The Automatic Cancel Circuit (I think it's called) button? Push and twist it to lock it down and that tells the strobe that the camera does not use a Pre-flash. That thing?
If your camera is doing a pre-flash and you have that pushed down, I think that's your problem. It needs to be up so that it doesn't fire a full flash when the camera fires a pre-flash.
Unless you have the flash on the camera set to manual mode.
In other words, as I understand it, even if the camera mode is M, the flash mode is still independent and can be set to Auto or Fill-in Flash or Off or 2nd-C. Or you can set the camera's flash to Manual, too, and then set its intensity to whatever value you want. On my camera, when I go into the flash menu where it shows a lightning bolt for Fill-in Flash, an icon for Red-eye reduction, a symbol for 2nd-C, etc.. The last symbol on the right is a lighning bolt with the word Full on it. That's the manual flash setting. If you arrow over to that and then pretty Info, you can cycle through the manual flash settings. And that is even when the camera itself is in Manual mode.
I THINK, in any of the automatic modes, it will pre-flash. And in manual flash mode, it will not pre-flash.
At least, that's how it is on my E-M10.
I have no idea of how Automatic Cancel Circuit works but I have tested it and this is how it works with me.
With these settings
- EM5: M mode and manual flash setting 1/64
- Z-240 Automatic Cancel Circuit locked down and using power settings with M position (left side knob)
everything works fine.
If Automatic Cancel Circuit is up, the results are the same no matter what strobe power I use with the right side knob (just above of Automatic Cancel Circuit button).
The conclusion is that Automatic Cancel Circuit needs to be locked down.
When using 2nd-C there will be two flashes of Z-240 (in the beginning and in the end).That means 2nd-C is not useful.