Slaved Flashes

I have a question to ask you all - one that has been puzzling me for a while.

I have a 10D, a 420EX, an older Cobra flash with the Canon mount, a hammer head flash and a couple of studio lights. Sometimes when shooting indoors I do not want to use the studio lights and prefer to use some of the battery operated flashes mentioned above. As a general rule I will connect the hammer head to the 10D with a sync cable and then slave the Cobra.

The problem I get is that all is fine for the first shot, but on subsequent shots the slave unit will not fire the Cobra (even the test fire button will not work?). I have swapped the slave unit onto the 420 and the same problem persists. I have tried a number of slave units (from different manufacturers) and the problem is still there. If you disconnect the slave unit from the flash and reconnect it will work once and then lock again. The only thing I can think of is that there is either something odd with the Canon mount (although I find this hard to believe) or there is a polarity issue, I cannot see any other commonality that could cause the problem.

Any ideas would be greatly appreciated as I do not want to have to use cables between all the flashes.

PS - I bought a second hand 70-200L F2.8 lens yesterday and cannot wait to try it out (really crap weather here in England at the moment). Only problem is that it is a tight fit in my gadget bag - but this is a good problem

Cheers,
Mark.

I have a question first. Does the flash attached to the 10D via a PC Cord fire each time? If that is the case, then there is something wrong with your slave set up as your slaved unit is not recycling. How are you slaving the 420? Canon flashes have an unusual foot that need a custom hot shoe mount.
Scott

Scott,

Yes, the flash connected to the camera does flash every time, and both the 420 and Cobra were/are slaved from a standard hot-shoe slave unit (the light sensitive slave is connected onto the flash via the hot-shoe).

I am curious to hear you say that the Canon flashes have a unique hot-shoe mount, this could be the problem.

Do Canon flashes need a special hotshoe slave to drive them?

Mark.

The feet of the Canon EX series speedlites don't fit most slave hotshoes. They don't fit Wein for sure. You can't even slide it on. I bought a couple of custom cords from www.paramountcords.com .
Scott

So, is there no way that I can slave a Canon footed flashgun without using a cord or using a 550EX (and slave the 420 from that)? I like the flexibility of not having the sync cable. I think I will have a quick look on the Canon website. There must be some way of doing it.

Mark.

You have three options:
1. Using a 5550EX or STE2 as a wireless master to another 550 or 420.
2. Use your 420EX on camera, and attach your Cobra flash to a Digital Slave that will ignore the 420's pre flash.
3. Buy a custom cord with a Canon hotshoe on one end and a Pocket Wizard mini plug connector on the other and attach the 420 to the hot shoe and a PW receiver on the other. Put a PW transmitter in the camera hot shoe. Set your Cobra flash as in #2. The 420 will only fire at maximum.

The 550EX will not work with a Wein Slave. It won't recycle after the first shot. You have to turn it off/on after each shot. I don't know about the 420.
Scott

The design of most slave triggers centers around the use of a Silcon-Controlled Rectifier (SCR), which functions as an electrically-operated switch to short the main contact on the flash to ground, thereby triggering the flash.

The problem with SCRs is that they can get stuck in the closed position, even when the triggering signal is completely removed. The only way to open the switch is to remove the current flowing through the SCR. In the situation involving flashes, the current is supplied by the flash. That's why power-cycling the flash seems to "fix" the problem.

The more expensive slave triggers, like the SSL series slaves made by Wein, use a more sophisticated design that is supposedly not prone to SCR lockup. The other option is to use a radio-controlled trigger like a Pocket Wizard.

Scott, the recycling issue with your 550 is exactly what I have with the 420.

Steven, very interested to hear that it could be a problem with SCR lockup on cheaper slaves - and would also explain why one worked for a while and then stopped. Any other manufacturers suggestions apart from Wein as there appears to be doubt if the Canon flashgun foot will fit.

Mark.