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HMI head scream


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#1 Paul Curtis

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Posted 17 November 2011 - 08:13 AM

Hi,

I have a couple of Dedo 400 HMIs, in general they're fine but a couple of times i've had one of the heads emit a screaming sound/very high pitched whine. Shut down for a while and restrike and the sound went away.

I've googled and i'm no expert on this but any ideas what this might be?

Could it be the bulb on its way out? Or something more serious, i assume the head includes an igniter, the ballasts are external and seem to be okay.

cheers
Paul
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#2 Guy Holt

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Posted 17 November 2011 - 09:55 AM

i've had one of the heads emit a screaming sound/very high pitched whine. Shut down for a while and restrike and the sound went away.


Paul, it is not clear from your post whether the head is running and suddenly emits a high pitch sound and then shuts itself down or whether you are shutting the head down. Also, it is not clear if the high pitch sound is during initial strike or after the head has been running a while. Please clarify.

- Guy Holt
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#3 Paul Curtis

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Posted 17 November 2011 - 10:49 AM

Paul, it is not clear from your post whether the head is running and suddenly emits a high pitch sound and then shuts itself down or whether you are shutting the head down. Also, it is not clear if the high pitch sound is during initial strike or after the head has been running a while. Please clarify.

- Guy Holt


Thanks Guy,

The few times it happened it was just after turning the lamp on. The head would 'scream' (I really can't remember whether the light actually came on), . It screamed for 5-10 seconds then i shut the lamp off, gave it a little while then tried again and all was well. The light hadn't been used before that first strike, so it would have been cold.

Any pointers?

Thanks
Paul
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#4 Guy Holt

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Posted 18 November 2011 - 09:54 AM

I really can't remember whether the light actually came on)


It sounds like a failed attempted strike of the bulb. The cause could be that the electrodes of the bulb have deteriorated to the point where it will strike some of the time but not all the time (it eventually won't strike at all.) If changing the bulb does not remedy the problem, then the "spark gap" on the head may need to be adjusted. The purpose of the Spark Gap is to apply a jolt of electricity to the lamp electrodes to strike the lamp. When striking an HMI, the ballast ramps up the voltage to the HMI head. When the potential created by the ballast becomes sufficient to jump this gap (two small electrodes in a clear plastic box) it continues to the lamp electrodes and strikes the lamp. If the spark gap is too narrow, the voltage to the lamp electrodes is not sufficient to strike the lamp. If the gap is too wide, the ballast may have difficulty creating a potential sufficient to jump the gap (possibly the case here.) This is an adjustment you can make yourself (just mark where the original setting was so that you can go back to it if that wasn't the problem.) You can use this link for more details on how HMIs operate.

- Guy Holt, Gaffer, C.O.O. of New England Studios - soon to be the regions first feature production stage complex

Edited by Guy Holt, 18 November 2011 - 09:57 AM.

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#5 Paul Curtis

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Posted 02 December 2011 - 10:03 AM

It sounds like a failed attempted strike of the bulb. The cause could be that the electrodes of the bulb have deteriorated to the point where it will strike some of the time but not all the time (it eventually won't strike at all.) If changing the bulb does not remedy the problem, then the "spark gap" on the head may need to be adjusted. The purpose of the Spark Gap is to apply a jolt of electricity to the lamp electrodes to strike the lamp.
- Guy Holt, Gaffer, C.O.O. of New England Studios - soon to be the regions first feature production stage complex


Thank you Guy for all your advice.

I'll get some new bulbs in as i need to anyway.

One question though that you *may* also be able to answer. These lights are supposed to use the black tipped MSR bulbs from Dedo which are obviously considerably more expensive than their Philips originals. I understand the blacktips cut down internal reflections? Is that the case or do i really need to get 'proper' Dedo bulbs. Exactly what to the tips do?

Many thanks
Paul
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