12 volts drops to 6 when amp is powerd on - diyAudio
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Old 17th January 2009, 09:26 AM   #1
DAKOS is offline DAKOS  Canada
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Default 12 volts drops to 6 when amp is powerd on

hello,
i've picked up a defective amp to fix it and i've got in to this issues that i don't manage to fix it
it's a classic amp ( about 800w rms bridged) build on TL494 and irfz 44n (8 of them for each channel - 2 total) ... every time i'm powering on the amp the, voltage drops to 6 volts and 4 of the irfzs - right side- are heating up...i've have replaced them with another 4 new ones but still same result....i don't have the schematics for the amp and i've tried to do a revers engineering for the pcb but is kind of hard to follow up the circuit
.any suggestion will be more then welcomed!
thanks in advance
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Old 17th January 2009, 12:37 PM   #2
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Check the 4 gate resistors of those FETs. Also between the gate resistors and the TL494, there is possibly a defective PNP transistor. If there are 2 transistors between the resistors and the 494 they both may be defective.

Don't power it back up until you replace the transistor or resistors. You may want to put an automotive headlight bulb in series with your powere wire to act as a current limiting resistor.
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Old 17th January 2009, 04:31 PM   #3
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Try to repleace new TL494 is small cost.Or measure them by osciloscope..
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Old 17th January 2009, 05:00 PM   #4
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Maker's name and model?

Have you looked for a schematic?

w
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Old 17th January 2009, 11:46 PM   #5
DAKOS is offline DAKOS  Canada
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hi,
the amp is a crunch gp1600.1 from ground pounder?
i did test it without the TL circuit and it did the same..
here is a picture of the inside of the amp, only the first 4 irfs on the left side up are heating up...
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File Type: jpg amp zout11.jpg (75.6 KB, 235 views)
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Old 18th January 2009, 12:37 PM   #6
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Can't find a schematic.

A picture is pretty much useless at that level of detail, but one thing I can tell, the layout is symmetrical.

If you go through the circuit with a DVM testing resistances and doing diode tests in both directions across the transistor pins, you should be able to identify the problem area by comparison with the side that is OK.

w
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Old 18th January 2009, 02:24 PM   #7
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Start by figuring out which of the 4 transistors are connected to the 4 gate resistors of the set of FETs that are getting hot. There are now 2 yellow rings in this picture. If one of the 4 transistors inside the rings connect to the gate resistors of that set. Change it. Did you check the gate resistors? The yellow rings are located to the west and to the south of the 494.
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File Type: jpg amp zout11.jpg (92.4 KB, 203 views)
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Old 18th January 2009, 09:06 PM   #8
DAKOS is offline DAKOS  Canada
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...sorry for late replays but since i'm new to this forum they have to check my messages every time i'm posting a replay..
k..here what i did by now: i've used a 12v bulb in series with the +12 , that was a great idea , the IRFs are not heating up that much..and...what i've noticed is that when i'm conecting the + wire the light on the bulb is fade( the voltage drops to 7V ) and after about 8-10 secs the toroid on the side that the IRFs are heating starts to make a zooming noise and after the 10 secs the voltage drops one more time to 3 volts and the light on the bulb gets brighter!!
can the toroid be broken or anything else??
i'm trying now to figure out the path for the +12 on the back of the PCB and see if there is no shortage or anything...
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Old 19th January 2009, 02:13 AM   #9
DAKOS is offline DAKOS  Canada
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....i've checked all transistors on the board and they are ok...
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Old 19th January 2009, 02:29 AM   #10
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The transistor will not be shorted, it may be possible to have it slide by the untrained eye. Did you identify the one that is connected to the gate resistors of the set of FETs that are getting hot?
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