How to measure PC Flash Sync Port OHMs? Hasselblad 500c/m

crumpet8 · Jan 21, 2016

Hi,

Does anyone here know how to properly measure if the flash sync port on a 500c/m is sending out signal during the entire shutter release?

I set my black and red contacts onto the outside of the PC port and got readings that jumped around, but should I see a change in this behaviour during shutter release?

Obviously, I have very little idea as to what I am doing so all of your help is much appreciated!

Thanks,

Daniel

shutterfinger · Jan 21, 2016

Flash sync should read infinity any time the shutter is not in operation.
X sync- should read 0 when the shutter blades are within 5ms or less of full open. Full open is the target but a few milliseconds prior to full open will not adversely affect exposure.
M sync- should read 0 when the shutter is 15ms to 20ms prior to full open.

See page 17 of http://www.cameramanuals.org/hasselblad/hasselblad_500c_m-swc_m.pdf

Shutter operation is so fast that an analog VOM needle will just jump 1/4 to 1/3 of full movement and most digital VOM's will not register the momentary flash sync contact.

Best way to test is to connect a working flash, point it at a light colored wall, and look through the lens with the aperture wide open and trip the shutter. If you can see the shutter blades on X setting the flash is firing early or late.

It will probably show 0 if locked open on B.

crumpet8 · Jan 21, 2016

Thanks for the info

MattKing · Jan 21, 2016

A synch connection on a camera doesn't send out a signal. It closes and then opens a circuit. The current comes from the flash (magicubes excepted).

I'm not sure about the source of current for additional, TTl circuits on cameras that offer them to flashes that make use of them.

Chan Tran · Jan 21, 2016

crumpet8 said:
Hi,

Does anyone here know how to properly measure if the flash sync port on a 500c/m is sending out signal during the entire shutter release?

I set my black and red contacts onto the outside of the PC port and got readings that jumped around, but should I see a change in this behaviour during shutter release?

Obviously, I have very little idea as to what I am doing so all of your help is much appreciated!

Thanks,

Daniel

The 500c/m is an old camera and it has simple dry contact that closes the circuit when the shutter fully open. I don't know about the 500c/m but checking my old Nikon FM, Canon AE-1 I can measure resistance with a DMM if I set the shutter speed long enough like 1 sec. Newer camera and the digitals use a solid state device for switching and they could burn out if subjected to high voltage as many already know but it won't conduct either if the trigger voltage is too low.

AgX · Jan 21, 2016

As said your camera does not send out a signal.

And if a circuit is believed to send out a signal, one would not meter it in Ohms, as that would necessitate to send a signal into it oneself.
Thus here you got a basic misconception.

Concerning the Duration of the switching: it must not start in X-setting before the leaf shutter has fully opened. The time it then stays open not necessarily has to be as long as the shutter stays open. It actually is a triggering. (I would have to look up minimum switching time.)

wombat2go · Jan 21, 2016

Here is how I would "properly" do it using a dual trace storage oscilloscope.
The lens/shutter is a Mamiya RB67 Mamiya Sekor C 1:4.5 f=50mm.

I put a 3.3 Ohm resistor in series with the pc cable and the scope measured the volt drop across the contacts.
( Caution: to avoid possible damage , know what you are doing, and set the bench power supply current limit to less than 2 Amp before doing this)

The shutter was set to 1/60th .
The trace ( 2 millisec per division) in the middle of the screen is of the fast photo diode recording the shutter opening and the ripple of the tungsten lamp.

The top trace falling low is the pc flash contact voltage. ( at about 0.66 Amp, as it turned out)
It is a bit disconcerting.
Firstly there is a lot of contact bounce in the first 4 milliseconds. The flash initiation time would depend on the flash circuit.
Secondly the contact resistance after the bouncing is over , is about 1.3 Ohm.

I suppose it all worked OK back in the day, or maybe the flash contact on this lens is a bit deteriorated.

shutterfinger · Jan 21, 2016

Vintage shutters used copper contacts which corrode over time. Compur, Rapax, Alphax, Acme shutters use a silver colored metal that oxidizes like silver, turns black, but not as fast as silver. All clean up fairly easily with metal polish. The degree of corrosion will determine if you get a make-break-make type action or no contact at all.
Early contacts were pressed together direct by the shutter blade controller while later shutters use a mechanism triggered by the shutter blade controller. Most if not all types are adjusted by the gap between the contacts. On a PC socket the outer ring is connected to the shutter case with the operated contact connected to the case then mated to the center contact of the socket when the shutter reaches full open.

crumpet8 · Jan 22, 2016

Chan Tran said:
The 500c/m is an old camera and it has simple dry contact that closes the circuit when the shutter fully open. .

Thanks everyone. Don't know if it matters but it's an older C lens. Chan Tran, does this mean the reading should go to 0 for the one second the shutter is open? Or longer if I use b?

crumpet8 · Jan 22, 2016

Thanks Wombat! Wish I had an oscillator. Super precise reading, exactly what I'm after. I don't know how to read that thing but does that trace in the middle of the screen mean that there is current whilst the shutter is open and not when closed? In contradiction to Chan Tran.

itsdoable · Jan 22, 2016

Most 500C/M's do not have a PC flash port (except for some early ones that synced the auxiliary barn doors - for bellow's/macro work with shutterless lenses).

The PC Sync is in lens, and is part of the Prontor/Compur shutter. It is a dry contact that closes when the shutter is open (or just prior to opening if you have an "M" sync C-lens)

The PC flash connector on the body of the 500C and C/Ms (that have them) is also a dry contact that closes when the barn doors are fully open.

AgX · Jan 22, 2016

What is the meaning of "dry contact"?
Same as "closing switch", thus open, unless activated?

Chan Tran · Jan 22, 2016

AgX said:
What is the meaning of "dry contact"?
Same as "closing switch", thus open, unless activated?

We electric guys call it dry contact when an electrical contact is made by metal to metal touching. Not by mercury or more often by a solid state device like transistor or SCR etc.. So a dry contact is a switch in its simplest form.

How to measure PC Flash Sync Port OHMs? Hasselblad 500c/m

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