JVC Shelf Component System Power supply

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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
 
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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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.
 
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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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.
 
attached photos of JVC Power supply
 

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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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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.
 
When I examine the board where the main power leads attach, all I see are the transformers, two resistors and the three fuses...the main cable that goes from the power supply to a much larger board looks to have capacitors and power diodes, etc.
Unlike a modern PC which has an individual, all in one power supply is it possible for the JVC to have the power supply divided up into more than one area????thanks for being patient with me...

see attached photo, power supply is on the left...
 

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I'm refering to the transformer (tranny).

Your photo above show what would normally be called the "power supply" (PSU). This is all fed from the low voltage secondary side of the transformer and has no direct connection to the mains.

In a unit like this it's common for the transformer to have several secondary windings that are used to provide different voltage rails within the equipment.

If you look at the board with the three fuses on it I think you will find that all the connections on the "top" of the transformer are on the primary or live side. This will connect via the fuse that has blown to the mains lead.

All the connections at the bottom of the transformer are the secondary windings that feed into the unit. I think you will find that the two other fuses connect to some of these windings.

Transformers usually have primary and secondary windings (the pins or leads) at opposing ends. They are never mixed together.

So the power supply in this unit in a way is "divided up". There is the main rails that feed the power amp stages and so on. This will be the windings that feed those four large diodes and two large capacitors. There are also other supplies though... look at all those smaller diodes and te smaller capacitors in the PSU. They will be lower voltage rails that feed the system control/standby circuitry etc and are derived from other windings on the transformer.
 
Mooly, thanks so much for all the information and help....In your opinion, if I find all the sockets, leads etc. OK then could the problem be on the main board with those diodes and capacitors? I know it's hard to say (I'm sure it could be anywhere) but what would be the next best troubleshoot....Many thanks again for all the help....Jim
 
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Normally a fault on the secondary side would not blow the primary fuse unless it were something like a dead short or shorted turns on the transformer windings.

The fuse blowing could have been something like a glitch in the mains supply... it happens.

You went on to replace the fuse... it blew. But you then went on to short the fuse out and found it all works normally so that really points to the fact the replacement fuse wasn't correct in some way. If the unit were drawing excess current something would be getting very hot.

If the unit had been genuinely faulty you would have known about it... with a shorted fuse enough current would have continued to flow to literally make the fault evident... smoke/heat etc

All you can do is check the AC supply as best you can. Wall sockets/plugs fuse holders etc. Replace the fuse with the correct one and see if it happens again.
 
Thanks again....left the unit on all morning and periodically checked the internal temps....seem all normal - 90's to the high on the right side transformer at 113 degrees Farenheit....(ambient temp now 86 degrees F)When I go to the city next I'll pick up the 2a/125V mini fuse....I'll post back my findings when I install it....thanks again for all the help and have a great day...Jim
 
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