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Old 25th March 2008, 08:29 AM   #1
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Default audiobahn amp A1500HCT

i have a a1500hct amp a couple of the fqa85NO6 are blown should i have to worry about anything else or just replace these.i am having trouble finding them are there any other ones i can use?
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Old 25th March 2008, 08:55 AM   #2
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IRF3205s should be a good substitute. If the gate resistors are 100 ohms, replace them with 47 ohm resistors if you use the 3205s. If they're 47 ohms or less, they don't need to be replaced.

If the gate resistors are not going to be replaced, check each one (with the FETs out of the circuit) to make sure each one is within tolerance.

The driver transistors may have also failed.

If you can post a photo of the power supply drive section, it would be helpful if further help is needed. If you can take high resolution photos, zip them and email me a set (only if additional help is needed).

babin_perry@yahoo.com
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Old 25th March 2008, 09:42 AM   #3
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how do i tell if the driver transistors have failed?
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Old 25th March 2008, 10:25 AM   #4
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You can measure across each terminal of the driver transistor. If it's shorted you read zero ohms or close to it. A better way would be to remove them from the board and measure each transistor out of the circuit.

I measure each transistor and compare the measurements with the transistors in the other channel. It's usually fairly obvious when one driver transistor is out of whack.
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Old 25th March 2008, 10:54 AM   #5
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If you have an oscilloscope, you need to confirm that the drive signal swings cleanly from ground to ~10v.

If you only have a multimeter, you should set it to DC volts and measure the drive signal voltage. It should read ~5-6 volts DC. Do this with the black lead on the chassis ground and the red lead on the gate pads for the FETs (leg 1 of the FETs). Do this for all of the FET gates. If you read approximately the same voltage for all of the gates, the drive circuit is probably OK.

Check the drive circuit after all of the original/blown FETs have been removed.
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Old 27th March 2008, 02:48 AM   #6
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do you where i can get a schematic for this amp?thanks
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Old 27th March 2008, 02:48 AM   #7
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my gate resistors colors are red brown and gold so is that a 2.1ohm.
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Old 27th March 2008, 03:40 AM   #8
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That's probably not correct. There should be another band on the resistor.

No schematics are available (they haven't been available when I've tried to get them from Audiobahn).

Photos would be helpful.
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Old 29th March 2008, 04:11 AM   #9
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i bought the IRF3205s.my gate resistors are 47 ohms i checked them with a multimeter.are the 3205's pretty close to the 85N06's.i would post some pics but they dont look very good.
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Old 29th March 2008, 05:19 AM   #10
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Are these the transistors that you have in your amp?

http://www.fairchildsemi.com/ds/FQ%2FFQA85N06.pdf

If so, the 3205s are the wrong part. The specs are close biut the case is the wrong style. I don't remember ever working on an audiobahn with the large case transistors in the power supply.

How many did you order? Where did you buy them from?

Photos help unless the resolution is so bad that it's impossible to make out anything on the board.
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