Film-Tech Forum ARCHIVE: Rectifier Ingnition problem

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Posted by Martin McCaffery (Member # 37) on 12-07-2007, 03:36 PM:

I was trying to get rid of a rectifier buzz by running the lamps for several hours and then tightening all the contacts in the rectifier while nuts and bolts were hot.

When I repowered the rectifier, the buzz was deeper and louder before even trying to strike the lamp. The lamp would not strike (though electricity was going through it)

Any suggestions as to what I may have screwed up?

Using Christie CC20U with Strong X60-D and Osram 2000w lamps. Manual ingnition

Posted by Steve Guttag (Member # 268) on 12-07-2007, 05:48 PM:

Check the open circuit voltage...sounds like you might have just blown a diode...they tend to make the rectifier sound much deeper and would cause the situation you described.

Steve

Posted by Martin McCaffery (Member # 37) on 12-07-2007, 06:17 PM:

That's where I've heard that noise before;>

Thankfully, we're closed for the month, so I won't be losing any shows.

Thanks

Posted by Mark Gulbrandsen (Member # 72) on 12-08-2007, 08:58 AM:

Be sure to change out all six diodes so you don't have a repeat performance. Three forword and three reverse 85 amp 1200 PIV are what you will need.

If you want the latest greatest diodes and not have to pay some cash happy dealer 35 bucks apiece get the SEMIKRON SKN/SKR 71 from Allied Electronics. These diodes are much higher quality than the crappy IR diodes that Christie uses and actually cost less than the IR's. You will need to drill the mounting holes out for the metric stud on these diodes. BTW: Irem uses Semikrons in all their new product.

If you replace the diodes and still have the buzz from loose windings which is a common Christie problem on older units it can be remedied by tapping in thin linoleum strips between the sides of the winding and the iron core. Loose core plates can be tended to with varnish or silicon seal but you will need to force the stuff in between the plates to get them to quiet down.

Mark

Posted by Martin McCaffery (Member # 37) on 12-09-2007, 12:30 PM:

Yup, a bad diode.

quote: Mark Gulbrandsen
Three forword and three reverse 85 amp 1200 PIV are what you will need.

Mark: The diodes I removed are marked 70H30A and 70HR30A which I'm guessing is 30 amps. Is that correct, and if so, why go to 85amp? Protection?

Posted by Gordon McLeod (Member # 33) on 12-09-2007, 12:41 PM:

That is a 70amp diode

Posted by Mark Gulbrandsen (Member # 72) on 12-09-2007, 01:09 PM:

Larger diodes = Better reliability. Of course some techs and dealers don't want you to know that....

The IR diodes are quite inferior to the Semikron stuff. The Semikron is closer in quality to a mil-spec type of diode but is not really a Mil-Spec part.... just made better.

The 70H30 and its mate are only 300 PIV rated.... quite low and typical of the diodes Christie used in years gone by. Christie still uses IR but has gone to a much higher PIV rating these days.

Mark

Mark

Posted by Martin McCaffery (Member # 37) on 12-09-2007, 05:10 PM:

Mark and Gordon:
Thanks. Obviously, electronics is not my strong point. I'll see what I can find in town (pickings are slim around here). I guess I have time to try online also

Posted by Mark Gulbrandsen (Member # 72) on 12-09-2007, 07:07 PM:

Just call Allied Electronics... makes it easy!

Posted by Dave Macaulay (Member # 813) on 12-10-2007, 09:10 AM:

The 70H30A is an IR diode rated 70A and 300V. IR rates the voltage differently from Semikron (peak vs continuous voltage), so this would be about a 600V Semikron rating.
The 70HR30 is the same rating but reverse polarity, you have to make sure you get the right ones back where they belong. It's common to use these reverse diodes in rectifiers because it eliminates a lot of expensive insulation and/or extra heat sinks. You absolutely must use 3 forward and 3 reverse diodes and install them in the right holes.
If you can get IR, I think the best choice would be 85H80 and 85HR80, larger amp ratings are in a different case that won't fit. The Semikron ones suggested will require a slight enlarging of the mounting holes, nothing serious and you can still use the 1/4-28 stud IR types on future replacements.
You will need some semiconductor heat sink goop to help keep the new diodes cool. This is available at Radio Shack (unless they needed the rack space for toys or ipod accessories and dropped the product). Just put a thin coating on the diode flat around the stud. Use the new washer(s) and nut, torque the nut tight enough to compress the spring lock washer most of the way if it has that type, if you get the nut with attached belleville washer it should be tightened almost flat.

Posted by Martin McCaffery (Member # 37) on 12-10-2007, 09:23 AM:

Thanks for the rundown.

quote: Dave Macaulay
The Semikron ones suggested will require a slight enlarging of the mounting holes, nothing serious and you can still use the 1/4-28 stud IR types on future replacements.

Nothing serious as in take it to a machine shop to rebore the holes in the diode rack?

Posted by Steve Guttag (Member # 268) on 12-10-2007, 10:16 AM:

IR actually makes the 85 series all of the way up to 1600PIV (85H160 and 85HR160) in the DO-5 package. They are not too much pricier than the lesser rated units.

Christie has gone to the 150A package (DO-8, I think its package style is) for all rectifiers...even the 2K.

What is weird about Mark's spec sheet is that they list both 1/4-28 (what a DO-5 uses) and 8mm...which is much closer to 5/16"

As for boring out the diode holes...no machine shop is needed...just a drill and a good bit (and a vacuum).

Steve

Posted by Martin McCaffery (Member # 37) on 12-10-2007, 11:45 AM:

quote: Steve Guttag
What is weird about Mark's spec sheet is that they list both 1/4-28 (what a DO-5 uses) and 8mm...which is much closer to 5/16"

Just talked to Allied and Semikron. The diode does come in both a metric and American size.

Allied is out of 1200 at the moment. Will 1600 work?

Posted by Steve Guttag (Member # 268) on 12-10-2007, 01:00 PM:

The higher the PIV the better (1600 is better than 1200) but normally it comes with a cost increase...can be a little or a lot...in large quantities even little increases add up but not in a 6-piece order.

If they have the 1/4-28 version...go for that, it will have the same thread/hole size as your current diodes.

Steve

Posted by Martin McCaffery (Member # 37) on 12-10-2007, 01:24 PM:

Yeah, ninety-six cents. I splurged and got the 1600's. 36 piece minimum for them to order more 1200 from the factory. I just don't have that many rectifiers.

Thanks for all of the advice

Posted by Mark Gulbrandsen (Member # 72) on 12-10-2007, 02:28 PM:

The main thing I find that Semikron has going for them over the IR stuff is the glass pasivation (seal) they use over IR's crappy green or blue epoxy. I've seen way too many failed IR diodes over the years with loose stems where the epoxy has let go after so many thermal cycles. Glass pasivation is used on all Mil-Spec parts and is a huge contributer to longevity of life.

Mark

Posted by John Hawkinson (Member # 1135) on 12-10-2007, 03:31 PM:

Also, IR doesn't make rectifier diodes anymore. They sold the line to Vishay. I don't know what that means for long-term availability of IR part numbers...

--jhawk