Film-Tech Forum ARCHIVE: Replacing blown diodes on Christies

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Posted by Frank Dubrois (Member # 3042) on 03-29-2006, 03:41 PM:

We have had several experiences with a diode going out in the middle of a show. We replace it and think nothing more of it. We have been noticing that when one diode goes out, it is usually follwed by another one...and another one...all on the same projector, sometimes weeks apart, sometimes a month or so, sometimes never. It sounds like the one diode going out might be weakening the others in some occasions.

My question is this: When one diode goes out, do you simply change that one diode and hope the others don't fail? Or do you anticipate the other diodes following suit and just change them all at the same time? It seems like a waste of "good" diodes, but then with the potential of the projector going out in the middle of another show...and the past track record of additional diodes going out soon after, it only seems logical to change them all and avoid future problems. Anyones input on this matter would be appreciated. Also, if anyone knows Christies official recommendation, I'd like to know that.

Posted by Louis Bornwasser (Member # 3063) on 03-29-2006, 04:19 PM:

Older Christie's had "low p.i.v. ratings" and were prone to failure. Best to use at least 1000 volt p.i.v on them.

There is no general agreement on your question. I usually replace only the bad one unless I am upgrading the p.i.v. Louis

Posted by Randy Stankey (Member # 64) on 03-29-2006, 04:48 PM:

Replace the one that's bad just to get the show back on the screen as quickly as you can. If you've got an "Emergency Kit" all ready to go with the tools and spare parts you need to do the job it might only take you a few minutes if you work smartly.

As soon as you have time, change all the rest of the diodes. If you don't you're bound to have them blow, one after another.

Make sure you use enough heat sink compound. If you don't they will blow even faster.

If you can, get diodes with a higher PIV. They'll last a lot longer.

There is a kind of diode that's made with glass instead of epoxy inside to seal the parts together. I believe it's called "Glass Passivated". (Please correct me if I'm wrong, here.)
They may be more expensive but you'll only ever have to replace them once.

Posted by John Walsh (Member # 168) on 03-29-2006, 04:57 PM:

I used to be in the 'change only the bad one' camp, and this worked OK on the many older IREM supplies I delt with. But for some reason, with the newer supplies, I've had the same experience; one goes, then another, etc. So now I'm a 'change em' all' guy. I don't want to risk loosing a show.

I think what happens is, when one shorts, the other diodes conduct "into" the shorted diode for only the time their phase is on, which weakens them. That's why they don't die right away. A suggestion might be; if you take out the bad diode and it's shorted, replace them all. If it's open, just change that one.

Posted by Michael Schaffer (Member # 1204) on 03-29-2006, 05:29 PM:

Also, there is a blower right underneath the diode heatsink of the Christie PS. In older machines, these are almost always out and that may be overlooked since you switch off the PS for diode testing and changing. But do check the blower, even though it is hard to see, with the PS switched on.

Posted by Jack Ondracek (Member # 1466) on 03-29-2006, 05:43 PM:

I'm with Louis on this one.
I had an ORC supply that would blow diodes at least once per year... usually between shows (back when we shut the lamps down). Moving to a higher PIV required drilling the heat sinks out a bit, but I never lost a diode on that supply after that.

Posted by Mike Blakesley (Member # 26) on 03-29-2006, 06:16 PM:

quote: Randy Stankey
As soon as you have time, change all the rest of the diodes. If you don't you're bound to have them blow, one after another.

You must have stock in a diode manufacturer!

Our Christie tends to blow a diode every couple of years, if that. I have always just changed the blown one. Following those instructions I'd've spent over 2000 bucks replacing perfectly good diodes by now. I think this is a classic case of "your mileage may vary."

Posted by Frank Dubrois (Member # 3042) on 03-29-2006, 07:21 PM:

we do have a kit ready ready to go, but I guess I dont know how to work "smartly". It seems that when a diode blows, its a crap shoot as to which one is blown. We do keep track of blown diodes, but i'm curious if there is a "smart" way to proceed when it comes to replacing them. Are there signs to look for? I'd love to save time replacing these things..

Thanks!

Posted by Brad Miller (Member # 2) on 03-29-2006, 10:20 PM:

I haven't had a diode fail since 1992 when I was working with CFS lamphouses. I would tell you my secret, but then it wouldn't be a secret now would it? [evil]

I'm in the "change them all and upgrade them while you're at it" camp.

Posted by Mark Gulbrandsen (Member # 72) on 03-29-2006, 11:13 PM:

I don't keep any secrets.... they are all divulged here on F-T!Always replace all of the diodes and only use Semikron diodes. They have a rating of 1600 volts 140 amps in the same size package that a normal 85 amp diode fits in. I never have had one fail and they are an inexpensive part available from Allied Electronics either with leads for Irem, or without leads for Christie, CFS, Strong. Big Sky and Newmaid charge over 30.00 apiece for these [Eek!]

