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Old 5th November 2009, 03:13 AM   #1
fongman is offline fongman  United States
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Default Rockford Fosgate 100x2

Hi, i have this old 1996 fosgate 100x2 amp This amp was working good when I stored it away and now only one channel works. I tested the speaker output and got 32.4v on the right channel? I check alll the output transistor and the 1 ohm resistor on both side. They check out okay. The left channel is fine and plays music fine. Nothing was burn. What am i missing here. I would like to say thanks to those of you that help me before. Thanks to Perry for all the help and time. I know he's busy i appreciate Perry your time.

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Old 5th November 2009, 05:42 AM   #2
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If you're absolutely sure that none of the output transistors is shorted, give me the DC voltage on each FET in the defective channel with the amp powered up (no audio). Place the black probe in leg 3 of the FET being tested and the red probe on leg 1 and then leg 2. For each transistor, post the circuit board designation.

Q#
Leg 1:
Leg 2:


Q#
Leg 1:
Leg 2:


Q#
Leg 1:
Leg 2:


Q#
Leg 1:
Leg 2:
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Old 5th November 2009, 05:53 AM   #3
fongman is offline fongman  United States
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Q#225
Leg 1: 65.3v
Leg 2: .261v


Q#215
Leg 1: 65.3v
Leg 2: .261v


Q#219
Leg 1: 0v
Leg 2: -17.66v


Q#224
Leg 1: 0v
Leg 2: -17.66v

Last edited by fongman; 5th November 2009 at 06:05 AM.
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Old 5th November 2009, 05:56 AM   #4
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Did you have the black probe on the 3rd leg of each transistor being tested?

What are the circuit board designations for each of the transistors?
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Old 5th November 2009, 06:10 AM   #5
fongman is offline fongman  United States
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Yes i just add the negative on the 17.66v. I start on the bottom left of the amp in the picture that i posted. The reading should be down to up.
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Old 5th November 2009, 06:25 AM   #6
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If you have 65v from gate to source on the top 2 FETs, they're VERY likely defective but I see no way to get that voltage. Confirm that you had the probes as shown in the attached drawing.
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File Type: jpg checkingFETsinpunch01.jpg (22.5 KB, 69 views)
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Old 5th November 2009, 06:37 AM   #7
fongman is offline fongman  United States
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Just like that. I might have some extra that i can change?
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Old 5th November 2009, 07:05 AM   #8
fongman is offline fongman  United States
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I try two of the irf540 and still the same?
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Old 5th November 2009, 07:12 AM   #9
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Are you sure it's 65.3v and not 65.3mv? The drive circuit can't produce that much drive voltage.
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Old 5th November 2009, 07:18 AM   #10
fongman is offline fongman  United States
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its 65.3vdc on my meter.
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