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Old 26th September 2004, 03:08 PM   #1
Tensop is offline Tensop  Lebanon
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Default quick! look, another dead car amp thread!! :D

hey all, heres the issue. the amplifier is a generic korean.

the PCB says the brand is BM Audio labs, but more specificly the casing says AVC.

its a 2x200 class AB amplifier using a (non feedback as far as i can tell) ka7500B SMPS driver, 6x 60n05 mosfets, a single core, twin primary, twin secondary transformer. the issue is it blows fuses. as soon as the amplifier is powered up it will blow a fuse. so far i've only tried it with a 20 amp fuse - but theres no way in hell one of these could draw that at idle. can anyone make any suggestions as to what is causing the large current draw? i've removed the rectifying diodes so the entire output(amplifier) stage so it's somewhere within the SMPS itself
any suggestions?
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Old 26th September 2004, 03:45 PM   #2
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Default idea...

Hi,

You probably already checked these trivial cases, but if it is a half bridge design any one of the SMPS output fets failing as a short will look like a dead short on the 12v DC input, the transformer has no DC resistance to speak of. If it is a full bridge design it takes two fets failing, on opposite sides of the transformer. There is normally a big diode connected across the 12v input. If the power is connected with the correct polarity it doesnt conduct and does nothing, if the power is reversed it acts as a short to blow the fuse and limit the reverse polarity voltage. Under these reverse polarity conditions if someone put in a large enough fuse and kept trying to connect the power backwards the diode could perhaps fail as a short...

Good luck

Stuart
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Old 26th September 2004, 08:04 PM   #3
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or amp amplifier output stage is shorted.
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Old 27th September 2004, 08:16 AM   #4
Tensop is offline Tensop  Lebanon
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the output stage is disconnected.
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Old 27th September 2004, 08:40 AM   #5
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I'd suggest checking the FET's.Most likely one (or more) is toast.
Also check the diodes on the output of the transformer (if they are still connected.) one of them might be shorted?
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Old 27th September 2004, 10:16 AM   #6
Tensop is offline Tensop  Lebanon
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yeah its the diodes on the output of the traffo i removed, as for the mosfets i tried the charge with a diode tester then short with a finger trick and they all responsed properly to it, so im now suspecting its the mosfet driver transistors or the SMPS controller.
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Old 27th September 2004, 07:52 PM   #7
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you could remove all the power mosfets and check the oscillator and drivers with an oscilloscope.
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Old 1st October 2004, 10:01 AM   #8
Tensop is offline Tensop  Lebanon
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i dont have a CRO on me, but im going to take a stab in the dark and replace the twin pair(bd139/140 equiv) mosfet drivers with a Ti 2812CN(P) mosfet driver, and replace the KA7500B with a TL494. the internal circuitry of both seem to be the same as is the output. i guess ill find out
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Old 1st October 2004, 10:02 AM   #9
Tensop is offline Tensop  Lebanon
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the real question is, there is 10-20 amps(depending on fuse rating) worth of current draw - i cant figure out where this massive amount of power is getting dissipated - if the mosfets were stuck on wouldent it they be dissipating in excess of 200 watts? they would explode at that amount of dissipation
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Old 1st October 2004, 11:43 AM   #10
Tensop is offline Tensop  Lebanon
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well, i replaced the SMPS and used my own mosfet drivers. there is power on the output of the SMPS(40-0-40 AC) however its still drawing large amounts of current.. I dont know what is causing this, is it possible for one side of the output stage to be dead, and the other active without causing voltage output problems?
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