This topic seems to always get recycled, so I don't like to see it get recycled
with misinformation.
Joe wrote contesting the high voltage capability of the OM4 trigger cct:
>>The OM-4Ti circuit diagram does show the SCR (Q110), and the
inspection criteria does list 500 volt insulation resistance. But I
haven't figured out how transistor Q101 figures in, and why it
doesn't let the 500 volts into the big IC (IC102). I doubt IC102 can
handle any such voltage.<<
Joe , Oly uses a rather small triangle to indicate chassis ground. The emitter
of Q101 you assume is possibly tied to trigger high voltage, is grounded to
chassis, The OM4T service manual shows one line less connected to the FP
switch cct +pc socket ,than shown on OM4, making this more confusing, but this
does not change the connection to ground of Q101 emitter. The OM4 **main**
schematic diagram is clearer, and page E-48 of the OM4 service manual makes it
much clearer, especially the lower assembly diagram. (unfortunately some other
diagrams in Mark's scan of OM4 are largely illegible) Q101 discharges C103
through thyristor gate to chassis, to trigger it. It needs to be set up to do
this because the 3V battery is otherwise the wrong polarity to trigger scr gate
without the +ve level shift provided by capacitor. The OM4T has a slightly
improved C103 charge cct for the capacitor using an added diode.
I always wondered about the FP switch. I assume this is how it is meant to
work: Because Oly wanted to allow external flashes to be used via the pc-sync
socket, but wanted compatibility with on camera T series flashes, they added
the FP switch timed so it opens just after the trigger is applied to scr. That
way the scr has already shorted out the Tseries flash trigger and so no high
voltage can be applied to the T32 trigger pin by the external flash at time
when FP switch closes. When only an external flash is connected via
pc-connector, the trigger pulse is long enough to still trigger the
scr+external flash. Very Obscure Warning : This means if you use a high
voltage flash and connect it via a hotshoe to pc-sync adater you could apply a
high voltage via the Tseries/F connector to an Oly system flash. (The hot shoe
sync terminal goes directly to the Tseries 5pin connector, not via FP switch as
it does for pc connector.) If it works as I assume, it also means by shorti
ng the
hotshoe sync pin to ground the external pc-socket should trigger an external
flash even when the camera is set to red manual 1/60 with no battery It would
be interesting to see if it also works like that on an OM3, as this would allow
flash with dead batteries or in very cold conditions whiich is not possible
normally.
An interesting bit of flash trivia for the OM4 is that on manual speeds, it
will trigger strobes at any shutter speeds (with poor results >1/60). It makes
a bit more sense on OMT with possibility of super fp, but even there, since the
super fp needs the extra dedicated pins, it is strange...
As noted before the only single digit Om I have not checked for high voltage
capability is OM3 but my guess is its design is similar to OM4, esp as it
requires a battery to fire. We need to get a schematic to verify.
As far as 2digit OM's, the OM10 seems ok, as is the OMPC/40, but I have not
checked others.
Regards,
Tim Hughes
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