How did is stopped working ? What did you do to it ?
The channels are isolated from each other so if you did not blew up the first channel and then the second, the channel asics will be good. The fault will then be where both channels use the same circuit.
Connect a test signal to a channel and follow it with a scope until it disappears. It goes to the channel asic, via an opto and transformer to the Sample-asic. From there to the ADC and then to the Digital Asic. But the latter controls the rest so lot of chances to run around in circles.
Be careful with the display and backlight connectors and flatcables because you need to "split" the case and measure on both sides of the PCB and turn pcb and display/front-case at the same time without damaging the connections.
They are a pain in the @#$ to repair, the D-asic is often a bga and the asics are expensive.