Happy Voyager. speed stuck on 500?!?

Hi there,

I have a happy voyager (NO LCD) 2005 model.

the machine speed stuck on 500 and even I try to speed it up ,it turns back on 500.

I Inisialized the setup and the speed .it speeds up till 1000 ,but when I start to sewing it gets stuck on 500.

I turned the machine on and off couple times ,even I unpluged it but still no results.

I will appreciated any help

Thanks:)