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Old 10th October 2005, 09:52 PM   #1
gti311 is offline gti311  United States
diyAudio Member
Join Date: Oct 2005
Question Rockford Punch 250x2

This website is loaded with information. It is great to see people willing to help others. Keep up the greatness.

This Rockford has given me many years of great use. I STUPIDLY allowed the speaker wires to short together cooking the amp.

I opened the amp and found all of the power supply mosfets (MTP50N06V) to be shorted. They also have 22 ohm resistor tied to pin 1 of the mosfets which one reads 2K and another 200 ohm.

Further inspection revealed the 10 ohm resistors which are connected to the IRF540's to be shorted. It appears that the entire output section to be shorted. I had the amp bridge when the wires touched.

I have received the schematics from Rockford but this still seems a little overwhelming. Any help in what to do next would be greatly appreciated.
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Old 11th October 2005, 04:22 AM   #2
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
If the power supply AND the output FETs are damaged, you'll have to replace the power supply transistors before you can do any in-depth troubleshooting of the audio section. Sometimes they only damage a few components, other times they make a mess.

After replacing the power supply components (FET, gate drive resistors, PNP driver transistors...), you can then begin troubleshooting the audio section. The output section will have several sets of transistors in parallel. Generally, only 1 shorts in each set of parallel components. Find the ones of each parallel set that have the lowest resistance from pin 1 to pin 2/3. The one with the lowest resistance can then be cut out of the circuit (cut at least 2 of the 3 legs and make sure that they are no longer making contact. Replace any obviously defective resistors. Check the surface mount driver transistors (connected to one-another via ~20 ohm resistors between their emitters). If they are OK and the 20 ohm resistors are OK and you've replaced the others you've found, you can try to power it up. I'd recommend that you have a 2 ohm 25 watt resistor in series with the power wire. If that's not available, use a 7.5 amp fuse. It's going to be safest (less likely to do more damage) if you have it mounted down into the heatsink.

If it powers up and plays at low volume, replace the outputs (all in parallel with the shorted one have to be replaced). Be sure to check the emitter resistors that were connected to the failed output transistors. It's important that they're closely matched. If they're out of tolerance, replace them.
After replacing the outputs and the transistors are again clamped down to the heatsink, then you can test it at higher power.
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Old 14th October 2005, 06:31 AM   #3
gti311 is offline gti311  United States
diyAudio Member
Join Date: Oct 2005
Thank you Perry Babin.

So far I have replaced the power supply components and the amp now powers up with a 1A current limiting power supply. I also replaced the output mosfets. Now when I power it up four transistors on each channel get very hot. Iv'e troubleshot and cannot determine the reason for this heat. It appears that my voltages are OK. The +/-17VDC, +/-24VDC. I'm not sure where to go from here.

The transistors(2 MPSA56 and 2 MPSA06) are tied to a potentiometer. Both the left and right channel transistors are getting hot. I figured what is feeding them to be at fault but I cannot find anything suspicious.

If needed I can forward a copy of the schematic. Is it legal to attach it in the forum?

Thank you for your help
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Old 14th October 2005, 06:51 AM   #4
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Join Date: Nov 2003
Location: Louisiana
Email the schematic to me at

I'll take a look at it tomorrow.


Does it produce any audio at this point?

Are the output transistors getting hot?

Voltage across the 0.1 ohm resistors when the amp is on but no signal or load?

Does the amp have any DC offset on the speaker output terminals?

What components have you replaced in the audio section?
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Old 14th October 2005, 01:50 PM   #5
gti311 is offline gti311  United States
diyAudio Member
Join Date: Oct 2005
I have not injected any signals at this point to see if it produces audio. Should I use a signal generator or just a radio?

It appears that the output transistors(IRF9540 and IRF540) are staying cool.

The voltage across the .1 ohm resistors is 0VDC.

Measuring across the speaker outputs I get about 30mVDC across the left +/- and 20mVDC accross the right +/- terminals.

I have replaced all 12 of the output mosfets. All of the 10 ohm gate driver resistors measured 10.3 ohms.

Thanks again Perry Babin.
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Old 14th October 2005, 03:05 PM   #6
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Join Date: Nov 2003
Location: Louisiana
To determne whether the amp is operational, you need to drive signal into it. It doesn't matter whether it's an audio signal or a sine wave.

If the amp will not produce audio, it's possible that the protection circuit is engaged. You can defeat it by lifting two legs (base and emitter) of Q316/317. Be aware that defeating the protection may allow the amp to be damaged if you allow too much current to flow into the amplifier. From the schematic, it looks like pin 7 of U302 should be at or above 17 volts and pin 7 of U301 should be at or below 17 volts. If either or both are near 0 volts, the amp is in protect mode or there is a defective component in the protection circuit.

If you find that the amp is working properly, mount it back into the heatsink and adjust the biasing. If your power supply has an amp meter on it, turn the bias controls clockwise just to the point where the current begins to increase. If there is no ampmeter, you can set the biasing by measuring the voltage across the 0.1 ohm resistors. A bias setting that gives ~1mA across the resistors should be OK. To confirm that it's enough, if you have an oscilloscope, run a sine sweep through the amp at low power into a speaker or dummy load and look for crossover/notch distortion. If it's clean, the biasing is OK.
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Old 14th October 2005, 07:26 PM   #7
gti311 is offline gti311  United States
diyAudio Member
Join Date: Oct 2005
U301 and 401 have negative 15.5VDC. U302 and 402 have +17VDC.

I set the biasing on the right channel via the .1 ohm resistor at 1mA and the left channel seems to fluctuate from 0-3mA continuously every second. I cannot get it to stabilize at 1mA.

If I dont have a scope, is there any other way to verify the biasing?
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