Mark

Posted by Frank Dubrois (Member # 3042) on 03-30-2006, 12:33 AM:

Well the exact replacment diode to get also confused me. The manual lists 3. I assumed they were different diodes for different power supplies. Our 4000 watt lamp houses use different diodes than our 3000 watt house.

Posted by Michael Schaffer (Member # 1204) on 03-30-2006, 03:35 AM:

quote: Brad Miller
I haven't had a diode fail since 1992 when I was working with CFS lamphouses.

What, cut an opening and mount a blower to ventilate the power supply better in that ridiculously small compartment? That's what we did and it seems to have helped. We used to have diodes out in our CFS consoles regularly, but not anymore since.

Posted by Brad Miller (Member # 2) on 03-30-2006, 04:55 AM:

It's much easier to just not work with CFS lamphouses. [Big Grin]

Seriously though, heatsink, damned tight connections and higher rated diodes (when/if the initial set fails) is all I do. To date I have never had a Christie SLC diode fail on me, but upgrading the diodes has worked wonders for me in Strong and other rectifiers.

Posted by Mark Gulbrandsen (Member # 72) on 03-30-2006, 06:26 AM:

Actually cfs power supplies are/were made by LP assocciates and they are not that bad.... many are just very old. They are in fact the only good part of the console. As Michael said a fan also works wonders... Replace the rest of the cf guts with a christie plenum kit. A bit of work but worth the effort. The high reactance rectifiers Strong supplies today are LP rectifiers!!

Posted by Frank Dubrois (Member # 3042) on 03-30-2006, 10:23 AM:

Well, would anyone have a part number on a REALLY good set of diodes? I am personally getting sick of replacing these. All our brisge fans work fine and are cleaned out regularly. We have just been ordering the replacment diode that the book calls for, but if theres a better part available, please let me know where we can order it and maybe a part number for neg/pos diodes for a SLC30. Thanks!!

Posted by Louis Bornwasser (Member # 3063) on 03-30-2006, 02:41 PM:

4 or 5 post above: Mark said 140 amp-1600 piv diodes work for almost everything. Remember there ARE forward and reverse diodes. Louis

Posted by Michael Schaffer (Member # 1204) on 03-30-2006, 05:07 PM:

Another thing I have quite often seen in the field and that needs to be gotten rid of is when you find diodes where somebody has put a piece of loose hose or tubing around the leads to insulate them. That is not a good idea since it creates a pocket of air which can be heated up and contribute to the early failure of the diode. If you add insulation, it should be heatshrink.
I have also found a few which weren't simpled open, but physically broken with the tail loose. In those cases, I think people may have held on to the tail of the diode while tightening them down against the heatsink. That is another bad idea because it can put mechanical stress on the diode and contribute to its early demise.
As Mark pointed out, the CFS PSs are actually fairly well designed units but the console design puts them into a tiny space with not much air circulation. Yes, they have a big fan in the compartment door but I found that improving the circulation adds to their reliability.

Posted by Dustin Mitchell (Member # 372) on 03-30-2006, 07:02 PM:

To speed up the process of emergency diode repair we keep a spare diode bridge on hand. That way if a diode fails during a show we only need to swap out the bridge and can worry about testing/finding the bad diode and changing it later. Of course we've only had one diode go bad in our 6 SLC lamphouses in the past 9 years.

I believe its been mentioned on this site before but excessive diode failures can be a seen of inconsistant electricity in your building-phases out of balance, etc. Its something you might want to have an electrician check out.

Posted by Mark Gulbrandsen (Member # 72) on 03-30-2006, 08:56 PM:

quote: Louis Bornwasser
Remember there ARE forward and reverse diodes.

The standard DO-5 Semikron come in both forword and reverse. The leaded type come all set to just pop right into your favorite Irem. I think cost was about 8.00 each in 100 quantities.

quote: Dustin Mitchell
I believe its been mentioned on this site before but excessive diode failures can be a seen of inconsistant electricity in your building-phases out of balance, etc. Its something you might want to have an electrician check out.

Surge protectors ono the booth panels today are an absolute must. Probaby why we rarely see Strong Switcher failures, or many other electronic failures. Sqaure-D makes some very good ones.

Mark

Posted by Paul Konen (Member # 41) on 03-31-2006, 09:16 AM:

At first, it was just change out the bad one. Sure enough, a week or so later, the sister diode would go out too. So the procedure became what Randy said. Do the bad one now, change out the sister diode at the next break. This was mainly on the SLC-30 consoles.

Been in the process of changing out those smaller diodes with "beefier" ones that Christie now puts in newer consoles.