Fuse fire?

ACTSTech

Well-Known Member
So during a show, I noticed one of my old MAC 700s show a code, but I couldn’t read it from far away. Then it shut down and was out, no big deal.

Set up the scaffolding and crawled up, went to pull the fuse and it won’t come out. Opened up the unit and apparently the fuse burned out and so did the wires. Anyone seen this before? I’ll put in a new fuse unit, but I’ll have to get a new connector for the leads. Are standard spade connectors okay to use?

Any help is appreciated.
 

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SteveB

Well-Known Member
Yes, I had a 700 do exactly this, main fuse failed and fried the fuse holder. I know that LightParts in Austin, TX "should" have replacement parts. 2 step though, you need to replace fuse and holder, then make sure nothing failed on the motherboard. As note that I was never able to actually fix my unit as I was unable to purchase the parts, as they are considered "used' (from old units) and my purchasing system will not allow us to purchase used. Good luck with it.
 

RonHebbard

Well-Known Member
Premium Member

TimMc

Well-Known Member
Premium Member

ACTSTech

Well-Known Member
How can a fuse "fail"?

It's either good -- closed -- and has no potential difference from one side to the other... or it's open -- failed, and has live potential from one side to the other, but is passing no current.
That's why I asked the question. The instrument wasn't acting wonky or throwing any codes prior to the failure. It had been maintained and cleaned and inspected, everything looked fine. During act one of the worst Christmas show ever (That's not the official name of the show but it could be...) I saw the screen blinking a code, but like I said I was too far away to see what it read. There was no flash or smoke or anything, just the typical light drooping when the power is out.

When I went to pull the fuse, it wouldn't come out. The end inside the instrument (away from the panel) had melted. The element inside the glass was still intact, but the end capsule was blackened from the plastic melting. I'm guessing bad fuse because I saw no evidence of corrosion or dirt or anything on my 750 hour inspection when I relamped the fixture, but I'd never seen this before. And by bad fuse, I mean poor quality from some cheap vendor purchased and rebranded. At this point, I don't really know, I'm just crossing my fingers that the circuit board isn't fried as well.
 

SteveB

Well-Known Member
In my case the unit was working in a very hot enviroment, and we suspected that excess heat caused the fixture to overload and have it's main fuse blow, taking out the fuse holder in the process.
 

cbrandt

Well-Known Member
I've replaced that fuse holder on at least a dozen Mac 700s. My suspicion has always been that when the lamp gets older, it pulls harder to strike. You can see evidence of this when doing so on old lamps and old ballasts, that the screen will fritz out or blank out when striking. My assumption is that this repeated hard pull is what causes that, and other heat and overcurrent issues on the 700 line.
 

ACTSTech

Well-Known Member
I've replaced that fuse holder on at least a dozen Mac 700s. My suspicion has always been that when the lamp gets older, it pulls harder to strike. You can see evidence of this when doing so on old lamps and old ballasts, that the screen will fritz out or blank out when striking. My assumption is that this repeated hard pull is what causes that, and other heat and overcurrent issues on the 700 line.
Interesting. Brand new lamp (12 hours on it at time of failure), do you think the fuse might have looked good but I overlooked the internals? It definitely wasn't hot in the building and the unit wasn't confined.
 

TimMc

Well-Known Member
Premium Member
I've replaced that fuse holder on at least a dozen Mac 700s. My suspicion has always been that when the lamp gets older, it pulls harder to strike. You can see evidence of this when doing so on old lamps and old ballasts, that the screen will fritz out or blank out when striking. My assumption is that this repeated hard pull is what causes that, and other heat and overcurrent issues on the 700 line.
While that may be the case, look at the single power connector to the left side of the first picture. The discoloration and its placement kind of screams "resistance found here" at the point it joined the other terminal.

This is not the result of an instantaneous failure, as the fuse would have opened. This is heat from resistance, at a load insufficient to blow the fuse. The Connector Wizards will have to conjure the failure mode that created the resistance.
 

ACTSTech

Well-Known Member
While that may be the case, look at the single power connector to the left side of the first picture. The discoloration and its placement kind of screams "resistance found here" at the point it joined the other terminal.

This is not the result of an instantaneous failure, as the fuse would have opened. This is heat from resistance, at a load insufficient to blow the fuse. The Connector Wizards will have to conjure the failure mode that created the resistance.
I would much rather conjure new equipment and a working budget...

Thanks for all the input everyone, gives me a reason to open up all the equipment again and tinker.
 

Lasermike

Active Member
I too think it looks like a poor connection that lead to thermal runaway but I thought I'd point out that fuses have a voltage rating along with the current rating. You can get glass fuses that all look the same and have, say a 5 amp rating but the voltage could be 32 or 125 or 250. Too low a rating and it may not be able to extinguish an arc if one forms as the fuse blows. And then there's the curve, is it slow blow, regular, very fast AKA rectifier or some other curve? Not saying it's the case here but always replace fuses with the same rating and type as that supplied by the manufacturer.

Michael
 

almorton

Well-Known Member
I always thought thermal runaway was what happened in amplifiers, particularly those using germanium transistors, where the collector current increases with temperature, which leads to an increase in temperature, which leads to an increase in current, which leads ...

That looks like what I'd called thermal stress damage.
 

FMEng

Well-Known Member
Fight Leukemia
Buy your fuses from reputable electronic parts distributors. Amazon has sold dangerous fuses that don't perform anything like their ratings. I stick to Newark, Mouser, Allied, and Digi-Key.
 

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