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Old 22nd July 2011, 12:30 AM   #1
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Default JVC Shelf Component System Power supply

I have a 1995 JVC MX-C550 Mini Shelf system that is blowing fuses...this is my first post here...
I have some experience repairing circuit boards/power supplies but not for many years...
I just returned home to find he the JVC didn't power up (it was off), when I removed the case I saw that the single 2A/125V fuse had blown...I replaced it with a 5A/250V fuse that I had (only mini in my supply) and it blew instantly...feeling that I had nothing else to lose before ordering the 2A/125V I bridged the fuse with tin foil....now it powers up normally but I don't have the speakers attached yet (I'm not leaving it on for more than two seconds)....anyway, what do you think the primary component on the PS would cause this??? Faulty capacitor??? shorted speakers??? Any help would be appreciated...I've gotten attached to this system in my shop....many thanks in advance for any help, Jim
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Old 22nd July 2011, 05:29 AM   #2
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You'll have to isolate and test each component separately, but an offhand guess is that one of the output transistors has failed to a short condition.

Desolder the output transistors then see if the circuit still blows fuses, or of course test them, a google search will show you how.

Before attaching speakers, measure for DC offset. Granted, it is over 15 years old, also inspect capacitors for signs of failure but if it draws enough current to blow a fuse right away, it's probably not a capacitor unless you see one that has exploded.
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Old 22nd July 2011, 07:07 AM   #3
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So it blew the fuse...

I think a little warning and instruction is called for here. To short out any fuse or overide any safety device is highly highly dangerous

So you fitted a bigger fuse and that blew instantly...

At this point I think most would realise something wasn't quite right............

but to then go on and short the fuse out with tin foil is highly dangerous, the fuse is there for a reason... to protect you, to protect others, to protect your property.

You see somebody somewhere will read this and do as you have done... and their problem might be a live to chassis fault... and shorting the fuse may "blow" some poorly wired earth system/wires etc apparently "fixing" their problem but leaving the unit live... and they or someone else may not live to tell the tale.

It happens.

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So that said...
When working on any fault like this you should use a "bulb tester" (search the forums for what that is ) as that will limit any current and prevent major damage to components... and you !

You haven't specifically mentioned, is this fuse on the PRIMARY side of the mains transformer or the SECONDARY side ?

As ! mentions... first suspect would be an output fault. Maybe it uses as STK type module, an IC or maybe it's discrete.
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Old 22nd July 2011, 04:54 PM   #4
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Thanks for the help...I love my house too much to leave the foil in permanently - I just wanted to verify if it was something more than replacing the fuse....I not sure which side of the power supply the fuse in on....it's on the top of the circuit board as a single fuse...there are two additional fuses on the left side as well which are OK...the power supply is pretty easy to remove so I'll start there with the output transistors....any additional suggestions would be appreciated since as I said I haven't done electronics work in 15 years....thanks again, Jim
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Old 22nd July 2011, 05:30 PM   #5
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Hi Jim... you could perhaps post some detailed photos of the internals. It may or may not help... but at least we see what it's like.

If the fuse in the primary side there will be no continuity between live and neutral with it removed.
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Old 22nd July 2011, 06:10 PM   #6
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attached photos of JVC Power supply
Attached Images
File Type: jpg JVC power supply 001.jpg (447.0 KB, 71 views)
File Type: jpg JVC power supply 002.jpg (378.2 KB, 61 views)
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Old 22nd July 2011, 06:12 PM   #7
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Also, I connected speakers and fired up the system with the foil fuse.....works fine...left on for about an hour...
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Old 22nd July 2011, 06:32 PM   #8
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Thanks for the pics. It certainly appears to be on the primary side. I assume it goes to the black mains lead.

Was the fuse you fitted that blew an "anti surge" or time lagg "T" rated ?
For a transformer primary it should be. If it works it works... so get the correct fuse. Also worth checking the security of any mains lead/plugs etc that there are no loose screws or dicky sockets. Anything like that can cause random fuse blowing. Check the tranny connections to the PCB too that the soldering is good.
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Old 22nd July 2011, 06:38 PM   #9
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The fuse was just a Littlefuse 2a/125v mini fuse, no slo-blow or t...
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Old 22nd July 2011, 07:00 PM   #10
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Well I'm not going to go against any manufacturers specified fuses.

All I can say is that for any power transformer such as this it's normal to fit a time delay fuse. You can tell these sometimes by looking at the wire inside. Quick blow ones are just a thin single wire of appropriate size. Time delay types are often coiled or wavy.
Transformers draw a large current at switch on (or if there is a break/interuption) in the mains. Against all logic the current is actually greatest when the mains happens to be at the zero crossing point in the cycle when power is applied due to the inductance and residual magnetism of the core.

As a service engineer I have come across fuse blowing like this on countless occasions... sometimes the manufacturer alters the spec of fuse that should be fitted.

Fit what came out for continued protection.
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