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Old 12th May 2011, 06:32 AM   #1
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Default Rockford Fosgate BD10001

Hello everyone.

I recieved this amp in for repair. Someone had changed the transformer, the driver transistors (A06 & A56), and in the process of trying to change the output fets destroyed the MESHA board.

I removed the MESHA board and applied Kapton M/T and replaced all the output fets, as they were shorted.

The amp powers up fine and idles. There is no DC on the output terminals at an idle but driving a 100Hz sine wave into the amp at full volume there is 2 vdc on the + output terminal.

The amp produces good bass if you will but there is a buzzing or frequency noise through the speaker. I can also hear a slight buzzing noise inside the amp, almost sounds like it is coming from the MIC4420CT driver but not sure.

All of the op-amps have +/- voltage on the supply pins.

The amp has a rail voltage of 88 vdc on the r/b wires.

The LM339D reads as follows:

pin 1: 4.51 vdc
pin 2: 4.51 vdc
pin 3: 17.45 vdc
pin 4: 0.314 vdc
pin 5: 2.347 vdc
pin 6: 4.34 vdc
pin 7: 5.01 vdc
pin 8: 0.733 vdc
pin 9: 0.733 vdc
pin10: 5.01 vdc
pin11: 8.21 vdc
pin12: 0.001 vdc
pin13: 4.51 vdc
pin14: -0.001 vdc

I have included 2 pics of the output on the scope, 1st pic is at low volume and 2nd pic is at 1/2 volume.

Scope settings are 20 vol/div 2 ms

Can someone push me in the right direction of what I should test first?

The output does not seem distorted but like there is interference with it if that makes sense.
Attached Images
File Type: jpg BD10001 distorted output 001.jpg (886.5 KB, 62 views)
File Type: jpg BD10001 distorted output 002.jpg (926.5 KB, 58 views)
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Old 12th May 2011, 06:56 AM   #2
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When there is DC on the outputs, what's the DC voltage on all of the pins of U17?
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Old 12th May 2011, 07:06 AM   #3
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U17 LM6171

pin 1: 0.015 vdc
pin 2: 0.005 vdc
pin 3: 0.003 vdc
pin 4: -9.68 vdc
pin 5: 0.008 vdc
pin 6: -0.245 vdc
pin 7: 9.74 vdc
pin 8: 0.008 vdc
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Old 12th May 2011, 07:09 AM   #4
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Did it have DC on the speaker terminals when you took these reading?
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Old 12th May 2011, 07:16 AM   #5
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Yes 2 vdc on the + speaker terminal but the volume has to be at full volume driving a 100Hz sinewave into the amp. The DC is only present at full volume.
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Old 12th May 2011, 07:19 AM   #6
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Viewing the output on the scope, the amplitude does not get any greater after 1/2 volume. I cannot get the amp to clip on the scope. Its like after 1/2 volume there is no more increase in output.
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Old 12th May 2011, 07:22 AM   #7
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The DC isn't showing up at the IC.

Try connecting the pot to it to see if everything is switching properly. Remember to pass through the transition point quickly. You can try it with the resistors in the circuit. I wanted to try it that way but haven't had a chance.

Check for DC on the speaker terminals when it's in both states (clock/counter-clockwise on the pot). It shouldn't have any DC in either position.
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Old 12th May 2011, 07:24 AM   #8
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Same two output fets Q104 and Q107 as the BD15001?
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Old 12th May 2011, 07:26 AM   #9
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It doesn't matter as long you select one from each group. You can use the ones on each end if you want. Check the gate drive and also make sure they're switching the drains properly.
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Old 12th May 2011, 07:41 AM   #10
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Quote:
Originally Posted by Perry Babin View Post
It doesn't matter as long you select one from each group. You can use the ones on each end if you want. Check the gate drive and also make sure they're switching the drains properly.
I checked one out of every group of three. Q110, Q104, Q107, Q101

When the pot is turned they are all switching from +/- 9.75 vdc

When you say check the gate drive to make sure they are switching the drains properly do you mean red probe on gate and black probe on drain and turn pot to see if drains are switching on and off or from + to -?

Not quite sure how to check that?
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