Two 600ex-rt's with 2 different faults

[Britincan]

I have 2 600ex-rt's with 2 different faults. The first one is unable to fire in ETTL. Each time I reset the menu to enable ETTL firing and try to take a photo, the flash returns to TTL mode and overexposes. I've reset all the custom functions and checked the flash hot shoe and it isn't loose. All the pins are free moving and clean. I've also used different batteries. This happens on my 5d mk 3 and a 7d. Also when I try to control the flash through the camera, it says  "this menu cannot be displayed. Incompatible flash or flash's power turned off". 

The 2nd flash has an intermittent fault when I try to bounce the flash. After 2 or 3 shots which are correctly exposed, the next one is underexposed to the point that it looks like the flash hasn't fired, but it has. Again I've checked all the connections, reset the custom functions and used new batteries, without any resolution. 

Any help or advice is greatly appreciated.

[diverhank]

The first one sounded like a real fault where it would fire at full power all the time.  I think you need to have it repaired.

The second one...not too sure.  What are your settings for the camera and flash?  I'm assuming you have freshly charged, rechargeable batteries?  It's possible that after the initial flash to measures the intensity (if you're using ETTL), the flash might run out of juice and didn't fire the second time.

[TCampbell]

I agree with diverhank.  The first issue sounds like the flash needs to be serviced.  

The second issue sounds like the flash just hasn't had enough time to recycle the capacitors.  Are you shooting several shots in somewhat rapid succession?

The batteries don't technically power the flash tube... the batteries just charge up some energy in capacitors and the capacitors release that energy much more rapidly than the batteries can and this creates the burst of light.  This does, of course, take some power from the capacitors but they usually don't have to fire at "full" power so they're often good for a few flashes in a row (as long as it isn't at full power) but you have to wait for the flash to be ready again before you continue shooting.

You can get an external battery back which will recycle them much faster.

If you've got fresh batteries (and you're sure they're fresh) and you're waiting several seconds between each flash (more than enough time to recycle the capacitors) and you still have the problem, then something else is wrong.