Film-Tech Forum ARCHIVE: Victoria 4 flicker

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Posted by Bill Yacey (Member # 6102) on 02-05-2011, 09:35 PM:

Hi, recently I have started doing maintenance on 5) Victoria IV projectors at a local theatre. Most of the problems are lamphouse power supply related issues, but I have one that appears to have an excessive amount of flicker related to the shutter. I checked and adjusted the shutter timing, but it almost seems like the shutter blades are slightly on the small side as The frame starts to advance slightly before the shutter is completely closed, or if I adjust it the other direction, the shutter is beginning to open before the frame is completely stopped. The gear that is driven by the motor shaft gear has a considerable amount of slop to it, but I would think once the backlash play is taken up it should run properly in time. The top and bottom loop sizes seem to be fine. I'll be ordering a replacement gear once I source one out. One thing I thought might be a possibility is the damper springs on the shutter plate hub appear to be compressed on one side more than the other, probably fatigued from age and heat.

Ripple voltage on the power supply doesn't look to be excessive, and I don't notice any flicker when observing the lamp through the lamphouse viewing port. When projecting on the screen with the shutter motor running / no film threaded, I don't really notice any flicker.

Any thoughts on this?

Posted by Marco Giustini (Member # 4544) on 02-06-2011, 06:13 AM:

I am not familiar with V4's, but to check whether the shutter is correctly timed or not, you should use a loop of print with a still white credit on black background. Show it in Scope on the screen and with the framing knob move the credit on the top and bottom of the screen: you must not see any ghosting above or under it. If you see it, adjust the shutter accordingly.

You may also want to make sure the shutter is still BLACK. V5's shutters lose their black colour after a while and I was told that the light they reflect back into the lamphouse causes noticeable flickering on the screen. I painted a few of them with high-temperature proof paint and they worked fine even on 6000W lamps.

Posted by Dave Macaulay (Member # 813) on 02-06-2011, 09:09 AM:

There's 2 types of visual oddity from the shutter. If the blades are unequal size, have holes burned in them, or are or bent, you get flicker that shows up with open white light. With undersized or maladjusted shutters you get travel ghost but that is not "flicker", it shows as comet tails on high contrast images or juts a general blurriness towards the top and/or bottom of the image and it does not show on open white light.
There are "drive-in" shutters available for most projectors that are smaller than normal. They let through more light but allow a bit of travel ghost. Some projectionists or techs make their own by modifying a normal shutter with tinsnip or file and often go too far.
Check your shutter for burn holes or other damage. Measure it and compare to an "OK" projector. Painting it black with high temp paint is a good idea if it's shiny.

Posted by Bill Yacey (Member # 6102) on 02-06-2011, 10:37 AM:

Thanks for the replies. The shutter doesn't appear that it's been modified in any way that I can see, and there aren't any holes, so I'm thinking it may be reflections as you mention. The shutter paint has burnt to a light gray color. I'll pick up some high temp flat black paint and see if that helps.

There doesn't appear to be any blur at the top or bottom of the frame, so the timing must be pretty darn close to where it should be.

Posted by Bill Yacey (Member # 6102) on 02-08-2011, 11:14 PM:

I painted the shutter today, as well as the inside of the cone between the lamphouse and the shutter, and I still get the same flicker. I'm going to check the electrolytic caps in the power supply, perhaps they are drying out. The projectionist was indicating every once in awhile the lamp will go out by itself which points towards the filters too. this is a Strong lamphouse with a 1700W bulb.

Posted by Louis Bornwasser (Member # 3063) on 02-09-2011, 06:29 AM:

Have them swap rectifiers with a correctly operating one first. Louis

Posted by Bill Yacey (Member # 6102) on 02-09-2011, 05:47 PM:

You suspect an open rectifier in the bridge? The DC voltage is running around 23 volts under load; I guess I could separate the bridge and measure the rectifiers.

