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Really dumb question about fuses, ratings. Still blowing fuses

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fastline

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I am trying to test an amplifier board that keep blowing fuses. I thought I got the bad components out which was a bad mosfet. Resistance across the DC side was 0 ohms, now with the mosfet removed, it is up in the mohms. I figured I would power up the board to make sure nothing was going to blow, I was wrong.

The circuit uses a 2A slow blow glass fuse. I figured it was a shot to try a 1A fast act fuse just to power the front stage. Blew instantly. So I did not have a 2A slow blow so I tested with a 5A ATC automotive fuse (going for broke here). Still blew that! Took a couple seconds though and that is what concerns me that inrush is not the cause, but rather a possible STILL existing short somehow. I just do not get it. I test for resistance at the bridge rectifier and have very high resistance yet blowing a 5A fuse at 120V.

My question is would the 5A fuse give this problem if it is not rated for 120V? I realize that would not be a proper permanent solution but just for a minute unloaded, I figured it would work. I am still wondering if the inrush is an issue but I would sure think a 5A fast act would be similar to a 2A slow blow in terms of peak current/time.
 
What amplifier do you have in hand (brand, type number etc.?

Without having any schematics, it's impossible to tell what it may be. It could be a cap as well. Go ahead and search for shorted caps on the board. Just try to send a little current through them (just make it really simple with a small battery and a resistor). But you must still check the surrounding print layout to rule out any other components that may act like a short.

Also, try this over each transistors emitter/colector - source/drain.

A tip that may help: Often when transistors goes bad, they appear to be ok when doing diode tests on them, but when applying more power, they fails and short.
 
Do you think a bridge rectifier could do this? It was just off the board and diode tested with a DMM and flew like a bird but I did not power it up. Always just thought they were either good or bad but when bad, they would sure blow fuses.
 
It's VERY, VERY rare that only a single transistor fails - it's a domino effect, one goes and bits fail everywhere.

I would imagine the FET you replaced has probably blown again instantly.

You need to locate and replace ALL faulty components, before you ever switch it on.
 
I met the same situation whenever I tarried to repair Switch Mode Power Supply. I find it like what Nigel stated, not only one component that goes wrong.
 
What is this for and is it in spec?

I am still fighting a hard short in an audio amplifier board. Found a dead shorted mosfet which took me from a measured 0 ohm resistance on the DC buss to the mohms. Problem is I am still blowing the fuse. Just cannot figure this out.

The pic shows a coil that is being used on the AC input stage. Each side of it is in series with the bridge rect. I am at a loss to honestly know what it is and what it is doing. Does not seem to be a transformer. Each side measures 0 ohms. Just wondering it's function.
 

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He's also got the added complication of having put a 9A fuse in, so probably destroyed even more bits.

LOL. That's funny Nigel :D And so true.

As you know, I am in good old South Africa. We regularly get TV's in for repair that have been "attended to by mechanics". Like in TV mechanic........

No problem for these "mechanics" to insert a piece of Copper wire worthy of carrying around 30 Amps in place of the normal fuse rated @ around 3.15 A.

I fix them all though. At one helluva price to the the customer after quoting him basically the price of a new TV. He rather want's the thing fixed.....even though a BRAND NEW set will cost around R200 more.......complete with a years Warranty. We offer a One month warranty on spares only......

Only, and only in Africa can people be so fulking stupid.

No wonder this beautiful country is going down the drain.

Cheers
 
It's VERY, VERY rare that only a single transistor fails - it's a domino effect, one goes and bits fail everywhere.

I would imagine the FET you replaced has probably blown again instantly.

You need to locate and replace ALL faulty components, before you ever switch it on.


I have not replaced the fets that I removed. I have them but expected to see a circuit NOT blow the fuse. Now that I see that it does, even with the fets removed, I am further confused. I am combing this thing to every detail right now and just cannot get any component to test really bad with a DMM. I realize powering up can change things. There is obviously a very hard short somehow, some where. I was wondering about the transformer on here since all leads test 0 ohms to each other but Judging by wire size and windings, that is probably about right. I pulled the main cap off and tested it with a cap tester. all good. Also have a line of 5 transistors for the output stage and tested all of them for a short with nothing. I guess I am combing for low resistance or a shorted diode simply due to how it reacting with power. No worky is one thing but popping the fuse instantly sure makes me thing input AC side or right after the Bridge somewhere.

Again, can bridges diode test good, yet short out with power?
 
I have not replaced the fets that I removed. I have them but expected to see a circuit NOT blow the fuse. Now that I see that it does, even with the fets removed, I am further confused. I am combing this thing to every detail right now and just cannot get any component to test really bad with a DMM. I realize powering up can change things. There is obviously a very hard short somehow, some where. I was wondering about the transformer on here since all leads test 0 ohms to each other but Judging by wire size and windings, that is probably about right. I pulled the main cap off and tested it with a cap tester. all good. Also have a line of 5 transistors for the output stage and tested all of them for a short with nothing. I guess I am combing for low resistance or a shorted diode simply due to how it reacting with power. No worky is one thing but popping the fuse instantly sure makes me thing input AC side or right after the Bridge somewhere.

