YS-90 dual setup advice

I’m still just getting into UW photo and looking for some advice on troubleshooting an issue. This is a pretty long post, I apologize, I’m trying to give as much info as I can right from the start. My setup is a Canon T3i with 18-55 kit lens in a Meikon housing using dual YS-90 strobes - strobe 1 is a YS-90 TTL Duo with TTL, 1/2 power and full power settings, strobe 2 is a ys-90DX which has TTL and 12 power adjustments numbered FULL through-7 with half-stop increments (1/2 power is labeled at -2). The strobes are connected to the housing via dual fiber optic cables and fired using the slave setting on the strobe.

I set up a test shot on a table to experiment with power settings, strobe aiming, etc. Test shots with strobes off got a correct exposure at ISO 400, 1/20, f5.6. Setting shutter speed to 1/125 got a sufficiently underexposed photo to eliminate ambient light in the room which was the initial goal; now I could experiment with the strobes and know the exposure was controlled by the strobe(s) and not the room light. Switching the camera to manual at ISO 400, 1/125, f5.6 I started shooting.

Setting both strobes to 1/2 got overexposure. Switching to TTL got underexposure by at least 2 stops. Both strobes at full got underexposed by at least 2 stops. At TTL and using the camera flash compensation at +2 was still underexposed. Turn flash ev off. Bumped shutter to 1/200, both strobes at 1/2, still overexposed. Changed to f/8, was still overexposed but much better, maybe .5 stop over. This was starting to be what I expected to happen. Get overexposed, reduce aperture and exposure improves - basic exposure triangle.

The next shot iso200, 1/200, f8, strobe 1 @ 1/2, strobe 2 off - near black underexposed. Next shot same settings, strobe 1 off, strobe 2 @1/2 - slight underexposure, maybe .5 stop. Let’s keep strobe 1 off. Adjusted strobe 2 position to directly over the camera. Strobe 2 TTL - underexposed/black. Strobe 2@1/2 - correct exposure. Strobe 2@full - underexposed/black. Strobe 2@-3 (slightly less than 1/2 power) - good exposure. Strobe 2@-7 (min power setting) - slight underexposed (less than 1 stop off). Strobe 2 @ -1 (call this 3/4 power) - underexposed by 1-2 stops.

Turn off strobe 2, position strobe 1 over the camera. Various strobe settings - full, 1/2, TTL, camera flash ev from -2 to +2 - every photo at least 3 stops underexposed.

Summary - strobe 1 (90duo) doesn’t seem to get anywhere near good exposure. I see it fire, but with the naked eye that’s about all I can tell. Strobe 2 (90dx) is really strange. At 1/2 power or less I get a reasonable exposed image. At full power and/or TTL it is severely underexposed. This is exactly the opposite of what I expect. Add more light to a good exposure and it should be washed out/bright, not black. Or am I missing something? Is there a setting in the camera controlling the on-board flash I’m missing? (I’ve gone through just about every menu and haven’t found anything relevant - those settings seem more pertinent to land shooting with speedlights.)

I googled the instruction manuals and note that the Duo strobe is either TTL or manual with no preflash function and the DX strobe the same but has a pre flash function. It appears the the Duo is electric sync cord only or a slave using the on board sensor in the strobe.

I see two ways you could setup in manual, First set the Duo strobe to slave and the DX to Pre with the on board flash in auto. The DX ignores the pre flash but the Duo won't if it detects it.

Second set the Duo to On and the DX to On and set the camera onboard flash to manual flash.

I noticed you didn't mention what setting you were using with the onboard flash on the camera. You have to match the camera to what the strobe expects to happen, so if the strobe is not expecting a pre-flash and the camera supplies one it will trigger on the pre flash and have little or no power left to supply the second flash which occurs at the correct time to sync with the shutter. The pre-flash fires before the shutter opens.and is not recorded by the sensor.

I think the safest option is having both strobes set to on and the on board flash set to manual at low power, it's only function being to trigger the strobes.