Difference between 经常 and 时常?
No. They are not interchangeable at all. 

经常 bears more frequency than 时常. E.g. 我经常去看你 Vs. 我时常去看你. 我经常去看你 indicates I go see you more often than 我时常去看你. 

If you say 我经常去打球，I'd think your frequency of playing ballgames is high. If you say 我时常去打球, I'd take that your frequency is normal/moderate (or probably a bit above normal), neither very high nor low.