PC-Sync not tripping in only a specific situation - Troubleshooting

[Fragomeni]

This is technically hybrid but I'm posting it here because the question is mostly about analog strobe and this seems like the right sub-forum to get the right eyes on the question. Anyway, I'm working on a project to build a strobe powered camera scanning setup. I've got everything done and it works exceptionally well except for getting the strobe to fire with the digital camera. I've used an old Honeywell Repronar 805-A strobe-powered slide copier and modified it to replace the original Pentax film body with a Sony A7R. I've so far been unable to get the Sony to trip the strobe and after testing each individual component (each of which works), I can't figure out why so currently I have to trip the strobe manually separate from the camera exposure which isn't ideal (but will work if I have to do it). Below are details around what I've tested so far:

The Repronar uses a standard PC-sync for its strobe. Originally, the Repronar PC-sync cord connects to the original Pentax body via a Kodak ASA style PC connector on the Pentax body. Having removed the Pentax body, I've found an adapter to converts the Kodak style PC-sync connecter to a standard one. Since the Sony body doesn't have a standard PC-sync port, I've attached a hot shoe adapter that contains a PC-sync port that should allow the Sony to trip older PC-sync strobes. I've tested all components in the system individually and in a number of test configurations:

Repronar + Pentax body = strobe trips
Repronar + other bodies with PC-sync (example: Nikon F) = strobe trips

To test if the Sony PC adapter works, I've connected two Sunpac speedlights via the PC-sync chords to the Sony with the PC adapter. In all cases the Sony is able to trip the speedlights.

So, the Repronar strobe trips fine in any condition other than with the Sony. The Sony hot shoe PC adapter works in all conditions except with the Repronar. Does anyone have any clue what is going on and how I might resolve the issue to get the Sony to trip the Repronar's internal strobe via the connected PC-sync cord that works with everything else?

 

[cramej]

Might the Sony have a safety in the sync circuit to prevent syncing with too high a voltage?

 

[AgX]

A typical safety means for an electronic circuit would be a Varistor. But such would become active the moment a charged synch-cable would be mounted or when the already connected flash would be charged.
I assume the resistance of the active Varistor to be low enough for the synch-circuit to work properly. Then the flash would be triggered, though prematurely.


Or can a too high synch -voltage affect an electronic switch in a way that it will not switch it, but resume proper working with a lower voltage (thus no intented safety feature) ?

 

[MattKing]

Can you attach an optical slave to the Repronar, and a small electronic flash to the Sony?
Or alternatively, use a radio or IR trigger and receiver?
Watch out for high trigger voltages from the Repronar!

 

[Fragomeni]

Thanks for the brainstorming folks. I don't know about any voltage safety on the Sony but I suppose something like that could be at play. No idea though.

Can you attach an optical slave to the Repronar, and a small electronic flash to the Sony?
Or alternatively, use a radio or IR trigger and receiver?
Watch out for high trigger voltages from the Repronar!

This worked! I attached a small optical slave to the Repronar and tested it by popping off a flash from a speedlight nearby and it tripped the Repronar strobe. Now to just find a tiny flash that'll work with the Sony and I should be in business. Thanks for this! If you have any ideas about a super small flash unit that may work please let me know. Thinking about something like a Nikon SB-15. Ideally I want something small that I can connect with a sync cord so that I can contain the flash and slave in a little box to isolate any stray light from the flash. Thanks again!

 

[cramej]

How about using a Wein safe sync?

Or, since the optical slave will trigger with IR, any small dumb flash on the A7 with some E6 exposed leader taped to the front will work.

 

[Chan Tran]

Could be reverse polarity. Solid state switching device may have polarity. Or they can conduct continuously which would cause the flash to fire as soon as you connect the flash but not firing after that. This condition could be caused by too high sync voltage.

 

[AgX]

A typical safety means for an electronic circuit would be a Varistor. But such would become active the moment a charged synch-cable would be mounted or when the already connected flash would be charged.

Well, unlikely in the latter case, here the varistor likely would not even let the synch circuit and thus the trigger circuit come to sufficient high voltage.

 

[reddesert]

You should not attach an old electronic flash to a new camera without checking the trigger voltage of the flash, and what voltage the camera is related to withstand. Newer cameras use electronic rather than mechanical switching. Although the current flowing through the trigger circuit is usually minuscule, on old flashes the voltage can easily be over 200 V and this is enough to cause breakdown and damage low-voltage electronic components if they aren't isolated. Early on, people damaged expensive cameras this way. I don't know what the voltage rating of the Sony A7R is, but it probably says in the manual. It is not hard to measure the trigger voltage, with a high-impedance voltmeter (almost any modern voltmeter).

See https://www.botzilla.com/page/strobeVolts.html

Many electronic cameras (virtually all Nikons, for ex) include protection circuitry to isolate the trigger circuit. I don't think this is usually a varistor, but it might be an optoisolator or a high voltage rated transistor. It is possible that the Repronar's trigger current is so low that the isolator is not conducting.

The easiest solution may be to get a Wein Safe Sync, which protects the camera, but you have to see if the flash will trigger through the Safe Sync's protection circuit.

 

[AgX]

Many electronic cameras (virtually all Nikons, for ex) include protection circuitry to isolate the trigger circuit. I don't think this is usually a varistor, but it might be an optoisolator or a high voltage rated transistor.

Interesting, I was not aware of this. (Varistors are quite big which already made me wonder about them being installed in an SLR too.)

 

[MattKing]

You should not attach an old electronic flash to a new camera without checking the trigger voltage of the flash

The warning about cameras also applies to electronic receivers in a transmitter + receiver remote flash setup. Of course, damaging a receiver is usually a less expensive proposition than damaging a camera.

 

[Jude the Obscure]

Originally, the Repronar PC-sync cord connects to the original Pentax body via a Kodak ASA style PC connector on the Pentax body.

Out of interest, what was the Pentax body that had a Kodak ASA connector? Apart from that, a PC connector is something different from an ASA connector so I'm confused by them being together in that description - or did you mean some kind of adaptor from one to the other?