Film-Tech Forum ARCHIVE: The fun of hot rectifier frames

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Posted by Rick Long (Member # 213) on 01-30-2004, 10:23 PM:

My first job at General Sound way back in '74 was assembling automation alarm/annunciator panels. I was routinely cautioned to make sure all the screws on the terminal strip were tight, before fitting on the cover.
"Even the ones that aren't used?" I asked.
"ESPECAILY the ones that aren't used" came the reply.
Occassionally something happens to remind me of the wisdom of this policy.

A week or so ago, I carried a switching rectifier (7K) to one of our booths to replace one that had been blowing its breaker once a day.
Our rectifiers are now repaired by an outside source, as we have a very limited number of technicians for field work.
As I was connecting this replacement rectifier, the booth projectionist passed behind me.
"Five minutes" she announced.
Great, dontcha just love working under pressure?
Nonetheless, I worked methodically, checked every connection, and decided to try it.
Powering up the console breaker, (Super Highlight II), POWER DOOR and AIR lights came on.
Switching on the lamp switch, however, resulted in a distinctive clunk,
the sound of fans slowing temporarily, followed by billows of dark grey smoke eminating from the rectifier, being blown out by the fans into the booth. This of course, set off the smoke detectors and subsequent alarm.
After I convinced management that it was NOT necessary to evacuate the whole complex and call the fire department, we switched the feature to another cinema.

Wondering what the hell I might have done wrong, I went to the still-smoky cinema and examined my connections. Everything was where it should be. I noticed however, that the ground connection (a 14-guage wire that runs between the rectifier frame and console ground) was burnt (vaporized). Although it had burnt the whole length of the wire, the rest of the associated console wiring appeared to be intact enough to work.

I began to wonder what conditions were necessary to make this happen. How could such a heavy current be made to flow through a ground wire?

Slowly the penny dropped. The ground had burnt because it had carried an extreme current, because the frame of the rectifier had gone high to ground when the contactor kicked in. (This despite the fact that the rectifier frame sits on a painted metal console frame).
Unable to beleive this at first, I removed the little that remained of the ground wiring within the rectifier. The rectifier was sitting in place within the console.
I turned on the console breaker. Nothing.
I turned on the lamp switch. A delightful fireworks display between the bottom of the rectifier and the console. Indeed, it was a hot ground issue.

Later examination of the rectifier, showed a small 6-32 screw that, although it belonged on a terminal strip, had come loose and lodged itself between one of the contactor legs and the rectifier shell.

If you send back repaired equipment, PLEASE tighten all screws. Thank you.

Hint also to console manufacturers. Shouldn't the ground wiring be of sufficient guage to act (instead of melt) in the event of such a malfunction?

Posted by Sam D. Chavez (Member # 1841) on 01-30-2004, 10:54 PM:

Reminds me of one time I did some work in a very old booth. I ran some control wire in the already packed raceway and put the cover back on the front. I noticed one of the screws went in kind of hard. I turned the main power back on and kaboom, a big flame shot out of the gutter. It was 30 minutes to show time and when I took the cover screws off, discovered I had put a sheet metal screw right thru the aluminum main and the subsequent explosion parted it in two leaving everything in darkness. (The balance of the cover screw ends were of the blunt variety).

There was no slack in the main to splice it and with what in any case.

I finally used the jumper cables in my car until I could find a large Kearney to make a proper splice.

The flop sweat was amazing!

Posted by Michael Schaffer (Member # 1204) on 01-31-2004, 04:14 AM:

quote:
Shouldn't the ground wiring be of sufficient guage to act (instead of melt) in the event of such a malfunction?

That reminds me of one time when a Xetron console had a short and a ground wire burnt up. Interestingly, the ground wire had different gauges along its run. Of course, the smaller portion started to burn. To my extreme delight, this portion of the wire was routed through several cable harnesses everywhere through the console and it had burnt and molten several of the wires it was bundled with also. So I had to recable half the console. Great.

Posted by Mark Gulbrandsen (Member # 72) on 01-31-2004, 11:29 AM:

I've actually been considering repairing the switchers myself. We don't have many failures of them vs. the number of them in the area but they look VERY easy to repair. The output transistors are nothing but power Mosfets and those are quite easy to match so they will all share the load evenly. The only "special" thing in there are the transformers and chokes which most likely never fail.

As for Sams sheet metal screw going into the wire, sometimes alumnium wiring will melt faster than a large breaker can react. I had a similar experieince once trying to shove a fishtape through a fairly tight 3/4" conduit that ran from a raceway to under the projector base. Didn't blow the main feed wire but it blew the hot wire (#12 Stranded copper) for the projector motor before the 20A breaker could trip.

Mark
Mark @ CLACO