Film-Tech Forum ARCHIVE: IMAX Booth & Techs - Erratic reading of digital display on Miller Power supply

This is topic IMAX Booth & Techs - Erratic reading of digital display on Miller Power supply in forum Large Format Forum at Film-Tech Forum ARCHIVE.

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Posted by Dan Reiter (Member # 1386) on 03-17-2010, 02:39 PM:

I'm trying to solve a possible control board problem on a modified Miller 812. The voltage is reading high by about 5 volts during operation. It reads high on both the display and hand-held nmeter at the lamp.

Done all the usual connection checks, cleaned all connections, etc.

Is there a calibration procedure I can follow to test the internal electronics? Or, a good pointer for the problem?

Manhy Thanks!

Posted by Tony Bandiera Jr (Member # 2365) on 03-17-2010, 09:06 PM:

Shooting from the hip here Dan, as I have no direct experience with that equipment, but first off is the current within specs? How old is the bulb?

My understanding of Xenons is they are a constant current arc, and the voltage will rise as the bulb ages and the arc gap lengthens.

I'd try a different bulb if possible and see if things go back to normal.

Posted by Gordon McLeod (Member # 33) on 03-17-2010, 09:12 PM:

Tony is correct in that as a bulb ages the current will need more voltage to maintain at the same level
Also the design of the rectifier also varries a bit especially on the amount of reactance in the output stage
Also how far is the rectifier away from the lamphouse

Posted by Dan Reiter (Member # 1386) on 03-18-2010, 02:37 PM:

We have two test chambers, and the bulb runs fine in the other chamber. So, that eliminates the bulb. As well, section by section, been either bypassing or disconnecting the IMAX modifications, no changes.

Still scratching my head on this one. [Confused]

Posted by Rick Raskin (Member # 1561) on 03-19-2010, 03:44 PM:

I also have no experience with this equipment, but from a purely theoretical perspective:

quote: Gordon McLeod
Also how far is the rectifier away from the lamphouse

Is this a new problem, in that it was operating fine before?

Is your operating current within spec?

Posted by Tony Bandiera Jr (Member # 2365) on 03-31-2010, 12:13 AM:

quote: Dan Reiter
...and the bulb runs fine in the other chamber.

So does the affected rectifier read properly when connected to the other chamber? If it does, then there is a loose connection somewhere associated with the first chamber.

Posted by Dan Reiter (Member # 1386) on 03-31-2010, 07:08 AM:

We can't swap these power supplies, due to the size, wiring, and complexity.

But, it runs fine for months, acts up, then returns to normal.

Posted by Tony Bandiera Jr (Member # 2365) on 03-31-2010, 08:41 PM:

Well if you can't swap the supplies then you may be stuck waiting for something to burn up or fail completely to find out what the cause is.

If you want to get to the root cause of this you're gonna have to take the time to swap the supplies.

What is so complex about the hookup that makes it we can't swap the units? I am aware that they're big, but what in world would be so difficult about swapping them out?

If unscheduled downtime would be a big problem, then I suggest you take some scheduled downtime to get this resolved. [Big Grin]

Posted by Rick Raskin (Member # 1561) on 04-01-2010, 07:39 AM:

I agree with Tony. The best way to get a handle on this problem is to split the trouble. Its either in the direction of rectifier or the load.
If it proves to be towards rectifier you should consider having the power company place a voltage graphing recorder on the line side. This may sound wacky but I once found an over-voltage problem caused by a faulty neutral connection.

Posted by Gordon McLeod (Member # 33) on 04-01-2010, 02:47 PM:

I have often noted difference in the voltage compared to amps with the miller conversion it was normal in many cases often the rheostat for current control contributes to it being eratic