Rockford Fosgate BD10001

[Perry Babin]

Are there any parts missing in the area of the switches or U4?

If not, Do you get a clean signal (same signal as pin 7) one the end of R20 and R22 that are NOT connected to U4?

One resistor will get the signal (sine wave) in one position. The other resistor will have the signal in the other position.

 

[seankane]

R52 and R54 are missing but I think they are suppose to be.

I was checking signal on R22 and R20 the buzzing was getting louder and louder, then Q110 and Q104 and there emitters went up in flames and smoke.

I am at a loss not sure why that happened.

 

[Perry Babin]

Cut the damaged FETs out of the circuit so it will power up. Don't repair it yet.

Yes, it appears that those two resistors were left out intentionally.

Remove R31. That will disconnect the amp from the preamp and won't allow a bad signal to damage the power amp section.

 

[seankane]

Ok how do I test the MIC4420CT when its out of the circuit as far as probe configuration and readings. Dont know if it could cause this but I am almost certain it is defective.

I removed the 2 fets that blew but already installed the new 15 ohm gate drivers, do I need to take them back out?

 

[Perry Babin]

There should be no low ohm readings between any of the legs of the driver. If you have new drivers, compare the readings from those. This only works out of the circuit.

You don't need to repair the output stage at this point. Leave the driver out, clean up the pads so that there are no solder bridges and connect a resistor across the gate and source of one of the transistors driven by the missing driver and it should power up and allow you to troubleshoot the preamp problem. After you get a good signal, then you can repair the output stage.

 

[seankane]

I powered up the amp after doing what you said to do everything is idling fine.

Something I dont understand is U4's signal is greatly affected by the cross over frequency and the sub sonic switch.

None of the other op-amps signals are affected by either switch or the cross over requency.

I replaced the switches with new ones, and replaced U4.

Could U17 be causing this?

Could a bad mosfet driver (MIC4420CT) cause these symptoms?

MIC4420CT is out of the circuit now but could it have been causing the frequency buzzing and output interference? I tested it and it was defective testing it against a new one.

 

[Perry Babin]

Removing R31 disconnected U17 from the preamp so it won't hum or stress the outputs until the resistor is reinstalled.

The noise was likely caused by the malfunctioning preamp and the distorted signal. It wasn't likely caused by anything in the power stage.

Check the signals on R20 and R22.

 

[seankane]

Removing R31 disconnected U17 from the preamp so it won't hum or stress the outputs until the resistor is reinstalled.

The noise was likely caused by the malfunctioning preamp and the distorted signal. It wasn't likely caused by anything in the power stage.

Check the signals on R20 and R22.

I get nice wave forms and perfect clipping on pins 1 and 7 of U2 and U3.

U1 and U4 confusing me, they have strange readings.

Pin 1 of U4 only deflects about a 1/2 a division above and below reference line then I see clip but it looks funny because its only a 1/2 of a division.

Pin 7 of U4 makes a nice wave form about 3 to 4 divisions then right when it clips the top deflection clips as it should and when the bottom starts to clip it jumps up between the top deflection and makes a square form completely.

All these readings are with the sub sonic off.

With the sub sonic on..............

Pin 7 of U4 does exactly the same thing.

Pin 1 of U4 just starts to make a audio wave form then clips right away, it looks like a bunch of X's and squares and does not deflect at all.

Pin 1 of U1 deflects 1 division and will not clip

Pin 7 of U1 deflects 1 division and will not clip

These readings are the same no matter what the orientation of the sub sonic switch is.

R20 and R22 are going to be really hard to explain they almost replicate pin 1 and 7 of U4.

 

[seankane]

U1 and U4 have been replaced with new parts.

Something does not seem right with U1 and U4 and R20 and R22.

I am kinda lost here not sure what to think.

 

[Perry Babin]

Remove C74. Does that allow pin 1 of U4 to swing the same as pin 7?

R20 and R22 are connected to either the output of U3, P7 or U4, P7, depending on the position of the subsonic switch.

Do you have a clean signal on the center terminal of the sub sonic switch with the switch in either position?

Op-amps sometimes 'invert' when the input is driven too close to the power supply rails. This is likely what happened when you drove the op-amp to clipping. There's no need to drive it that hard.

 

[seankane]

Well to do this testing the board has to be out of the sink for the signal test on the sub sonic switch.

Q105 and Q106 fets are getting really hot!!!

Should I just remove all the fets in that half of the output stage and then do my testing?

I dont want to cause further damage. I dont know why they would be getting hot since they are disconnected from the pre-amp stage. The fets in the second half of the output stage are not getting hot.

 

[seankane]

On another note I recieved the IC today the LM6171, so thought I would let you know that as well. Maybe I should change it or wait until further trouble shooting is completed?

 

[Perry Babin]

Yes. Remove those outputs.

Wait on the 6171.

 

[seankane]

Will the other "good" side of the output fets get hot when powering it up out of the sink?

Do I have to solder resistors across the gate to sources on the removed outputs or just one?

Just want to be sure I dont inflict anymore damage before we figure out whats wrong ya know?

 

[Perry Babin]

1. They shouldn't. They are more likely to get hot if they're not on the MEHSA strip (didn't you say you did away with it in this amp?)?

2. That's only when the driver IC is out of the circuit.

3. If the transistors are not on the MEHSA insulators and they're easy to remove, remove them all if you want to eliminate the possibility of further damage. Be sure to clean up the solder pads so there aren't any solder bridges.

 

[seankane]

With one half of the outputs removed and one half still in powering up the amp it now is drawing 5 amps current after the protection LED kicks off. It was not doing this before.

Do I need to de-solder the gate drivers of the missing outputs as well?

 

[Perry Babin]

No.

What's getting hot now?

 

[seankane]

The other half of the outputs that are still in are getting hot, but that does not explain the sudden current draw. It didnt have current draw with the remaining output fets still installed in the failed side.

I dont get it?

 

[Perry Babin]

If they're not soldered to the MEHSA insulator, remove them. It will just take a minute to do.

 

[seankane]

Remove C74. Does that allow pin 1 of U4 to swing the same as pin 7?

R20 and R22 are connected to either the output of U3, P7 or U4, P7, depending on the position of the subsonic switch.

Do you have a clean signal on the center terminal of the sub sonic switch with the switch in either position?

Op-amps sometimes 'invert' when the input is driven too close to the power supply rails. This is likely what happened when you drove the op-amp to clipping. There's no need to drive it that hard.

C74 made no difference.

The subsonic switch has 6 pins. I tested the midlle two. One pin has no signal either position. The outer middle pin towards the edge of the board has a good signal in one orientation and the same funny looking signal in the other position. The signal clips up top then the bottom wave form jumps on top and gives a square signal in between the wavey signal of the top wave form.

Its got to be something related to the switch or components of the switch.

I replaced the switch with a new one but I dont know???