Prospot 250 lx capacitor goes boom

[millamber]

I was on a service call yesterday to address a prospot 250 lx. The client told me he thought the lamp had exploded in the fixture. During a service, they "heard a pop and then the light went out". I opened the head of the fixture to sweep out the broken lamp pieces but found the lamp intact and the head fairly clean. So I pulled down the fixture to see what the problem was.

It was about this time the client told me that after they heard the popping sound, there was an odor of melting wires and when he said the light went out he meant the fixture didn't power up at all. I pulled open the cover on the transformer/ballast/capacitor side and found the below images.

 

[Chris15]

That, folks, is what they call a catastrophic failure

 

[carllib]

Faulty construction or ??

 

[Lambda]

Now the real question is, was the capacitor defective, or did some other problem cause it to be pushed way past its safe operating voltage?

 

[jonliles]

What Venue in the ATL?

 

[JD]

A Poly cap none-the-less! They were suppose to be the answer to exploding oil caps! Guess not. They are also suppose to have internal protection (fuse/melt link.) Guess not again!

 

[kiwitechgirl]

Had a Mac600 capacitor do exactly the same thing - except that when it exploded it set the wiring on fire. My chief LX emptied a powder extinguisher into the thing to put it out. It got sent back to the Martin Oceania office, and apparently they sent it straight back to Denmark because no-one had ever seen one do that before - the hire company got a replacement unit from Martin for free...

 

[DrPinto]

I don't know if this is relevant, but...

Soon after the conversion from analog to digital TV, I found myself repairing quite a few of the Apex DT-250 converter boxes. Most seemed to fail after only a year. The problem was always a bad electrolytic capacitor. And not always the same one. Some would fail on the video board while some would fail on the power supply board. While researching the problem, I came upon information on something called "Capacitor Plague". Here's some info on this:

Capacitor plague - Wikipedia, the free encyclopedia

I wonder if this has anything to do with the failing caps on these instruments.

 

[JD]

I don't know if this is relevant, but...

Soon after the conversion from analog to digital TV, I found myself repairing quite a few of the Apex DT-250 converter boxes. Most seemed to fail after only a year. The problem was always a bad electrolytic capacitor. And not always the same one. Some would fail on the video board while some would fail on the power supply board. While researching the problem, I came upon information on something called "Capacitor Plague". Here's some info on this:

Capacitor plague - Wikipedia, the free encyclopedia

I wonder if this has anything to do with the failing caps on these instruments.

Poly caps used in ballast circuits do not contain the electrolyte chemical that electrolytic capacitors do. Basically, a poly cap is just foil and a thin sheet of plastic. This is due to the fact that Electrolytic capacitors are DC, with a distinct + and -, and poly caps are AC. (Although some have taken electrolytic caps and put them back-to-back and called them AC, crossover networks come to mind.)

Because the electrolytic cap cheats a bit by using the electrolyte, they can be DRASTICALLY smaller in size than a poly cap for a given value. It is almost a half breed between a cap and a battery, but lacks the chemical change a battery uses to generate a voltage. In this case, the fluid is used to exaggerate the surface area!

I am almost surprised that the wiki article did not touch upon "Fish Caps." These were so bad they drove me into retirement! First introduced as "surface mounted electrolytic capacitors" in the late 80s, these were suppose to be "environmentally friendly" as they used an organic fish oil as an electrolyte. Unfortunately, the oil would go rancid. They are still in use, but apparently they have the rancid fish oil under control.

 

[DrPinto]

Poly caps used in ballast circuits do not contain the electrolyte chemical that electrolytic capacitors do. Basically, a poly cap is just foil and a thin sheet of plastic. This is due to the fact that Electrolytic capacitors are DC, with a distinct + and -, and poly caps are AC. (Although some have taken electrolytic caps and put them back-to-back and called them AC, crossover networks come to mind.)

Because the electrolytic cap cheats a bit by using the electrolyte, they can be DRASTICALLY smaller in size than a poly cap for a given value. It is almost a half breed between a cap and a battery, but lacks the chemical change a battery uses to generate a voltage. In this case, the fluid is used to exaggerate the surface area!

I am almost surprised that the wiki article did not touch upon "Fish Caps." These were so bad they drove me into retirement! First introduced as "surface mounted electrolytic capacitors" in the late 80s, these were suppose to be "environmentally friendly" as they used an organic fish oil as an electrolyte. Unfortunately, the oil would go rancid. They are still in use, but apparently they have the rancid fish oil under control.

Fascinating. I thought the failed caps were electrolytic. And the "Fish Caps" are a new one for me. You learn something new every day around here...

 

[JD]

Fascinating. I thought the failed caps were electrolytic. And the "Fish Caps" are a new one for me. You learn something new every day around here...

