Does anyone know how to service a 'Kern anniversary/400 day' clock?

Hi there,

Does anyone know how to service this clock made by Kern?

I could not find service manual for Kern clocks, and it is difficult to search a service manuals as the company does not exist any more and they produced a lot of clock models which do not have model names or numbers, which made my search impossible. I tried to change the suspension spring twice, but first spring made the clock run 4 times quicker and the last spring was too heavy as the clock stopped after few minutes of operation.