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Old 25th December 2005, 07:44 AM   #1
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Default strange amp problem!!!

hi there
i have a 2 channel car amp(110WrmsX2) that i've been using in bridged mode as a subwoofer amp for a long time now with no trouble at all but suddenly it started acting strange

it turns on fine and sounds perfect at low levels but when i raise the volume it cuts and goes into protection and then it tries to come back again and then cuts..... and so on unless i reduce the volume then it plays normally !!!

it overheats with the load connected even with no input! but if i disconnect the sub it doesn't heat at all .

i took it apart and removed the heat sink and power it up for 5 minutes with no load and no transistors(power or drivers) got even warm!!!! so i removed all power transistors and mosfets and even the power supply's diodes and measured them with my multimeter with no sign of failure for any of them,and the pcb looks clean with no sign of overheating or cracks,and there are no pots at all to set the bias .

i measured the power supply which seems to be ok at +/- 42 volts.

i noticed that one of the channels output is -8 volts dc with no input or load connected (that's a huge offset)!! the other channel is fine.
what could be the cause of this problem??
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Old 25th December 2005, 08:48 AM   #2
AndrewT is online now AndrewT  Scotland
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Hi,
-8Vdc offset is enormous. Sounds like something has broken, it's just the smoke hasn't escaped yet.

-8vdc through a 4ohm speaker will be pulling 2A through the negative transistor of the output stage as well. Both the amp and speaker will overheat. Disconnecting the load and running it open circuit stops the current flow and apparently cures the overheating.
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Old 25th December 2005, 08:57 AM   #3
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Its most likely one of the driver transistors or one of the predriver transistors that have started to give up.
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Old 25th December 2005, 09:03 AM   #4
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Probably dry or leak electrolythic caps in feedback...
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Old 25th December 2005, 02:12 PM   #5
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thnx for all the replies.

i've changed the output power transistors with no change at all, is it possible for one of the driver transistors to do this while still giving a good reading on the multimeter , i thought it could only be good or bad but no (in-between) states!!

do i have to take all drivers and predrivers out to test??

i'm also confused because the amp works at low power level (20watts) and then it cuts when bushed harder , i've fixed many amps before with blown power stages but i never faced that situation

widomaker , how could i spot the dry or leaky elctrolytic ??
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Old 25th December 2005, 02:23 PM   #6
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Sometimes bad caps have grown thicker, or leaked onto the board, but this can be hard to see as they often have glue around them on the board.
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Old 25th December 2005, 03:13 PM   #7
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well i've looked alot at the pcb and i can't find any sign of oil leaking from a cap , and by the way the caps arn't glued to the board...
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Old 26th December 2005, 01:16 AM   #8
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c'mon guys i need ur help , i have another amp but this one means alot to me
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Old 26th December 2005, 01:26 AM   #9
anatech is offline anatech  Canada
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Hi Moonchild,
Look for cracked solder joints near the heavier components on the board. A capacitor may leak a little directly underneath the cap. You may not see it. Look in the voltage amp section (you didn't have high current draw with the load disconnected).

-Chris
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Old 26th December 2005, 01:53 AM   #10
Eva is offline Eva  Spain
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These failures are quite common and are usually due to cold solder joints that end up losing contact due to car vibrations. Don't bother too much with the output stage, the problem is probably in the input stage or the VAS.
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