modding yongnuo ctr-301 flash trigger for 420ez speedlite

so i've finally received my new yongnuo ctr-301 wireless flash trigger set today. i've ordered a set of 1x transmitter and 2x receiver from a hong kong based supplier, which arrived in less than a week via registered airmail. total damage: 40usd. the package has actually been sitting at the local post office for a few more days because i was too lazy to drive all the way from my workplace to collect it, so i waited for the weekend instead.

so here's the bad news: the trigger appears to have problem working with my ancient 420ez speedlite. setting the shutter speed at 1/200s, which is the max sync speed of my cam, i get dark images - the flash appears to have a lag and only fires long after the shutter have closed. after a couple more tests with different shutter speeds, i can conclude that the trigger only managed to sync correctly at up to 1/30s max. 1/30s??! this is pretty much useless for handheld shots, and not what i bought it for. uh...

i dug out my pentax af200sa flash for a quick comparison, and indeed, the af200sa managed to consistently sync at 1/200s. so this is certainly an issue with the 420ez flash. compatibility issue? well, the product actually listed 420ez as being one of the compatible models, so...

test subject: canon 420ez speedlite and pentax af200sa flash mounted on the receivers, with the transmitter on canon eos 450d. at 1/30s, both flashes are firing correctly.

upping the speed to 1/40s, it now appears from the resulting image that only the af200sa is firing. note the small red dot in the picture, which is an l.e.d glow and indicates that the receiver on the 420ez is actually receiving the transmitter signal as well. i can also confirm visually that the 420ez did indeed fire, but slightly later than the af200sa - which explains why the 420ez flash did not show up in the image. uh... this is not good.

a quick googling managed to turn up some pretty useful info on this issue. apparently i'm not the only one discovering that this trigger model can only manage 1/30s max with the 420ez speedlite. some users reported that the flash can succesfully sync at 1/200s by using freshly charged batteries, and only starts showing problem after a hundred plus shots, when the batteries voltage gets lower. well, no luck for me here - my set of freshly charged maha imedion 2100mAh batteries still fails to make the camera sync faster than 1/30s.

another observation is that if i discharge the flash successively using the test button until the ready lamp on the flash turns to green instead of red (indicating that the flash is still charging and have not reached full capacity), i can succesfully get 1/200s sync speed, but after the ready lamp turns red again, the same sync issue crops up again. interesting...

fortunately, there's another suggested workaround for this, which involves a small modification to the receiver unit. so, let's get on with it.

* * * * * * * *

there are 4 screws that need to be removed to open up the receiver casing. the wires connecting to the hotshoe are socketed and can be disconnected. the 'faceplate' is held in place using double-tapes and need to be pried off if you want to separate the 2 parts of the casing (not really necessary for this mod), else do not bother.

here, i'm making a temporary wiring by wrapping one end of a short wire to the negative battery terminal, and the other end is fixed to the hotshoe ground plate by attaching the wire to one of the small screws holding the hotshoe in place. this makes the mod reversible easily if needed, but for reliability it is better to solder down the wires. i'm just too lazy to dig out my old solder gun, plus i don't know if it even works anymore after many years unused.

and... that's it. just a minor mod. tuck the wire aside nicely, close up the casing and replace all the screws and it's now ready for testing again. what does the wire do anyway? well, it appears from various user reports that the trigger is mainly having issue with flashes that have very low trigger voltages, and the wire mod is bypassing a rectifier onboard which has an inherent diode voltage drop.

test firing at 1/200s: success! yay... ^_^.
(ignore the fact that the af200sa flash is not firing instead. idiot me forgot to switch on the receiver on that flash)

disclaimer: try at your own risk. i don't really know if there'll be any damages in the long term. this mod should also work with 430ez/540ez speedlites as well.

the trigger is specified to work up to 1/250s, while my eos 450d can only sync at 1/200s max. how about attempting to work at shutter speeds higher than the rated max? first, at 1/500s - not good, the shutter only managed to open 1/4th before the flash finished firing.

next, at 1/320s. this time we're getting close to 2/3rd of the sensor exposed.

now at 1/250s. only a small black band is visible at the bottom of the image, which can be removed with a little cropping. so i'd say that 1/250s speed is pretty much usable if you don't mind doing a little post-processing to crop away the bottom of the image.

a note for those looking for cheap old flashes to use for strobist work: the af200sa flash is not useful for this purpose, as there's no manual power control available, plus the head position is fixed. i got it as part of a package for my pentax me super (that's another story), so it really cost me next to nothing. although, considering that it's using a thyristor to automatically sense the flash output and cut off the flash once sufficient lights are available, i'm thinking of modding it by adding a potentiometer in place of the thyristor to provide manual adjustable power control - that is, if i can drag my lazy ass off to get it done.

and, speaking of more mods - the 420ez speedlite has an annoying power-saving feature whereby the flash will go into standby mode after 5 minutes if the flash is not fired or no buttons are pressed, and you'll need to wake it up again by pressing the 'ready' button. and this oh-so-useful feature, which is not useful at all for off-camera works, has no way to be disabled at all. i'm thinking of adding a simple mod to periodically simulate a button press on the 'light' button to prevent the flash from going into standby, since the 'light' buton is the only button that doesn't change any of the flash settings. of course, i can always use the test button on the transmitter to fire off the flash at short intervals, but that still requires manual intervention and frankly is a waste of battery power.

useful links:
> original ctr-301 mod discussion on flickr group
> general ctr-301 discussion on flickr group

also, yongnuo has apparently released an updated version, ctr-301p, which adds a mono sync port in place of the test button on the receiver. right after i purchased mine, ish... although i don't own any flash with sync port capability, it's still a nice-to-have feature just in case...