Repairing HMI's

[Evan Woss]

I just started as a repair technician for a rather large rental house. Just like anyone with a handful of common sense I'm pretty decent with tungsten lights, grip gear, fixing up rags and such, but I have never really torn into an hmi. I hardly have an idea of what they look like inside. I've tried searching the bowels of Google and still have little grasp on what really goes on in there. In the next few weeks I have a few arrimax 18k's, a bunch of cinepar 6/12k's and a few various ballasts, from a joker 400 to a 20k, that I'm going to need to start repairs on. I know I'm in a few feet over my head, but it would be a great help if someone could throw me a paddle.

 

I would love any input or advice on hmi's in general. A link to a site or some pictures would be a great help too!

 

Thanks in advance.

 

-Evan

[Dom Jaeger]

Hi Evan,

 

do you have any electrical training? Are you working under the supervision of someone experienced with HMI heads and ballasts? They are not the sort of thing you just 'tear into'. When I worked for Panavision I used to assist an experienced electronics tech repair HMI gear - it is seriously dangerous stuff. If you're fishing around the innards of a ballast, even unplugged, and touch the wrong bit the discharge can throw you across the room. The striking voltage can be in the tens of thousands of volts, lamps can explode, not to mention the compromised safety of the crew who uses the gear after you've been 'repairing' it.

 

Seriously, stay away from HMI equipment if you don't know what you're doing, or aren't under the close supervision of someone very experienced.

[Phil Rhodes]

I once repaired a head cable from a 1200W HMI PAR, and it's worked fine to this day, but I don't think you could really do component-level repairs on ballasts without a decent idea what you were doing. Simple cabling repairs, connector replacement and general cleaning and lubrication of any moving parts I might try (with due deference to the safety hazards, and I am expressly not encouraging you to try it) but as to actually hacking on the ballast electronics itself, I'm not sure there's much you could do outside of the manufacturer's labs.

 

Modern HMI ballasts are basically very large (very large) current regulating switch-mode power supplies with lots of extra fiddles and tweaks to provide all the features we like in the film industry - the high efficiency, the hot restrike, the flicker free modes, the quiet 50/60Hz modes, the decent power factor, and so on. Switching power supplies are difficult enough to work on even if you do have the drawings and someone around who knows how that particular design is supposed to work, and even if they don't have enormously potent high voltages flying around into the bargain. The tesla coil ignitor in the lamp head is often a potted module which can only really be replaced anyway.

 

Even old iron ballasts are tricky to repair as many faults come down to insulation breakdown in the enormous transformer, which can't really be fixed without completely rewinding it, which is something that would almost never be worthwhile for an older piece of gear.

 

P

[Guy Holt]

…. stay away from HMI equipment if you don't know what you're doing, or aren't under the close supervision of someone very experienced.

 

I don’t know if I would go this far since most HMI repairs don’t require servicing of electronics. I have operated a lighting rental house in Boston for over twenty years and, in all that time, there were only two problems that I could not fix myself and both had to do with blown IGBTs on 18ks.

 

Most problems with HMIs have to do with the Head Cable, the Power Supply, or the Safety Loop. If you understand how HMIs work, you can usually identify the cause of the problem to be related to one of these three and repair it easily without having to get into component level repairs.

 

To repair anything you must understand how it works, so you need to start by learning as much as you can about how HMIs operate in principle. Harry Box, author of the authoritative trade handbook “The Set Lighting Technician’s Handbook” has established a companion website to the Fourth Edition of the Handbook, called “Box Book Extras.” On it he has posted a very good article on trouble shooting HMIs (you can access the site with the password “set lighting.”) Our company news letter article on “Portable Generators in Motion Picture Production” also has a lot of useful information on HMIs. The article is available on-line at www.screenlightandgrip.com/html/emailnewsletter_generators.html.

 

To make shop bench repairs easier, many manufacturers are now designing their equipment with easily swappable parts. For instance, a lot of the ARRI head electronics (igniter, spark gap, etc.) are a factory sealed block that you can only swap out; Power Gems is now designing their ballasts with swappable Power Modules; and Power-2-Light has designed their ballasts with diagnostic LEDS that will identify the board that has the problem so that you can simply swap the board.

 

 

- Guy Holt, Gaffer, ScreenLight & Grip, Lighting & Grip rental in Boston MA

[Neil Snape]

Hi.

 

I am looking all over the place to find out something about repairs for a magnetic ballast.

This last summer I bought some old HMIs, a Kobold 575w, and an LTM 1200w. Since I bought an almost new 200w Kobold too.

 

The Kobold 575w works fine, but I noticed a smell from the unit but couldn't really localise it. The light and ballast were always stored in an aluminium box with some very old deteriorated foam ( Poly urethane I think).

I thought the problem may have been the foam dust melting on each use.

 

On the last session with maybe 2-3 hours on time, the smell was getting worse, the ballast running a bit hot (indoor use) and eventually I saw some smoke coming from the Ballast. I immediately turned off the unit.

I took off the case days latter to see if anything looked burned. I understand the dangers of high voltage electronics and did not want to touch anything in the unit, only take off the cover.

The transformer looks to be melting off the varnish (insolation I presume) off the coils. I took pictures of the unit, then put the cover back on. Upon a brief trial the unit again worked fine, no smoke or other signs of anything wrong.

 

Yet I am almost sure it will fail.

 

Now I have no money to replace the ballast with a new or electronic ballast, nor if the transformer is priced sky high.

I found this transformer on Ebay Germany. http://cgi.ebay.fr/ws/eBayISAPI.dll?ViewItem&item=150934339386&ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649#ht_819wt_1104

 

So the big question is, could the transformer be the result of another component, or do these things sometimes just die of old age and break down of the transformer insolation?

 

If so IS the Ebay HMI transformer a good option for which I might take it to a film production rental and repair and have them change it?

Or are there way too many variables and this is basically a junker.

Edited March 24, 2013 by Neil Snape