Vivitar 285HV trigger circuits, disassembly notes

Resistances across pins 1 and 2

M open 1/2 181k 1/4 81.2k 1/16 23.2k

According to Loren Winters, flash duration on the Vivitar 283 is given as follows:

for R=2 to 100k, t=17R, where R is resistance in kohms, and t is duration in microseconds.

for R<2k, duration falls less rapidly, reaching a minimum value of 20 us.

Using these formulas, the duration of a 1/4 power flash would be 1380 us, or 1/724 s. Extrapolating to a full discharge, assuming a linear relationship between discharge time and power, the duration of a full discharge would be approx 1/181 sec. I would have guessed about 1/500s, based on the discharge time of my old Honeywell Strobonar. Nevertheless, this is in the ballpark of a reasonable number. It is quite possible the time/resistance relationships are the same for the 283 and the 285. Dunno.

To modify this circuit for continuously variable power is impossible, if full discharge is to be achieved. However, one could use a toggle switch to change between full power mode and attenuated power mode. The toggle would open the connection between pins 1 and 2. When switched in (atten mode), a 200k potentiometer in series with a 10k fixed resistor could adjust continuously between roughly half power and roughly 1/32 power. An 200k audio taper would achieve a roughly linear scale of stops.

Funny, the 283's VP-1 goes continuously from 1/1 to 1/64. Why should a tricked-up 285 be different?

If I understand correctly, the VP-1 has selectable power, but not continuously variable power. For instance you couldn't set 1 1/3 stops attenuation, because there's no switch setting for it. I'm guessing it's also missing the 1/8 power setting, just like the 285. Any thoughts as to why Vivitar would omit the 1/8?

Anyway, it's quite easy to do the mods. I have a small gaggle of 285HVs I use for architectural photography, so I'll be making myself several modules. Each module will have a 1/4" jack for the sync, a switch for manual vs. attenuated, and a potentiometer for dialing the output level I want. It's just easier that way. I hate the module that comes on the thing. It's hard to see/use/set in dim light. I'm also tired of fussing with tiny PC connections that don't work. ;-)

Ack! These are the resistances in more legible format:

M open

1/2 181k

1/4 81.2k

1/16 23.2k

So... I made the box. It was pretty easy. On one side is a 1/4" jack for the slave. On the other is a 37-detent 500k potentiometer, in series with a 10K resistor, with a built-in on/off switch, that controls output power. Works like a champ. I couldn't be happier with it. I'll post pics and diagrams on another thread, just as soon as I get back to my other computer. :-)

Sarah, the VP-1, or at any rate my 3, has power level continuously variable from 1/1 to 1/64. The last level isn't marked, but it is there.

Thanks for the useful info, saves me some time! Here are some illustrations of what the wiring looks like.

Here is a view from inside the case for the front control circuit socket.

Attaching a replacement metal foot (ebay) to the 285HV. The white and green wires were just snipped and protected with heatshrink. This was a worthwhile upgrade: for the first time, the flash is actually solidly mounted on the shoe instead of wobbling.

Vivitar 285HV output measured with Sekonic L508 flash meter: I found the range is controllable over 10.5 stops using R down to 2k. The difference between "1/2" and "M" is only 0.5 stops, so "M" might as well be called the "3/4" setting.

stops=kohms
0.6=2k, 3.1=4.7k, 4.2=6.7k, 5.2=9.97k, 6.5=16.7k, 7.4=23.92k ("1/16"), 9.5=81.1k ("1/4"),10.5=179.7k ("1/2"),1 =(open) ("M")

Diagram indicating which two pins the resistor goes between, in my above graph of exposure. (previous data has a typo: "M" open setting is exposure 11.0 stops, not 1. Graph is plotted correctly.)

Update: note that Sarah Fox put much additional info on her 285HV mods on her web page here: www.graphic-fusion.com/vivitar285mods.htm

Hi,

Thanks for all the info - really useful as I'm working on a 285 trigger mod (yet another one). I was just wondering if you had traced down where the center pin/orange wire goes in the flash? Sure would be neat if it happened to be the +ve supply line :)

James.

The center pin #6 (orange wire) in the front socket is connected to the hot side of the HV transformer. On my unit, I measured it at +331 Vdc relative to pin 2 (hotshoe ground). There is a small AC ripple voltage, around 0.1 Vac. Careful, the DC voltage stays high for quite some time after you turn the flash off.

Thanks, John! :-)

James, how are you modding the trigger?

Hi Sarah,

I'm working on a wireless remote with power control, very much along the same lines as http://www.flickr.com/groups/strobist/discuss/72157605520115509/

The plan is to build a controller for four flash units with independent continuously variable power from full to approx 1/128 and decent range. That should cover most of my needs for the foreseeable future!

I enjoyed looking through your website, by the way. Thanks :)

Regards, James.

The above doesn't describe Pin 6. I just found out the hard way that it carries the gazillion-volt output of the capacitor used to discharge the flash.

Hi Joe,

The easy way was about 3 posts above. Hope it didn't hurt too much. I measured mine at about 350V. Strange line to have brought out to the sensor, I wonder what they had in mind for it. I'd also love to know the purpose of pin 3. It's connected to the base of a transistor on the vertical pcb, but I didn't spend the time trying to work out what that transistor does.

Regards, James.

Doh! I had read the thread earlier, particularly Sarah's information. When I came back to it, I search for "pin 6" on the page. Not seeing it, I didn't bother looking further. (3, 4 posts above, it's referenced as "pin #6.") Gee, I feel dumb. Moreover, my curiosity was the same as James's: I was wondering if pin 6 carried Vce. I want to make a trigger/power mod, too, but I want to avoid opening up the flash (to reduce the risk of shocking myself!). Anyway, besides a pinpoint 3rd-degree burn and some burst blood vessels in my eyes, nothing too bad happened.

As to the strangeness of the lines, I find it strange that the trigger lines are also brought out. My guess is that the "automatic" modes use a photon transistor embedded into the vari-power module and directly control the shunting of power from the capacitor to the light tube. My guess is that the trigger lines start tell the transistor to begin switching current from pin 6 to pin 3. The amount of light received controls when the switch turns off.

My plan is to bring out +6v, convert to +3v (for the trigger), and rope in a potentiometer to control power. I wasn't going to remotely control power, but that could be done with a second trigger--each time this trigger is fired, it ratchets up the power, rolling over to the lowest power after reaching the top. You would need some kind of circuit (e.g. a microcontroller, but it's possible a simple counter would work) to handle this.

JB