8mm Forum: Lamp query

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Posted by Mike Peckham (Member # 16) on July 13, 2005, 03:55 AM:

I recently replaced a blown lamp in one of my Sankyo projectors only to find that it still didn’t light up. The lamp was new but as I had three spares I tried another, still no joy so I started down the route of checking firstly the connection at the lamp holder and then getting the meter out and checking for current.

I checked right through the machine from the transformer through the switch and on to the lamp holder. Eventually I pulled the lamp holder away from the lamp just enough to get the meter prongs on the pins, there I found a healthy 13volts.

I then checked the lamp only to find it was almost, but not quite open circuit. The other new spare was the same. I then tried the third to find that that was ok and popped it in and Hey presto!

So what are the chances of that? These are admittedly old lamps but never used, visually they appear perfect, the filament shows no sign of damage or use and the reflectors are bright and clear. They were all three in their original Phillips boxes.

Is it possible that some sort of oxidation or other ageing can occur to a projector lamp when it is left unused for a number of years or were these from a long since forgotten faulty batch?

I’m still scratching my head…

Mike [Cool]

Posted by Jan Bister (Member # 332) on July 13, 2005, 08:42 AM:

Maybe they had been shaken and rattled so much over the years that the filaments simply broke?

Posted by Winbert Hutahaean (Member # 58) on July 14, 2005, 08:15 AM:

In the country that so humid like Fiji, oxidation always happen to the long unused bulbs, no matter they are in the box or installed. What I always do to solve the problem is to clean the metal part (+/- poles) using a sand paper.

I am now asking, it is OK to check the bulb by using AV Meter? Somebody says that since AV Meter has a current (despite it is very small) then it can harm the fillamen. It is true?

Thanks.

Posted by David Pannell (Member # 300) on July 14, 2005, 08:58 AM:

Hi,

Not true!!

Any simple continuity meter is absolutely fine for checking lamps. The current is so small that there is no way it can damage the lamp.

As for a damaged filament - hardly. If the filament is broken a test will show it as open circuit, not partially so. A lamp filament will either be healthy or not - nothing in between.

I would go for Winbert's diagnosis 110%. Oxidisation - or rather dirt/contamination on the pins. However, I don't think I would use glass, sand or emery paper for cleaning, as often the pins are silver plated, and this would remove the plating. I would go for an electrical solvent switch or contact cleaner, such as Super Servisol or similar. Silver plated pins are used due to the excellent low contact resistance which it provides, thus helping to keep the lampholder cool whilst transferring maximum power to the lamp.

BTW, - a fine point here, - silver oxide is a conductor of electricity, so it's not the actual oxidisation of the pins causing the problem, but the atmospheric dirt and grime over the years which attaches itself to the silver oxide.

Dave.

Posted by Mike Peckham (Member # 16) on July 14, 2005, 03:40 PM:

Thanks for the input chaps.

Jan the filaments don't seem to be damaged, although it's not possible to examine them to closely they look perfectly intact.

As regards oxidation on the pins, the confusing thing is that when the lamp was in the projector I pulled the lamp connector away from the base just enough to get the meter prongs on the pins and it registered 13 volts across the pins, this would suggest that the lamp is receiving power, wouldn't it? But the lamp itself wouldn't light and when I checked it seperately with the meter it appeared to be [almost] completely open circuit.

I can only think that either they were somehow faulty from new or, there has been some sort of chemical reaction over the years within the lamps that has corroded the pins, perhaps within the lamp base?

At the end of the day it really doesn't matter because I shall just throw them away and get some new spares in stock, but I am curious as to why two unused bulbs which visually appear sound won't work...

One of those puzzling dilemmas that make the crazy hobby of collecting cine films all the more interesting [Smile]

.

Mike

Posted by Tony Milman (Member # 7) on July 14, 2005, 04:56 PM:

Mike

But I assume you won't throw away the boxes [Wink]

Posted by Mike Peckham (Member # 16) on July 15, 2005, 01:08 AM:

Tony,

You know me too well, the lamps are in their boxes sitting on the edge of the kitchen worktop next to the bin,but I haven't yet had the courage to open the lid and put them in.

Even if they don't work, they look so nice sitting on the shelf in my cine cupboard [Roll Eyes]

.

Mike

Posted by David Pannell (Member # 300) on July 15, 2005, 01:31 AM:

From what you have said, Mike, it certainly DOES sound like faulty manufacture - especially as you are getting NEARLY, BUT NOT QUITE an open circuit reading across the filaments - AND there is voltage on the actual pins.

Well diagnosed! 10/10

Dave.

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