Problem with hot shoe on Canon XT

I posted earlier about a Vivitar 3500 flash I was given, but have
uncovered a bigger problem.

I determined that the voltage on the flash's trigger pin was ok for
my Canon XT, so this morning I mounted the flash on the camera.
And it doesn't fire. I bought the camera used, and have never
mounted anything in the hot shoe before, so it may have been dead
from the beginning.

So, in troubleshooting, I've found that:

The built-in flash pops up and flashes properly.

When the flash is mounted in the hot shoe, the built-in flash will
not pop up. That means the little switch in the hot shoe is
working, and the camera is responding properly.

When I short the flash center pin to its shell, the flash fires.
The test button on the flash also works.

I inserted a popsicle stick on one side of the hot shoe to depress
the "external flash is present" switch, then tested resistance
between the center pin and ground. I get infinite resistance
measuring one way, and a diode drop the other way - I assume this
is a protection diode of some sort for when the current shuts off.

I then measured resistance when the shutter is triggered, and see
nothing on my analog meter. Not a twitch. But, I don't know how
fast it would be, so I don't know whether it should twitch or not.

Well, unless there's a camera setting I haven't found to enable
external flash, it seems my hot shoe isn't operating.

Anybody have any suggestions on this? I assume it wouldn't be
worth it to send an XT to Canon for repair, but are there any other
possibilities, or things I should try?

Peabody wrote:
I posted earlier about a Vivitar 3500 flash I was given, but have
uncovered a bigger problem.

I determined that the voltage on the flash's trigger pin was ok for
my Canon XT, so this morning I mounted the flash on the camera.
And it doesn't fire. I bought the camera used, and have never
mounted anything in the hot shoe before, so it may have been dead
from the beginning.

So, in troubleshooting, I've found that:

The built-in flash pops up and flashes properly.

When the flash is mounted in the hot shoe, the built-in flash will
not pop up. That means the little switch in the hot shoe is
working, and the camera is responding properly.

When I short the flash center pin to its shell, the flash fires.
The test button on the flash also works.

I inserted a popsicle stick on one side of the hot shoe to depress
the "external flash is present" switch, then tested resistance
between the center pin and ground. I get infinite resistance
measuring one way, and a diode drop the other way - I assume this
is a protection diode of some sort for when the current shuts off.

I then measured resistance when the shutter is triggered, and see
nothing on my analog meter. Not a twitch. But, I don't know how
fast it would be, so I don't know whether it should twitch or not.

Well, unless there's a camera setting I haven't found to enable
external flash, it seems my hot shoe isn't operating.

Anybody have any suggestions on this? I assume it wouldn't be
worth it to send an XT to Canon for repair, but are there any other
possibilities, or things I should try?

It could be that the contacts which are shorted in the shoe to fire
the flash aren't shorted completely enough to count as a short for the
flash. That sometimes happens when old film-era flashes are mounted on
modern cameras which use semiconductor switches. If so this is a well
known problem, and you should be able to google up a simple cheap
circuit which you can build into the flash, or alternatively you may
be able to find someone selling a specialised rather more expensive
shoe extension with the circuit built-in.

--
Chris Malcolm

Chris Malcolm says...

It could be that the contacts which are shorted in the
shoe to fire the flash aren't shorted completely enough
to count as a short for the flash. That sometimes
happens when old film-era flashes are mounted on modern
cameras which use semiconductor switches. If so this is
a well known problem, and you should be able to google
up a simple cheap circuit which you can build into the
flash, or alternatively you may be able to find someone
selling a specialised rather more expensive shoe
extension with the circuit built-in.

Yes, that sounds like a possibility.

It just bothers me that I can't detect any activity on the
camera's hot shoe center pin when the shutter trips.

Well, I guess the thing to do is find someone with a genuine
Canon E-TTL II flash, and see if it works on my camera.

Thanks very much for the information.

I got out my breadboard stuff and rigged up a 74LS74 D flip-flop so
that if the center contact on the hotshoe did anything when the
shutter was pushed, the flip-flop would change state.

But nothing changed. So it appears the hotshoe is not functioning.
Now I need to see if I can get to anything to repair it. It's
possible the hotshoe assembly is removable, but the circuit is
probably buried.

I've confirmed that the hot shoe on my XT isn't working. The
built-in flash pops up and works fine, but the hot shoe won't
trigger anything. I called Canon, and they think the hot shoe is
"burned out."

Canon wants $190 plus shipping for any repair, but it doesn't make
sense to spend that kind of money on this camera.

Does anyone have experience taking this camera apart? I'm pretty
good at electronics, and was thinking if perhaps the transistor
that drives the center hot shoe pin could be located and replaced,
that might fix it. But I have no idea where that transistor might
be.

Alternatively, is there an independent repair shop for Canons
where I could send it in and get it fixed at a lower cost?