Film-Tech Forum ARCHIVE: ORC diode issue

This is topic ORC diode issue in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Richard May (Member # 2627) on 10-04-2011, 06:11 PM:

Really strang. I have never had this happen before. ORC 1600 watt console. I had a diode short. I changed it. A few weeks later, one of the wires burnt off the contactor. 2 weeks after that, the same diode shoted AND the other diode on that plate was open. Now I changed all 4 diodes. Last night the same diode shorted again for the 3rd time. Amps are 69. Voltage is around 22. 120VAC on each leg coming in after the contactor. All connections seem tight. The plate which holds the diodes is clean. Any ideas?

Posted by Mark Gulbrandsen (Member # 72) on 10-04-2011, 06:29 PM:

Intermittent or clogged cooling fan...?
Is this leg of the rectifier on a high leg of three phase AC as measured to neutral....?
Bad transformer winding....?
I've seen the area where the connecting wires are attached to the heatsinks oxidize and cause excessive ripple and slightly varying current.

Posted by Richard May (Member # 2627) on 10-04-2011, 07:01 PM:

Cooling fan is running perfectly. Felt very free. 123.1 VAC on all 3 legs in the panel. I was wondering about where the wires bolt onto the plate myself. Even the diodes themselves. I was thinking about drilling new holes for them. Maybe I'll do it for the wires also.

Posted by Tony Bandiera Jr (Member # 2365) on 10-04-2011, 11:22 PM:

Richard, did you use silicone heatsink grease on the diodes? VERY important to use it or the new diodes will burn up fast.

Also, ALWAYS change at least ALL of the same-polarity diodes as a set! (When one blows it can weaken or damage the others of the same polarity.) I would change all diodes on both positive and negative sides to be safe, remembering the heatsink grease. [Smile]

Posted by Richard May (Member # 2627) on 10-05-2011, 05:05 AM:

Yep. Used the grease and after it blew the second time, I changed all 4 diodes.

Posted by Steve Guttag (Member # 268) on 10-05-2011, 06:45 AM:

If you can, measure the current on all four diodes to see if one is being stressed more than the others...likewise, measure the current of the AC coming in...the voltage rarely tells a good enough story.

-Steve

Posted by Bruce Hansen (Member # 281) on 10-05-2011, 05:03 PM:

Do not use too much heat sink grease. It is there only to fill the microscopic voids in the metal. Too much grease will hurt heat transfer.

Posted by Tony Bandiera Jr (Member # 2365) on 10-08-2011, 08:17 PM:

Good point Bruce.

Of course the best solution is to get rid of that ORC junk in the first place.

But not always the practical one.