I design and build these for a living for a North American Company. Check the following before opening the TD:
1. Ensure that the control box is functional...check all fuses, ensure each channel (if present) is functional (use a voltmeter).
2. Check all your external wiring for bad connections or corrosion. A lot of imported TD's use aluminum wiring and it tends to fail.
Now inside the TD:
3. If it's modular and connected with a harness check the wiring on each module and the harness. If it still doesn't work that's it (modular ones suck for repair).
4. If it's connected to a terminal block check for corrosion and check all the connections.
If you still have LEDs out then the circuitboard has an issue and your screwed.
Whelen & Federal Signal make pretty good TD's. I work for another company in Canada and we make excellent TD's and arrows, but I do not post to promote. If you want more information PM me.