Rockford Fosgate 100x2

[fongman]

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.

An externally hosted image should be here but it was not working when we last tested it.

 

[Perry Babin]

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:

 

[fongman]

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

 

[Perry Babin]

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?

 

[fongman]

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.

 

[Perry Babin]

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.

 

[fongman]

Just like that. I might have some extra that i can change?

 

[fongman]

I try two of the irf540 and still the same?

 

[Perry Babin]

Are you sure it's 65.3v and not 65.3mv? The drive circuit can't produce that much drive voltage.

 

[fongman]

its 65.3vdc on my meter.

 

[Perry Babin]

With your meter on ohms, do you read infinite resistance from leg 1 to the other legs of the FETs you removed?

 

[fongman]

Leg 1 reads infinite but leg 2 reads 10 to 15 m ohms

 

[Perry Babin]

Try it again but this time, place the black probe on chassis ground.

Q225
Leg 1:
Leg 2:
Leg 3:

Q215
Leg1:
Leg 2:
Leg 3:

Q219
Leg1:
Leg 2:
Leg 3:

Q224
Leg1:
Leg 2:
Leg 3:

Are there any burned SMD resistors near the bias pot? The ones marked 200 are the ones you need to check. Are they within tolerance?

 

[fongman]

Q225
Leg 1: -32.7
Leg 2: 32.47
Leg 3: -32.29v

Q215
Leg1: -32.7v
Leg 2: 32.47v
Leg 3: -32.29v

Q219
Leg1: 32.47v
Leg 2: 32.47v
Leg 3: 14.79v

Q224
Leg1: 32.47v
Leg 2: 32.47v
Leg 3: 14.79 v

 

[Perry Babin]

Are you using the numbering scheme shown in the attached image?

224 and 219 should have + rail on leg 3.

 

[fongman]

Sorry i had them in the wrong place, but i have them like it would be on the amp.

Q225
Leg 3: -32.29
Leg 2: 32.47
Leg 1: -32.7v

Q215
Leg 3: -32.29v
Leg 2: 32.47v
Leg 1: -32.7v

Q219
Leg 3: 32.47v
Leg 2: 32.47v
Leg 1: 14.79v

Q224
Leg 3: 32.47v
Leg 2: 32.47v
Leg 1: 14.79 v

 

[Perry Babin]

Is Q221 shorted?

 

[fongman]

I can not find it? how to test it?

 

[fongman]

Im using a 100ix schematic. Are they the same

 

[Perry Babin]

I don't know where the transistor is but it's likely near the bias pot.

I don't know if it's the same as the ix. I have the x2 schematic. Email me if you want it.

babin_perry@yahoo.com

Go to the following page and click the circled link. Read the circled text also. Send me the screen cap for Q221.

Basic Amplifier Repair - Transistor Test Applet Link