Sarah Fox 
, Jun 12, 2008; 01:47 p.m.
Hi all,
I'm putting this up so that I can find it later and so that others might find it, if they
need it, with a search. Sorry, no diagrams or photos. No replies needed! ;-)
To disassemble:
Remove two screws on foot.
Pop off decorative aluminum disc at hinge joint, and remove copper clip.
Remove one screw on this hinge joint and two screws on the opposite one.
Pop apart shell.
Unplug capacitor and head assembly, and set aside.
Remove 3 screws on circuit board.
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Sensor assembly connector:
Note: Pin numbers elsewhere on Internet are wrong. I am using the pin numbers
printed on the connectors themselves. There is one set of numbers on the inside
of the flash shell, and there is another set of numbers inside the sensor assembly,
which is disassembled via two screws on the backside. When looking at the front
of the flash unit, there is a notch in the lower right. At the lower end of the
connector, the first pin clockwise from the notch, is pin 1. Pin numbers are 1
through 5, ordered clockwise from that point, not counting the center pin, which I
presume would be pin 6 (but for which I can find no label).
Pin 1: Blue wire internally, Red wire in sensor assembly.
Pin 2: Hotshoe GND. Black wires internally and inside sensor assembly.
Pin 3: Red wire internally.
Pin 4: White wire internally
Pin 5: Green wire internally
Center pin: Orange wire internally
---------------------------------------
Trigger wiring:
Green wire passes from center contact on hot shoe to Pin 5 of sensor assembly.
Pins 5 and 4 are shorted in sensor assembly. White wire from Pin 4 passes back
to the PC connector in the foot. When PC cord is unplugged, this wire connects
to the red wire, which is the trigger input to the main circuit board. When red is
shorted to ground, the flash fires. When the PC cord is plugged in, the connection
between white and red is broken. The hot from the sync cord connects to the red
wire, and the white wire is disconnected. The test button grounds the red wire (i.e.
to the black wire).
Trigger points:
(1) Ground the red wire. (requires modification to foot)
(2) Ground the white wire by shorting pins 4 and 2 on sensor socket.
Disadvantage: connects through switch in hotshoe foot. Subject to corrosion or
bad contact.
-----------------------------------------
Power settings:
Controlled by resistance between pins 1 and 2.
Sarah Fox , Jun 12, 2008; 04:18 p.m.
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.
Dan Fromm , Jun 12, 2008; 05:13 p.m.
Funny, the 283's VP-1 goes continuously from 1/1 to 1/64. Why should a tricked-up 285 be different?
Sarah Fox , Jun 12, 2008; 05:50 p.m.
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. ;-)
Sarah Fox , Jun 12, 2008; 05:52 p.m.
Ack! These are the resistances in more legible format:
M open
1/2 181k
1/4 81.2k
1/16 23.2k
Sarah Fox , Jun 14, 2008; 03:19 p.m.
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. :-)
Dan Fromm , Jun 14, 2008; 03:36 p.m.
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.
John Beale , Jul 18, 2008; 01:30 p.m.
Vivitar 285HV wiring photos
Thanks for the useful info, saves me some time! Here are some illustrations of what the wiring looks like.
Vivitar 285HV flash foot wiring
John Beale , Jul 18, 2008; 01:34 p.m.
Vivitar 285HV trigger circuits wiring
Here is a view from inside the case for the front control circuit socket.
Vivitar 285HV front socket wiring
John Beale , Jul 18, 2008; 02:50 p.m.
Vivitar 285HV replacement foot
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.
attaching a replacement foot on the Vivitar 285HV
