Camera hot shoe trigger - Sherlock, help me deduce what this component may be

[Peabody]

Yes, I know. It's elementary.

Well I have a Canon XT/350D DSLR, and the hot shoe doesn't work.
The built-in pop-up flash works fine, but nothing fires in the hot
shoe. So the issue is whether to attempt to take it apart, attempt
to fix it, and reassemble without having parts left over. Aside
from it just being a loose connection, fixing it would depend on
the bad part being a discrete component that could be replaced with
something similar. There are no schematics available, but here are
some clues:

1. When I measure resistance between the center contact and ground,
with the positive lead on the contact, I get very high resistance.
But with the leads reversed I get a diode drop.

2. Since the popup flash works, it seems logical that the failure
is fairly near the hot shoe in circuit logic terms. Otherwise,
neither would work.

3. There are warnings about trying to mount old flashes on modern
cameras because the old flashes can have trigger voltages of
several hundred volts, which can "burn out" the hot shoe. The Canon
XT is rated to handle up to 250V.

4. When I took the camera to a local tech for an estimate
($115-145), he showed me the "top cover" assembly from a similar
camera, and I saw up near the hot shoe a small three-terminal
discreet device that looked like an SOT-23 package, with two leads
on one side, and one on the opposite side. Of course it could have
been related to the built-in flash, not the hot shoe.

So, back in the 80's when I was keeping up with this stuff, I would
be thinking - it's a HexFET. I guess today it would maybe be an
IGBT. But in any case, very probably a discrete component, and not
a tiny one. And the replacement wouldn't have to be an exact
match. In fact, all I really need it to do is fire a radio
trigger, which is a 3V trigger drawing very low current. So I
would have wide latitude in the replacement part.

And I found a video on YouTube showing the disassembly of a Canon
XTi, which is a similar camera. It looks complicated, but not
impossible. So I'm thinking I might want to give this a try.

Anybody have experience in this area who can give me some words of
wisdom on what I'm dealing with here? Any info or suggestions
would be appreciated.

 

[Baron]

Peabody Inscribed thus:

Yes, I know. It's elementary.

Well I have a Canon XT/350D DSLR, and the hot shoe doesn't work.
The built-in pop-up flash works fine, but nothing fires in the hot
shoe. So the issue is whether to attempt to take it apart, attempt
to fix it, and reassemble without having parts left over. Aside
from it just being a loose connection, fixing it would depend on
the bad part being a discrete component that could be replaced with
something similar. There are no schematics available, but here are
some clues:

1. When I measure resistance between the center contact and ground,
with the positive lead on the contact, I get very high resistance.
But with the leads reversed I get a diode drop.

2. Since the popup flash works, it seems logical that the failure
is fairly near the hot shoe in circuit logic terms. Otherwise,
neither would work.

3. There are warnings about trying to mount old flashes on modern
cameras because the old flashes can have trigger voltages of
several hundred volts, which can "burn out" the hot shoe. The Canon
XT is rated to handle up to 250V.

4. When I took the camera to a local tech for an estimate
($115-145), he showed me the "top cover" assembly from a similar
camera, and I saw up near the hot shoe a small three-terminal
discreet device that looked like an SOT-23 package, with two leads
on one side, and one on the opposite side. Of course it could have
been related to the built-in flash, not the hot shoe.

So, back in the 80's when I was keeping up with this stuff, I would
be thinking - it's a HexFET. I guess today it would maybe be an
IGBT. But in any case, very probably a discrete component, and not
a tiny one. And the replacement wouldn't have to be an exact
match. In fact, all I really need it to do is fire a radio
trigger, which is a 3V trigger drawing very low current. So I
would have wide latitude in the replacement part.

And I found a video on YouTube showing the disassembly of a Canon
XTi, which is a similar camera. It looks complicated, but not
impossible. So I'm thinking I might want to give this a try.

Anybody have experience in this area who can give me some words of
wisdom on what I'm dealing with here? Any info or suggestions
would be appreciated.

Is it possible that the flash head uses a trigger voltage of the
opposite polarity to that required by the hot shoe ?

 

[The_Giant_Rat_of_Sumatra]

My only other suggestion would be to see if one of the smaller pins on
your hotshoe performs an "x-synch" function.
Maybe you could trigger off this? (Not sure what Canon might call
this. I think Nikon refers to it as FP-Synch?)
Anyway, just a thought.

-mpm

Flash Program synch?

Sheesh.

 

[josephkk]

Perhaps you will find something useful at this link:

http://kzar.net/wiki/Photo/CanonE-TTLProtocol

Or perhaps: http://www.conraderb.com/flashrepair/

My only other suggestion would be to see if one of the smaller pins on
your hotshoe performs an "x-synch" function.
Maybe you could trigger off this? (Not sure what Canon might call
this. I think Nikon refers to it as FP-Synch?)
Anyway, just a thought.

-mpm

Ahhh. So much information that has effectively been lost. X-sync and
FP-sync refer to old flashbulb days time versus intensity characteristics.
The functions remain the similar but the timings have changed to reflect
electronic strobes. Hot shoes date back nearly as far and were originally
just metal contacts with timing related to shutter movement. About mid
1980s more hot shoe contacts were added to integrate exposure control with
the flash controls. Those other contacts do not carry normal flash
trigger information. FWIW my first camera was a hand me down Kodak
Autographic 120A folding bellows type.

 

[The_Giant_Rat_of_Sumatra]

Sheesh indeed. FP-synch is Focal Plane synch. Refers to the type of shutter.
Of course, you wouldn't know that, being Always Wrong.

You probably had to google it.

Likely not the same function (or name) on all cameras either.