up. He said he has defrosted it twice and there doesn't seem to be any leakage. 

It is an older home and the air vents are the kind without filters so dirt is bound to be sucked into the coils. 

Does anyone know if this type of system has an air filter somewhere else that I may know about? 

Or should I just call an hvac repair person?