Yamaha CX-800 power on but channel selector not work

Hi,
I own this pre-amp sind more than 25years. Now some problems occured, in first one channel did not work anymore. After open the amp to see for dust, contact problems... cleaned solded some points where a cold contact seems to be. But now it does not work anymore, I can switsch on, the power LED lights, but I cannot even select a channel. There is no reaction on channel selector and even no selectid channel is indicated by a light.
Could you give me some advice where to start ?
Best regards, Steve