Film-Tech Forum ARCHIVE: Strange Xenon problem on home setup

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Posted by Matthew Taylor (Member # 2310) on 03-15-2005, 10:56 AM:

I have just acquired a Prevost P70 for my home setup. I also got a cinemacannica(?) CX445 small lamphouse for it. I run this from an old Elmo rectifier designed for a 500W 16mm xenon projector. The lamp seems in good shape - no coating on the inside of the bulb.

The problem is that I can strike the lamp up and if I pull open the dowser on the non-running projector, the light output looks great. BUT..when I turn the projector on, the movement of the shutterblade makes it look like the light is 'pulsing' about 2 times a second. If I close the dowser while it's running you cannot see any obvious flickering of the lamp.
Also, the ammeter on the lamp shows a healthy 28amps after striking but then after about ten seconds it just instantly drops to about 21 amps. The pulsing is there even when the lamp is running at the full 28 amps on startup.

Also (...sorry...bear with me!), I left the lamp running for about 10 minutes yesterday and then I heard a loud 'pop'. After leaving it to cool, I opened the housing and a 0.1uF 630v capacitor had exploded. The lamp still worked and would still strike ok after this happened.

Dare I say.....any ideas?

Posted by Gordon McLeod (Member # 33) on 03-15-2005, 11:20 AM:

The pulsation is the beat difference between the flicker of the lamp 120hz and the shutter 48hz
most likely a bad filter capaticor or diode in the rectifier

Posted by Dick Vaughan (Member # 540) on 03-15-2005, 02:43 PM:

Gordon

May have my strange head on but why why 120Hz?
Matthew is in Essex using 50 cycle AC

Posted by Jeff Taylor (Member # 443) on 03-15-2005, 02:50 PM:

You don't state what lamp you are using with the 500w Elmo rectifier, but if it's 500w or more 21 amps is light.

Posted by John Pytlak (Member # 331) on 03-15-2005, 03:19 PM:

Let the stray light leaking from the lamphouse shine on a resistively loaded silicon photocell, and look at the cell's output as a waveform on a scope. Any ripple in the DC power supplied to the lamp should be obvious. Not only will high levels of current ripple cause flicker from the "beat" with the shutter frequency, but it will also shorten the life of the lamp electrodes.

Posted by Matthew Taylor (Member # 2310) on 03-15-2005, 04:35 PM:

Jeff, I am running a 450W lamp. This should run at 28 amps but as I said earlier, it starts at 28A then drops suddenly to 21A after 5-10 seconds.

Posted by Gordon McLeod (Member # 33) on 03-15-2005, 05:50 PM:

Dick you are correct so it would be 100Hz as there would be 100 pulses per second

Posted by Dave Macaulay (Member # 813) on 03-15-2005, 07:10 PM:

You have some AC ripple on the DC to the xenon lamp, like Gord says this beats with the shutter and leaves the difference visible (48hz film + 50Hz power = 2Hz flicker). The little capacitor probably exploded because the ripple causes high current flow in a capacitor and that makes heat, it should be replaced as it probably helps bypas the ignitor voltage. Make sure the rectifier polarity is correct for the lamphouse though, connecting a DC capacitor backwards can lead to pyrotechnics too. Inside the power supply box there should be some much larger capacitors, probably 10,000 uF or more. If the wiring to them looks OK assume they are bad (unless you can test them) and replace them... your excess ripple is almost certainly bacause of failed filter capacitors, they are reasonably cheap to replace. Make sure the voltage rating on the replacements is equal or higher than what you have. You can increase the uF rating by about 50% without much worrying.

Posted by Matthew Taylor (Member # 2310) on 03-16-2005, 02:37 AM:

Guys, thanks for your help. I will look into replacing the caps and report back. Thanks!

Posted by Gilbert Travin (Member # 2138) on 03-16-2005, 04:49 AM:

Hi

I think you have a problem with a bad filtering capacitor.
You can verify the current with a DC current clamp. If the clamp indicates 21 A, then the lamphouse Currentmeter is all right and the problem is in the rectifier.
Pay attention ! a too much important current ripple may damage lamp !

Posted by Matthew Taylor (Member # 2310) on 03-16-2005, 08:56 AM:

I forgot to ask - would it be ok to replace the blown capacitor in the striker unit with a 1uF 1000v bi-polar electrolytic rather than the 0.6uF 630v bipolar that was originally in there?

I ask because I'm finding it hard to track down an equivalent.

Posted by Gilbert Travin (Member # 2138) on 03-16-2005, 09:27 AM:

OK !
The impedance of the 0.6 uF capacitor, at 50 Hz, is 5300 Ohms : the current that flows in this component is negligible with respect to lamp current.
This capacitor was probably defectuous and has exploded (it is frequent with an old item).

If lamp is ignited without this capacitor, it is probably a device for interference reducing, thus I think that a 1 uF capacitor would be all right for your ignitor.

Prevost P70 is a good machine !Have you the 70 mm gate and accessories which are usually lost ? [Big Grin]

Posted by Gordon McLeod (Member # 33) on 03-16-2005, 09:34 AM:

If it is a RF supression cap too large of value negates some of its effectiveness at high frequencies

Posted by Gilbert Travin (Member # 2138) on 03-16-2005, 09:59 AM:

OK Gordon, that's true, high values of "chemical" capacitors are inoperative for RF erasing !
But between 0.6 and 1 uF, there is not a big difference, I think that the two capacitors are based on the same technology.
It remains the problem of DC arc current ripple : a "big" cap defectuous or a diode out of order ???

Posted by Matthew Taylor (Member # 2310) on 03-16-2005, 10:01 AM:

Gilbert, that makes sense about it being an interference capacitor - one end is connected to ground.

No I haven't got the 70mm parts - they are.....lost [Roll Eyes]

I would love to set it up as a 70mm machine but I fear the cost of obtaining the parts from Prevost would be too much to warrant doing it just for a home setup (that's if Prevost can still supply those parts).

Posted by Gilbert Travin (Member # 2138) on 03-16-2005, 10:29 AM:

OK Matthew !

It is very difficult to obtain 70 mm spares for P70 : these machines were often operated only in 35 mm mode. And if you find a set of accessories, you probably must adjust it on your projector. Good luck [Smile]

Posted by Matthew Taylor (Member # 2310) on 03-24-2005, 09:09 AM:

Just an update to this thread. I tried another one of these Elmo rectifiers and got exactly the same results. I'm not sure if there is some incompatibility problem or whether both have a diode gone in them.

Anyway, I obtained another old beast of a rectifier and it works perfectly now. 35mm on a budget [thumbsup]

Posted by Gordon McLeod (Member # 33) on 03-24-2005, 10:44 AM:

Years ago I remember haveing had an issue with an Atlantic AV rectifier from a 16mm conversion they had built and the filtering caps were in there lamphouse not in the rectifier box maybe that is the issue here since I don't have any experience with the ones you have