At 3:36 AM +0100 1/4/05, Listar wrote:
>Date: Sun, 2 Jan 2005 21:44:15 -0800 (PST)
>From: Tim Hughes <timhughes@xxxxxxxx>
>Subject: [OM] Re: Vivitar 550FD M/P/O discussion
>
>Joe wrote:
>
>>>If the voltage is less than twenty volts, and the same polarity as
>>>OM flashes, you're OK on all OM bodies. If the voltage is more
>>>than twenty volts (a hundred or more being typical), the flash can
>>>be used only on the OM-1 for sure. I don't know about the OM-2,
>>>but the OM-3 and OM-4 are out.<<
>
>This is a popular misconception, pretty much all the OM's take high
>voltage triggers.
I'd like to believe this, but "pretty much all" isn't "all". Which
ones are the exception?
As for the single-digit OM cameras, do the camera repair guys agree?
Olympus? The users guides are silent on this. I'd hate to blow
something out.
>The OM2 service manual says the mechanical flash trigger switch has
>a test voltage of 500V. The OM2N uses a 1N4004 diode to isolate the
>mechanical switch and the diode is rated at 400V. The OM2S, OM4 and
>OM4Ti all use a high voltage thyristor (scr) trigger which is good
>for all modern high voltage flashes (which run < 330V). Some really
>old studio units and Metz mech vibrator units used 500V but even old
>Sunpak units like 611 (international version Autozoom 5000) or the
>old manual Sunpak ringflash are less than 300V. I assume the OM3
>also uses a thyristor, but I have not verified this. In the OM4 it
>is mounted up near the eyepiece.
I've verified (by reading the service manual) that the OM-2 (no
letters) uses mechanical switches only in the flash synch circuit.
I don't have a OM-2N service manual, and the expoded OM-2N pdf I do
have doesn't show the circuit diagram clearly enough to be read.
I have no data on OM-3.
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.
I have a SunPak GX8R ringlight. The trigger voltage is 185 volts,
center pin (of the PC connector) positive, shell negative, as just
measured with a 10 megohm DMM.
>Because of the thyristor or diode in OM2N and later devices polarity
>of trigger voltage is important for triggering. In OM1 or OM2, it
>does not matter.
Yes.
>As mentioned on this thread, measuring the high voltages is often
>inaccurate because of voltmeter loading. A flash like the 611 has
>sync pin charge resistor in excess of 1MOhm, giving rise to quite
>bit more than 10% error with a typical digital voltmeter and much
>more with an analog meter.
Yes. It is easy to buy a digital multimeter (DMM) with 10 or 11
megohms input resistance, and even with 20% error we can tell what we
need to tell, so this ought to be a solvable problem.
Joe
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