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Old 13th June 2009, 03:27 PM   #1
hew128 is offline hew128  Singapore
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Question Blown fuse when power on without connections internally

Hi,

I have a DAC and was working fine when I was using it last night - abt 30min. This morning used it for about a few minutes then no power to the unit.

Checking it, found a blown mains fuse. Other internal fuses were ok. So I replaced the mains fuse - 250V 0.5A and it blew again.

So I disconnected the power cables from the transformer to the internal PCB(s), replaced the fuse and tried powering on again. Some results - the main fuse blown upon pressing the power on button. Checked the fuse using a multimeter beforehand and it was ok before pressing the power on button. The fuse was blown after pressing the power on button.

Does this mean the fault is in the transformer?

Thank you in-advance for any response on the matter!
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Old 13th June 2009, 03:46 PM   #2
mjf is offline mjf  Austria
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hello.
can you disconnect the mainscable at the transformer (primary)?
.........and try it again.......if the fuse is not blown it should be a bad transformer...............
greetings........
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Old 14th June 2009, 09:16 AM   #3
dddac is offline dddac  Germany
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just small experiment, try a 1 and 2 amp fuse and see what happens. May be inrush current or DC component on mains is causing an issue. You do not tell how large the transformer is, so hard to guess, but you might give it a try

Did you measure any current? Like is it a real hard short or mis design or some error causing to high current draw?
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Old 19th June 2009, 04:51 PM   #4
Mooly is offline Mooly  United Kingdom
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Mains input filter cap would be prime suspect !
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Old 10th August 2009, 06:03 AM   #5
hew128 is offline hew128  Singapore
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Hi everyone,

Finally managed to get some time and a digital multimeter to perform measuresments on the khaki coloured capacitor (looks like one), labelled "DH11" on the board with markings 103N74 and what looks like a triangle followed by letters "NS" on it (not able to Google information on it) on a different line.

Performing a voltage (in 200mV range) test on the legs of GH11. When I touch one of the leg of GH11 with say the black multimeter needle, the reading goes up to a certain value. When the other multimeter needle touches the other leg of the capacitor, the value goes up a bit further? When I perform the same test on other capacitors, the reading goes down to zero when the 2nd multimeter needle completes the circuit.

Using the resistance mode (all settings) and when switch to 'sine wave' mode, the multimeter reading shows as infinite on GH11.

Does this mean Mooly is onto something?

Thank you again in-advance to everyone who has responded to my request for assistance!
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Old 10th August 2009, 11:46 AM   #6
Mooly is offline Mooly  United Kingdom
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Firstly you can only test the caps out of circuit.
In practice what happens is one of a number of things,
1. The cap "may" genuinely be faulty but they fail intermitently. Have you heard of self healing caps. It's a common term, if a breakdown occurs internally the "short" actually vaporises and clears.
2. More common, the manufacturer gets it wrong. This happens countless times on various equipment. A transient mains spike occurs and the the fuse blows. Is it an anti surge fuse ? or "T" rating.
I cannot go against recommended values but it does happen. A well known make of VCR had this problem and the fuse was around 315ma from memory and eventually the official mod was to increase to a time delay T1.6 amp.
I would either replace the cap if it happens again, and if it still does it look to altering the fuse first making sure no dry joints or poor connection exist in the mains input supply side as this kind of thing causes this anyway.
Google and read up about Class X and Class Y capacitors.
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Old 10th August 2009, 02:17 PM   #7
hew128 is offline hew128  Singapore
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Hi Mooly,

I managed to get some extra fuses and decided to do elimination.

It was not the cap, and, it was the second of the three transformers. Hence now I have to source a replacement transformer.

Thanks!
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Old 10th August 2009, 03:58 PM   #8
Mooly is offline Mooly  United Kingdom
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Hi, didn't read back to your first post that it blows straight away. It was a while ago
Does sound like the tranny especially as you mention disconnecting the secondaries and it still blows the fuse. Not common though.
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Old 11th August 2009, 01:22 AM   #9
hew128 is offline hew128  Singapore
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Hi Mooly,

No worries. Had a similar experience with a Carver power amp many years ago.

Thanks!
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Old 30th August 2009, 04:18 AM   #10
hew128 is offline hew128  Singapore
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Hi everyone,

Confirmed it's a bad transformer.

Special thanks to "Mooly" for his guidance with the transformer replacement.

My Marantz CDA94 is now working like a treat again!!!
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