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Old November 16th, 2012, 09:57 AM   #1
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Chinese "As ARRI" 1.2k HMI

Some time back, I posted an unauthorised how-to on the Steven Studio 1.2K HMI light with electronic ballast.

There are now two package options posted for sale on eBay by a number of vendors. There is the original package with a heavy solid electronic ballast going for ballpark of $3,000 and now another with an electronic ballast which appears to be of sheetmetal construction, going for a significantly lower price.

The following comment relates to the heavier electronic ballast.

This ballast which is sold by a number of vendors is pretty much a sealed affair with passive cooling by radiative finned heatsink around the casework.

The following comment is based on my own personal experience with an individidual ballast and is not endorsed by the manufacturer or any vendors. Furthurmore, I have no formal qualification in this industry so appropriate furthur research should be undertaken by readers who might elect self-service over using a tradesman to fix a loose knob.

Readers of the text below, should act in the knowledge that their intervention in the external mechanisim of their ballast is at their own risk of unsuccessful or damaging outcomes. Readers who do not have dextoral skills in handling mechanical repairs and do not understand tapered-collet fixture systems should not undertake this task.

Recently, on my ballast, the frequency select switch knob worked loose on the switch shaft. It could be made to function by drawing outward on the knob as it was turned. It would pick up enough friction to overcome the resistance of the internal detent spring.

When I went after the cause, my first thougth was a loose grubscrew. However there was none to be found. Instead, the knob is attached to the shaft by a tapered collet arrangement. Why? I first asked myself.

Then it became apparent. The switch shaft itself is not metal but plastic. Any grubscrew fixture would tear it to pieces soon enough.

So why did it work loose? The plastic shaft is actually finished with splines and the splines eventually deform and wear enough to allow movement, then wear some more. Re-tightening the collet fixture solves the issue soon enough.

I was a bit perplexed as to why a plastic shaft was used when metal is most commonly found in this style of switch. Then I thought about the vendor's claim to the unit being compliant with CE standards. Ensuring double insulation is the likely objective.

So. - How do you get at it? The knob is hollow and has a thin cap on the outer end. This cap can be levered away. If you have a close look you can see the seam. Poke a jeweller's screwdriver under, give it a bit of a lever and it should come away from the knob body cylinder.

Inside you will observe a brass hex nut with two slots in it. You can either use a lens spanner to turn it by the slots or make your own custom wrench out of sheetmetal or find a small socket wrench with a deep enough reach and side clearance.

You will find that as you tighten the nut, the switch may move to its most clockwise setting. Let it do so, then loosen the nut, reposition the white mark on the knob to match the selected position, then retighten.

If the collet collar turns with the nut without tightening the wedge pieces on the shaft, your might need to remove the entire collet and smear a mere trace of grease on the cone surface and on the underwside of the nut.

Why both and not the nut undersurface only? Lubing the cone surface appears to aid the wedging action.

As I mentioned above, just use a mere trace of lube and keep it off the plastic shaft. Petrochemicals and plastics are sometimes antagonistic and you don't want to chance the shaft dissolving or becoming brittle.

Test the switch and then replace the little cap. Best to use a small dop of contact adhesive on two sides to make sure it does not drop out with use and that's that. Don't use a solvent adhesive like pipe glue or two-pack, because that may dissolve the plastic or change the properties of a mechanically stressed part.

Don't overdo the glue because you might want to retighten the knob again some time.

This comment should not be taken to apply to Richard Andrewski's Cool Lights 1.2K HMI product.

Richard. If you are there, you might like to add some comments or cautions relating to the generic Chinese 1.2K HMI lamps which I may have overlooked.

Last edited by Bob Hart; November 16th, 2012 at 10:05 AM. Reason: error
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