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Topic: Diode Failure on Christie

Randy Stankey
Film God

Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99

posted 03-15-2000 10:12 PM

No, these babies get hot. That's the way they've always been. If you take the panel off the side and wave your hand around, (Careful not to touch anything live!), It's pretty warm. Diodes and all metal surfaces get too hot to touch. Right after a show is over, you could get a nasty burn. That's where you have to develop talent. How do you change HOT diodes inside a HOT lamphouse on a sold-out Saturday night on opening weekend?

1) open the panels first. (shut off breakers!)
2) go get your tools. let things cool off.
3) put shop rags on the hot metal parts. (dont' forget to take them out!)
4) learn to endure pain!

I tell everybody it'll take 1/2 hour. When it takes me 15 minutes they're all impressed at how good I am. I learned that from watching Scotty on that episode of Star Trek!

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John Pytlak
Film God

Posts: 9987
From: Rochester, NY 14650-1922
Registered: Jan 2000

posted 03-16-2000 06:13 AM

Randy:

If the heat sinks get so hot that you burn your fingers when you have to touch them, you definitely have a cooling problem. Increase that air flow! Despite the difficulty in getting at them, you will need to maintain the fans on a more regular basis. You've noted the quick failure of the diodes when the fans fail.

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Professional Motion Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com

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Tim Reed
Better Projection Pays

Posts: 5246
From: Northampton, PA
Registered: Sep 1999

posted 03-16-2000 09:28 AM

First things first when replacing diodes:

(1) USE HEAT SINK COMPOUND

All the cooling in the world won't help, if the diode can't get the heat conducted away from it in the first place.

Heat kills 'em for sure, but I have a hypothesis that the PIV gets 'em more than you might imagine, when the power supply is turned OFF. That's where Gordon's caps across the individual diodes help (that, and it also shunts the ignition voltage).

When the power is shut off, all that heavy current flow comes crashing to a halt, which induces a pretty good reverse voltage spike as the field collapses. This, of course, is why you can't shut down a rectifier by opening the DC line (like we used to do with motor-generator sets). The change is too great, and you'll pop the diodes. Shutting the primary power down at the AC input, as we do now, is better, but I believe it is not perfect. It is probably the source of a lot of diode failures.

They really need to build a power supply that shuts down in quick steps; gradually decreasing output over a second or two until the arc breaks, and the input voltage returns to zero.

Just a thought, from my meanderings...

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Randy Stankey
Film God

Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99

posted 03-17-2000 03:08 PM

I oughtta' buy stock in a silicone factory! (Yes we do use heat sink compound) I also use alcohol and a clean rag to get all the old stuff off the heat sink too.

As for the caps on the diodes, I like the idea but I'd NEVER get people to go along with that one! I have a hard enough time getting people to keep using FilmGuard. (It was too much work to rewind those pads! -- When they found out they didn't have to clean projectors every day, they changed their tune!)

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 03-18-2000 10:19 PM

Just a possibility at the top of the two heat sinks there should be a cap bridging the two assemblies is one of the lead broken or burnt off
I just had a ORC console loose a diode and I found the cap of

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Randy Stankey
Film God

Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99

posted 03-20-2000 02:08 PM

I'll look for the cap. to see if it's missing.

BTW, they are mostly 4500W but there are some 2500W. We rarely have trouble with the 2500's

I'm pretty sure it's heat. The little blower down by the rect. never seemed like it was enough to do the job. In fact one person told me that the blowers were an afterthought by CFS. Until they added the blower, I'm told they REALLY went through diodes! Maybe cutting a hole in the side and sticking a blower in would help.

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 03-20-2000 04:45 PM

The XP45 rectifier is "usable" from 2K-4500 watts
The higher the current occours the hotter the stacks will get
hence the initial question is there enough airflow and I think we have an awnser to that
also diodes are more transient voltage sensitive the hotter they get
Also the PIV part usually doesn't get exceeded as much as the DV/DT (change in voltage vs change in time) exceeds there hot rateing
The console should have a minimum of 500CFM of exhaust and of course that much air must also be able to get into it as well

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Jason Burroughs
Jedi Master Film Handler

Posts: 654
From: Allen, TX
Registered: Jun 99

posted 03-20-2000 09:05 PM

Randy,

On the 7000 Watt version of the CFS Console they did just that, they added an additional fan to blow on the backside of the heatsink, in addition to the one already mounted. I too have experienced the CFS consoles going through SOME diodes but never to the scale that Randy is reporting.

I have also seen the fans below the heatsinks on the new Christies fail, causing them to eat diodes as well, even roasting the connection wiring. I would have to agree with everyone here, biggest killer of diodes is heat.

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Jon Bartow
Master Film Handler

Posts: 287
From: Massachusetts
Registered: Nov 1999

posted 03-20-2000 10:36 PM

Hey everyone,
this is only sort of related, but I had an interesting diode situation with a ORC console. The theater called me and said there were sparks coming from inside the console (near the rectifier) After telling them not to touch the thing and shut the power off I high-tailed it over there. Turns out the vibrations caused by the projector (Century SA) were causing just about every bolt in the console to work loose. Including the diodes. Four diodes were almost falling off. On those diodes the electricity was arcing from the heat sink through the washer and bolt and then onto the stud. Welded the stud, bolt, and washer together on all four. Intersting to get off.
I retighted most of the bolts on the whole console (and projector) and it has been fine ever since. (The motor mount bolts were loose causing the motor to vibrate, in turn vibrating everything else.)

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Gordon McLeod
Film God

Posts: 9532
From: Toronto Ontario Canada
Registered: Jun 99

posted 03-21-2000 08:20 AM

ORC's have always been notoriuous for loose connections. Some of there standalone rectifers for 1600watt had a plate that connected the top of several caps togather as a square bus well they used to vibrate loose and arc as did the connections on the breakers
It has always been our service policy if ORC (or the like) retighten every connection on every service
Some is due to vibration other loosenings can be due to thermal expansion and contraction of the bolt threads

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Randy Stankey
Film God

Posts: 6539
From: Erie, Pennsylvania
Registered: Jun 99

posted 03-21-2000 12:40 PM

Yes, tightening those connections is important. I always crank 'em down. It's also a good idea to check them all every time you change a lamp.

We've had two instances of the taps/jumpers smoking themselves because they came loose from repeated heating and cooling cycles. The whole terminal block had to be replaced!

The exhaust fans are good enough. All of our lamps are lasting to at least 1000 hours, now. We had to have the exhaust blowers cranked up right after we first opened. After that, no problem. I just think that down there in the bottom of the console there's too much metal blocking the air flow.

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