When you google "Fish Caps" you find a lot of hats fishermen wear! So, here is what they look like:

Those that were used from the mid 80's to about the year 2000 are trouble! I am hoping they improved because they would not only go bad, the juice would destroy the surrounding PCB. Some camcorders had over 100, so how could you morally charge someone to replace a few when you knew the rest were not far behind!

This guy shows the damage done to a board. (Although I would see them fail at more like 5 years.) http://www.repeater-builder.com/motorola/spectra/spectra-caps.html

 

[OTO1702]

Hi!

Been a long time since I posted something !

This type of capacitors if I remember are what we call Dry Type capacitors, they do not use any oil. These caps normally have a thermal/pressure switch inside that pop under certain conditions. The picture show that the cap internal protection never went active, so heat create the mess, then a small blast occured I think. This is typical in Power Factor Correction in large circuits.
Usually the capacitors faillures in this situation are caused by voltage notchs, short circuit or resonnance. You need to make sure that the power quality in this place is ok because you can certainly have an other major faillure again. Also verify that the transformers in the other lamps are still giving the right values.

Othmane

 

[Mike9]

I've serviced tube amps with cap failure, but never saw anything like that. Thanks for the thread.

 

[Aman121]

Yikes! Ive seen a computer power supply with exploded capacitors like that. I am now glancing over in fear at my crown amps with the huge filter cans on the back.....hope those dont decide to go boom.

 

[JD]

Yikes! Ive seen a computer power supply with exploded capacitors like that. I am now glancing over in fear at my crown amps with the huge filter cans on the back.....hope those dont decide to go boom.

Like the DC300's with the twin bombs on the back? Ha! Actually, a good electrolytic will last a long time. (Got some 30 year old Crowns myself.) As long as they run every now and then. Having voltage on them stops the decay. Usually, if a cap fails in a few years, there was something wrong from word go. The reason an electrolytic will blow is different then why a paper or poly cap will fail. Roll out a 20mfd poly cap and you have about 60 feet of very thin plastic with a foil coating on either side. All you need is a small crack in that plastic film and an arc-out starts. Usually, this will pop the internal fuse. If it doesn't have one, then the arcing will be amplified by the discharging/recharging of the film in the cap. The results are very pyrotechnical as long as voltage is present.

Electrolytic capacitors fail when they develop leakage hot-spots and boil the electrolyte until you have a good of fashion "boiler plate" explosion, usually resulting on the contents spewing all over the place. That why these caps usually have a designed weak spot at the top to channel the blow-out.

By the way, those caps used on the Crowns have a small rubber blow-out plug to release pressure should a problem occur, thus preventing "flying amplifier" syndrome.

 

[Aman121]

Like the DC300's with the twin bombs on the back? Ha! Actually, a good electrolytic will last a long time. (Got some 30 year old Crowns myself.) As long as they run every now and then. Having voltage on them stops the decay. Usually, if a cap fails in a few years, there was something wrong from word go. The reason an electrolytic will blow is different then why a paper or poly cap will fail. Roll out a 20mfd poly cap and you have about 60 feet of very thin plastic with a foil coating on either side. All you need is a small crack in that plastic film and an arc-out starts. Usually, this will pop the internal fuse. If it doesn't have one, then the arcing will be amplified by the discharging/recharging of the film in the cap. The results are very pyrotechnical as long as voltage is present.

Electrolytic capacitors fail when they develop leakage hot-spots and boil the electrolyte until you have a good of fashion "boiler plate" explosion, usually resulting on the contents spewing all over the place. That why these caps usually have a designed weak spot at the top to channel the blow-out.

By the way, those caps used on the Crowns have a small rubber blow-out plug to release pressure should a problem occur, thus preventing "flying amplifier" syndrome.

Yep 2 dc300s, some of the earliest production runs. (silver plate, big black knobs) Thats good to hear about the safety plug, those amps are mostly original and ive had some minor issues with them, but they sound epic with my old Altec bass cabs and horns. Your the guy with the shed of Altec right? Good stuff.....

 

[JD]

Yep 2 dc300s, some of the earliest production runs. (silver plate, big black knobs) Thats good to hear about the safety plug, those amps are mostly original and ive had some minor issues with them, but they sound epic with my old Altec bass cabs and horns. Your the guy with the shed of Altec right? Good stuff.....

Not so much a "shed of Altec" as much as "some Altec back in the shed!" Great sounding stuff. Always loved the DC300. Only problems I saw were a couple of transistors on the drive board that would get ring cracks where they were soldered in due to vibration. Sometimes a noisy op amp. Some were in sockets and just had to be reseated. Hey, at 30 years it still works!

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