Best way to determine which one is a bad ballast

[Eddy Current]

If you've got 2 ballasts in a light fixture that's flickering?

 

[fmtjfw]

trace the wiring

 

[electricalist]

The ballast closest to the window where power comes in is the middle 2 lamps on a 2x4 4lamp.

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[Electric-Light]

If you've got 2 ballasts in a light fixture that's flickering?

What kind of ballast? It's not necessarily the ballast's fault that lamps are flickering.
If you have a two lamp magnetic ballast, one bad lamp will cause two lamps to flicker.

If the lamp cathode filaments are not getting power due to bent pins in socket, loose wires, bent chassis, it will also cause problems. If replacement lamps are severely blackening quickly or only on one end, this is the first thing you should check.

 

[Eddy Current]

What kind of ballast? It's not necessarily the ballast's fault that lamps are flickering.
If you have a two lamp magnetic ballast, one bad lamp will cause two lamps to flicker.

If the lamp cathode filaments are not getting power due to bent pins in socket, loose wires, bent chassis, it will also cause problems. If replacement lamps are severely blackening quickly or only on one end, this is the first thing you should check.

I haven't checked for any loose wires yet, but the fixture has two 3X32 HO ballasts for a total of 6 T8 HO lamps.