Err 24 -

Hello, I have Denver Instrument make analytical balance model SI-234. When I power on the balance then ERR 24- immedietly displays on the screen and none of the keys working after this error. Checked voltage output of power supply and +5Vdc, +15Vdc, +7.31Vdc and -7.34Vdc meaured. What could be the problem? Kindly suggest further action.