I have done a lot of research on the neon VC heads, both Zone VI and Aristo. The HV supply in the Zone VI head is a solid state dimmable neon power supply. The older iron core transformers were used in the Aristo units. Though I have never seen a Zone VI 5x7 head I do have the 8x10 and have made two major modifications over the years to it.
What I found to be the short coming (as mentioned here) of the Zone VI VC head is the lack of full contrast control. This puzzled me until I went into the 8x10 unit I have and took a very close look at how it was designed. What they did was design two control circuits. One for the blue grid and one for the green grid. They use a dimmable solid state neon power supply on each grid, B & G. What they added was a photocell sensor to control and stabilize the light output of each grid. The problem is, there is no way to keep one sensor from seeing the light from the other grid. So the two circuits interact and fight each other. My 8x10 had blue and green filters added in front of each photo cell, I believe in an attempt to minimize the crosstalk. But this is still not adequate to stop all interaction. So as you increase say the green, the blue gets turned down. And the opposite happens when you turn up the blue. I was never able to totally isolate the interaction. I tried moving the sensors and even adding a light pipe directly to the associated grid. There was always interaction. I ended up removing all of the original circuitry and using two iron core transformers with associated dimmers like Aristo used. I used one photo cell sensor and a Zone VI Compensating Timer and all was well. We used this setup for over 10 years and it worked as expected.
In 2016, after a lot of research, I finally was convinced there were suitable green and blue LEDs that would work for VC printing. I tested several LEDs and then built a replacement head for the original modified Zone VI on our 8x10 enlarger. It has worked very well. If you are interested in that project, here is a link to the BLOG post.
http://www.jbhphoto.com/blog/2016/07...ds-vc-printing
Hope this helps. . .
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