Vivitar thyristor flash manual power control mod

A little over a week ago I learned that I can very easily modify my Vivitar 3500/35-D flashes to have manual power output control. Previously I tested the different settings on the flash. To give me full manual control I have replaced the photo resistor with a potentiometer. This should work with all Vivitar flashes that use a photo resistor to adjust light output. This mod is quite easy and only took about 15 minutes the second time I did it. The 3500 has a separate module where this circuitry is housed. I know some Vivitar models are one unit, so be aware that flashes have a high voltage capacitor in them and you can shock yourself.

Vivitar 3500 Thyristor Flash

I have two Vivitar 3500 flashes (one is a 35-D, but they are identical as far as I can tell). For the longest time I could not figure out how these things actually worked. There are four settings on back; red, blue, orange, and manual. From the 'calculator' up top you can tell the three color settings are two stops apart, and manual is full power. But until recently I did not know that there is a photo resistor up front which actually adjusts the power of the flash based on the amount of light the sensor sees. The more less light, the more resistance, and the stronger the flash. Thanks to this discussion on the Strobist Flickr discussion group I was able to get a better understanding on how this flash works. I decided to set up an experiment to see how this sensor works in practice. This will apply to all Vivitar thyristor flashes.