Zone VI VC Head, trouble

[J.B. Harlin]

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. . .

[dajaab]

Thanks J.B
I'm gonna check this link
BTW, I love your work

[J.B. Harlin]

Thanks for the kind words!!!

One other thing, those two gold 1.2K ohm, probably 25W resistors are heaters. Neon is most efficient (most light output) at a temp of about 112-115 deg F. The round button with the disk cap across it is the thermostat. This is why the head needs to warm up in order to help stabilize the light output. If the grids get cold enough they will not start immediately either. Neon, or cold cathode lighting is fascinating.

[David E. Rose]

I use my Zone IV head in split exposure mode. I expose the green and the blue lights separately, using the length of the exposure for each to vary the contrast. I did this partly because the individual lamps never had equal output, even when new. You might try this as an alternative to trying to get balanced output from both tubes at the same time.

[J.B. Harlin]

I use my Zone IV head in split exposure mode. I expose the green and the blue lights separately, using the length of the exposure for each to vary the contrast. I did this partly because the individual lamps never had equal output, even when new. You might try this as an alternative to trying to get balanced output from both tubes at the same time.

Split printing will work great. You only use one grid at a time and you have a built in stabilizer for each which will assure repeatability. They should have made this a feature, since the adjustable VC mode did not work very well.

[dajaab]

It's a very good idea.
Just need to find a high voltage switch to alternate blue or green neon since I have only one High voltage transformer in working condition

[J.B. Harlin]

It's a very good idea.
Just need to find a high voltage switch to alternate blue or green neon since I have only one High voltage transformer in working condition

I would be very hesitant switching that kind of voltage. Are there any markings on the failed power supply?

[Eric Woodbury]

BTW, it's NOT neon. It's a cold-cathode florescent lamp or CCFL.

I agree with Harlin, although switching these voltages with MOSFETs is possible, working on such things is dangerous. Do we know yet that the xfmrs are bad? Have you reversed them in the setup? Is there any number or symbol on the xfmr? If the xfmr is bad, it may be replaceable. Could be the easy solution since so far there is no schematic.

(For various reasons, my money is on the electronics being bad.)

e

[dajaab]

High voltage but low amperage, no ?
I admit it's a weird option.
I didn't have time this week to try to reverse the xfmrs, I'll do that tomorrow
But no number, no symbol, nothing, nada

[Eric Woodbury]

Yes on the volts amps thing. Volts times amps is power (watts).

I'd guess these CCFLs are hundreds of volts with a bit of a kicker at turn on. Once the tube starts, it becomes low impedance and the voltage would drop. Neon is typically 10 to 20 kV. If you touched, either would both make you tingle and let you know you're alive. A most unpleasant sensation.