One thing I don't understand is why they used such light gauge wire on the filter cap terminals. I imagine the ripple current is quite high with the bulb current involved; If I was designing this supply I would have used a #12 0r #14 AWG as a minimum.

Posted by Marco Giustini (Member # 4544) on 02-10-2011, 05:18 PM:

I am sorry that you haven't fixed the issue by paiting the shutter.
About ripple in the power, if the mains' frequency is 50Hz shouldn't you see a 2Hz pulsing on the screen?
Sure, with 60Hz it would be 12Hz...

Why don't you check the ripple, or check the waveform with a scope?

Posted by Bill Yacey (Member # 6102) on 02-15-2011, 10:50 AM:

I'll scope it and see. Does anyone know what an acceptable level of ripple would be?

Posted by Bill Yacey (Member # 6102) on 02-16-2011, 09:57 PM:

I scoped the output today, and after the lamp is fired, I read 2V p-p ripple; although I think it may be more of a ground loop. The scope is grounded through the power cord and the ripple component appears as a sine wave. If I was reading DC ripple it should appear as a slight sawtooth with good filter caps. I compared it to another working power supply on another projector, and I get the same readings, except for the DC arc voltage difference between the different lamps.

This thing really has me wondering what is causing this, I've examined the shutter, If it is indeed `120 Hz ripple from the lamp beating against the shutter frequency I should be able to verify this by putting a high power incandescent bulb in the lamphouse for testing purposes.

Posted by Kenneth Wuepper (Member # 1174) on 02-17-2011, 09:15 AM:

Bill,

Here is a safer way to see arc modulation. The scope need not be connected to the lamp house in any way to make this test.

If you have an old solar cell, simply connect it to the scope input with approximately 1,000 Ohm resistor across the terminals.

Place the cell in the light leaking from the lamp house BEFORE the shutter interrupts the light.

The scope will show you the variations in the light from the arc and you can see if there is any modulation of the light from the power source.

KEN

Posted by Bill Yacey (Member # 6102) on 02-17-2011, 11:12 AM:

That's a very good idea Ken; thanks for the tip. That will also rule out and induced noise in the scope leads from magnetic fields around the power transformer.

Posted by Al Biers (Member # 1961) on 02-17-2011, 12:53 PM:

Replace the shutter springs, I had similar problem years ago from oscillating ghosting and that fixed the problem.

Posted by Bill Yacey (Member # 6102) on 02-17-2011, 07:24 PM:

I thought about those springs, but I figured that would only affect the top or bottom of the image if that was the problem.

I discovered today it was an open rectifier; I should have listened to Louis sooner. I pulled the block out and measured the rectifiers, and sure enough, one was open. I wouldn't have suspected that because usually the gear I work on, when a rectifier shorts it usually pops the fuse or circuit breaker. I guess these supplies have more than enough reserve current to burn it open after it shorts.

As a side note, the audio previously had some distortion but now it's crystal clear. There must have been some lamphouse light leakage getting to the reverse head pickup and modulating the audio.

Thanks to all for the good advice.

Posted by Louis Bornwasser (Member # 3063) on 02-17-2011, 08:46 PM:

I got lucky.

Really I was just starting there since you didn't have to buy anything to make that test. Louis

Posted by Marco Giustini (Member # 4544) on 02-18-2011, 02:11 PM:

Interesting.

And with an open diode you couldn't see any ripple on the output? Anyway Irem recommends to check current ripple and not voltage ripple. Kenneth method is definitely right.

Happy you sorted it out though!

Posted by Bill Yacey (Member # 6102) on 02-18-2011, 02:28 PM:

That's the strange thing; I expected to see ripple, and all that I saw was a 60Hz 2 v p-p sine riding on top of the DC level. That's why I surmised I was reading some ground loop current or leakage instead of the ripple. I would have lifted the ground on my scope to verify, but I didn't have a ground lift block with me.