Again, can bridges diode test good, yet short out with power?



NO. Main Smoothing Caps yes. Not a Bridge Rectifier.
 
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In some expensive power supplies, They include VDR that suppose to open if voltage spikes occurs, It some times shorted instead.
 
Instead of a fuse, put a 200 watt lamp in series with the amplifier to limit the fault current.

While that reduces the current to about 2 amps at 110 Volts, you might be able to do some measurements without everything going up in smoke.

Check the VDR if fitted what aljam ri suggests, these do occasionally short out although they usually split open.
 
I am still fighting a hard short in an audio amplifier board. Found a dead shorted mosfet which took me from a measured 0 ohm resistance on the DC buss to the mohms. Problem is I am still blowing the fuse. Just cannot figure this out.

The pic shows a coil that is being used on the AC input stage. Each side of it is in series with the bridge rect. I am at a loss to honestly know what it is and what it is doing. Does not seem to be a transformer. Each side measures 0 ohms. Just wondering it's function.
The coil is part of a power filter circuit. I am confused! Is the fuse for the AC power input blowing, or is it between the power-supply and the amplifier??? Smps (switch mode power supplies) can be cery hard to fix!
 
The fuse it at the input for the board so there is no knowing at this point whether the PS or the output amp is causing the short.
 
An smps will shut down (and not blow the fuse) when the output is overloaded. The problem is probably with the smps. Did you check the power supply's mosfet, or the main filter capacitor?
 
Yes, the main cap was removed and tested out of circuit. There are 2 mosfets that run is parallel best I can tell. One was toast and other was good. I installed just the one for testing knowing that i would not put any load on it. It immediately blew it and it's driver resistor. I was suspecting something on the driver daughter board but have pulled the zeners from that board and all are testing fine though I just used a diode tester for them. I am definitely getting educated on power supplies on this one...
 
Did you check the rectifier diode on the tranny's secondary?

Note: it is a special kind of diode, called a schottky diode, and it has a lower voltage drop than, say, a 1N4001(regular diode),etc..
 
Ben, here is a couple pics. As well, there is 2 more here. https://www.electro-tech-online.com...d-audio-amp-where-should-i-be-looking.110285/

If you want the full rezzy pics, shoot me your email via PM. I very much agree with you that the driver or PS anyway just seems to be a problem and the fact that the mosefets toast instantly points me directly to the driver board BUT how the heck do those mosfets get controlled? By looking at the driver, they surely do not get switched by any type of IC.

If you look at the pics, you will see that I have removed the mosfets along with the heat sink. In the two recent ones, the driver board is out. There is another daughter board that has a glob of epoxy covering something so that might be the mystery IC that is running the show.... However, it does not appear that it has anything at all to do with the mosfet drivers.

On the mosfet driver board, You can clearly see that of the 5 diodes, 2 are marked "Z1, Z2", and the others are Ds. I will assume that only 2 are zeners and the others are ????

markings are
307
0
F4

I am not familiar with them. I am hoping I can find them bad.... Let me know what else you want/need.

I tested all the diodes with a DMM diode test. the "schottky" (??) diodes all test about .5V drop one way and OL the other so I am assuming they are not shorted out. As I understand, that is how you test a schottky, if that is what they are...


In the pics, you can see where the PS ends about where the large coil is and the output stage begins. Just not much going on in the PS and a bunch of stuff has already come of the board for testing. All the resistors, caps, etc have been tested. I did not remove the 2 large, fast acting diode but they test in circuit fine. Basically power comes in, makes its way to the bridge, then over to the mosfets and transformer coil and power cap. From there it goes out into the world. I have also removed several components on the output side for testing and everything is coming back fine.

IMO, I just cannot see a transient making it's way past everything in the PS and taking out the output stage. Amps commonly burn up from overdrive and that usually shorts the the output side. I think this one took a quick surge or possibly is just failed for no good reason. Only thing I have to go on is the PS mosfets are toasting and I do not even know how they work in this circuit....
 

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Also, before reinstalling the single mosfet for testing, I checked the board for power to the gates. both were around 150VDC. I realize this could just be "surface voltage" and would possibly drop once a small load was there and probably should have tested that... My thought was if there was excessive voltage to the drivers, that would certainly kill the mosfet and possibly short it, causing what we have here. Problem is I just cannot find a bad component driving the gates...
 
In a smps, there is always a feedback circuit that monitors the voltage on the secondary of the transformer. The problem MUST be there, since everything on the HOT (high voltage) side of the power supply is fine.

Usually, there is an optocoupler that isolates, and couples the feedback circuit to the oscillator/mosfet driver. As far as I can see, there is no optocoupler, am I correct?

Optocouplers normally are a 4 or 6 pin dip chip.

Were getting very close to figuring this out!

PS:thx for the pix, they always help:)
 
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