Long-term photographic photometry of the active long-period RS CVn binary HK Lac (HD 209813) was obtained from more than 2000 Sonneberg Sky-Patrol plates taken between 1956 and 1996. We achieve an internal accuracy of 0.07mag. The correspondence with contemporaneous high-precision photoelectric photometry from automatic telescopes is striking and successfully demonstrates the feasibility of our approach. Based on a Bayesian periodogram analysis, we improve previously published cycle periods to 13.37+/-0.8 and 6.7+/-0.1-years, and present evidence of an additional period of 9.48+/-0.13-years. This establishes the multi-periodicity of dynamo action in these overactive stars as compared to the Sun. Of course, the 6.7-years cycle may be an overtone of the dominating 13.4-years cycle. Our long-term photographic photometry even allowed the detection of the star's mean rotational period of 24.35-days.