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Old 10th April 2011, 10:41 PM   #1
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Default Rockford Fosgate Punch 400a4

Hello recieved a Punch 400a4 in for repair. The power supply was fried. I replaced them with IRF 3205, replaced all the gate drives 47 ohm. Also replaced the driver transistors. The amp powers up and produces nice clean loud audio. Sometimes it powers up just fine other times it draws full current for a second or two the clicks real loud and returns to normal. You can play it on the bench for hours with no issues. All the channels bias fine. I noticed it has been repaired before and somebody used IRF540N mosfets in the outputs.

Reading some of Perry's post he said he found the "N" version to cause wierd problems in Rockford amps. I didnt know if anyone else had these wierd problems with the "N" version of the IRF540 in Rockford amps. Do you think that this could be a problem with the amp powering up properly?

It will power up fine alot of times and then sometimes full 60 amp current on my power supply for 2-3 seconds then the loud click and returns to normal. This is a very strange issue. There is no DC present on any of the speaker terminals.

Wierd thing is alot of amps I work on the rail caps will hold voltage along time, but in this amp they do not. As soon as the power is disconnected they discharge. Is this normal? Most amp you have to discharge them before you work on them. Just wondering if anyone has any ideas as to what could make this amp sometimes draw so much current upon start up and other times not.

Any help would be great, thanks again.
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Old 11th April 2011, 03:23 AM   #2
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It's mainly class D that will hold a charge for a long time.

I'm not the only one to have problems with the N version of the 540s. There have been at least two repairs here with the 540Ns being the only problem. I'd suggest replacing them (IRF540 or IRF540A preferred) to eliminate them as the culprit.

If you can get the amp to draw current regularly, clip your meter probes to the 0.1 ohm source resistors to see if one channel has significantly more voltage across the resistors when the amp is drawing excessive current.

If you don't have a large current limiting resistor, you should get one. If you have a 2 ohm dummy load, you can use that as the limiter in the B+ line.
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Old 12th April 2011, 12:19 AM   #3
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I replaced all the ouput fets with IRF540 and IRF9540. Now the amp draws consistant current upon powering up, about 10 amps. I will try and get the reading off the emitters but dont want to damage the power supply. How should I configure the probes? Amp ground terminal to black probe, and red probe to 1 leg of emitter?
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Old 12th April 2011, 12:35 AM   #4
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The LR+ bridging terminal has +29.96 vdc and none of the other terminals have any dc. Thats the side of the power supply that has the clicking noise also. I hooked black probe to amp ground and touched red probe on one leg of emitters they all have the same voltage roughly -29.96 on the 540's and +29.96 on the 9540's.
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Old 12th April 2011, 12:39 AM   #5
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If I touch the red probe to one 540 emitter and the black probe to the other 540 emitter (in the same channel) it jumps to -59.90vdc and +59.90 on the 9540 emitters. All channels read the same. Not sure what you were wanting EXACTLY so I thought I would post both readings.

Thoughts?
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Old 12th April 2011, 01:12 AM   #6
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Did you set the bias pots fully CCW?
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Old 12th April 2011, 01:17 AM   #7
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Quote:
Originally Posted by Perry Babin View Post
Did you set the bias pots fully CCW?

Yes I always do before and after a repair is performed until it is time to set them. I found a missing diode D401 a 5D marking. Went ahead and replaced it. Also found Q413 a 1A marking with what looked like a burn spot on it.

Amp still has same issue.
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Old 12th April 2011, 01:21 AM   #8
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What is the DC voltage measured directly across the 0.1 ohm source resistors in the channel with DC on the speaker terminals?
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Old 12th April 2011, 01:42 AM   #9
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Quote:
Originally Posted by Perry Babin View Post
What is the DC voltage measured directly across the 0.1 ohm source resistors in the channel with DC on the speaker terminals?

-0.115 vdc
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Old 12th April 2011, 01:44 AM   #10
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All the other channels read 0.000 vdc across the emitters
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