# rockford fosgate 301m amp overheats



## lowrider2962 (Feb 26, 2014)

i have a RF 900w punch 301m amp that started overheating regularly last summer. its been a good amp for me over the years and ive repaired it once or twice before. it seems like one or more of the output MOSFETs are faulty. i know i would have no trouble replacing them but as a mechanic i know all to well that fixing the failed part doesnt necessarily fix what caused it. pervious repairs to this amp have been mostly due to wear and tear (its been in 4 different vehicles). the one thing that failed electroniclly was a diode located near the inputs. it burnt in half and damaged nothing else. that was years ago, i replaced it with another comparable diode and it worked like a champ since then. the FETs are IRF9540's and IRF540's, 3 of each. should i just replace them and see where it gets me?

any help and advice is appreciated. thanks for reading.


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## ATOMICTECH62 (Jan 24, 2009)

Could be the load,idle bias,defective speaker,defective opamp or cap causing high dc offset.Have you ever replaced a mosfet with the MEHSA strip?


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## lowrider2962 (Feb 26, 2014)

well the speaker is pretty new and in good shape. im not sure what a MEHSA strip is but i can say i havent installed one. all of the MOSFET's are original. the two caps at the power connection are not original though. it originally has 2 parallel 330uf63v caps, one of them broke at the positive pole and i had 2 matching 1000uf50v caps laying around that i installed. being located before any of the complex electronics i didnt think they could cause a problem, that it would be comparable to installing a cap between the battery and the amp, just with a much smaller effect. also what is the idle bias? 

thanks for the help


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## soccerguru607 (Nov 4, 2009)

MEHSA stripe is where transistors soldered to.
When replacing caps you should go with equal voltage caps or higher.
If your amp draw more current than specs at idle, then you can turn idle bias down to match original spec. I have repaired few RF amps where amp draw current upon power up and if you let it keep going it can blew outputs.


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## soccerguru607 (Nov 4, 2009)

If you send some pictures will be easier.
When RF outputs gets output blown, many times emitter resistors and other resistors gets blown plus sometime driver get blown also.


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## lowrider2962 (Feb 26, 2014)

how do u adjust the idle bias?


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## soccerguru607 (Nov 4, 2009)

That comes after you repair the amp and doing testing with power supply.
I leave it alone unless adjusting is need it.
Most times there will no bias problem.
Then if amp is repaired and working when power up at idle you can re-adjust.
I only use testing power supply with current and voltage meters
But its the blue with white circle that you can use a small flat screw driver to turn.
Located middle lower section of picture or output section
This looks like a mono amp so only one is in there.


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## soccerguru607 (Nov 4, 2009)

Also, note Atomictest62 asked you about if you ever replaced transistors on MEHSA. 
The reason is that this amp is unique when replacing them.
To me these older RF amps are probably the easiest for me to do repair because I probably repaired close to 1k of them past dozen year.
Repair car amps is only a hobby for me. I believe Atomictest62 is pro in repairing.


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## soccerguru607 (Nov 4, 2009)

@Atomictech62:
Sorry I did not mean to take your thunder


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## lowrider2962 (Feb 26, 2014)

all of the transistors on this amp are original. im curious about the idle bias though because everything on the amp appears to be in good shape and it still performed well when it started tripping the overheat safety. could it be that the idle bias is the only problem and just needs to be adjusted? at the very least perhaps its worth readjusting and retesting the amp to see if that fixes it.

thanks for the help guys


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## soccerguru607 (Nov 4, 2009)

Do you know how to measure those MOSFET transistor both on power supply and output section with a voltmeter? If you don't I can't really help you because its basic. 
Look can be very deceiving. 
If you do and they are good you can try to turn the idle bias lower but you need to power up the amp to adjust bias control. I never seem a RF has that problem without amp been repaired. I would also recommend that you replace those 50v caps to at least original 63v caps.
Also this is a class ab amp so it suppose to run hot when driven to extreme.
If your amp is working as it should then do this first:
Clean all the old dried up silicon paste off and apply new paste
Every RF amp I repaired I put new silicon so heat transfer is much better.
When I do 'burn in' test near clipping on class AB amps it always get very hot. 
If you want to learn repairing car amps you can google "perry babin" and purchase his dvd tutorial. Trust me it will be worth it if you enjoy car audio. His a buddy of mine and best car amp repair guy I know. Years ago I use to send amps out to few amp repair places and they can not fix it then I send to Perry and he fixed all of them for me. As long as the board is good there was not an amp I send him he could not repair.


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## lowrider2962 (Feb 26, 2014)

i suppose i should explain alittle about myself so we're all on the same page. im a professional mechanic at a Chevrolet dealership where i do diagnostics and repairs on all aspects of cars, including electrical systems ranging from airbags to audio entertainment and everything in between. i understand electronics quite well and have built circuit boards from scratch in the past. ive repaired audio equipment before but i figured id get some input for someone with alittle more experience before i go replacing everything on this amp. lol on that note i do appriciate the input you guys are giving me. all of the output MOSFETs show about 10k ohms between the gate and drain, and about 2.5k between the gate and source. so how do test/adjust the idle bias. ill swing by radio shack this evening and see if they have some caps i can use.


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## soccerguru607 (Nov 4, 2009)

measure them with voltmeter at 'diode check'


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## ATOMICTECH62 (Jan 24, 2009)

soccerguru607 said:


> @Atomictech62:
> Sorry I did not mean to take your thunder


 Your input is appreciated.There is only a hand full of us on here with on hands on experience that can help others in detail.Ive been so busy I dont check in like I use to.
I dont know how Perry does it all.That man has to be a genius.He makes his own how to videos,manages his own website,does who knows how many repairs a week and still has time to help others out on DIY.His personal touch is unmatched by anyone I have ever dealt with.He has sent me much needed schematics when I was in a jam and helped me numerous times with some very technical issues via email.His knowledge base is so vast I seriously believe he his the best.
Anyone not familiar with his website needs to check it out.

Basic Car Audio Electronics

Back to the issue at hand.
I would power the amp up with no speakers connected and see if it gets hot or even slightly warm at idle.Connecting it to a 12volt power supply with an amp meter would be even better to see how much current it pulls at idle.If heats up or pulls more then about 3 amps the idle bias might be the the problem.Without a scope and a high frequency sine wave the best way to adjust it would by measuring the DC voltage across one of the emitter resistors.10 to 5 millivolts is common but you will have to be very careful when turning the pot.Do it very slowly.I always back it down,then bring it up till it reaches about 10 millivolts.If you go too far,too fast the power supply mosfets can explode.

Best to put a 5-10 amp fuse on the power wire first.


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## soccerguru607 (Nov 4, 2009)

Please go with what Atomictech62 suggest.

Yes, I am so happy to see someone appreciated Perry Babin. To be honest with you if he did not help me through the years I probably would have quit. At a time when no one wanted to help me to see me succeed he was there for me. I bought his dvd back in 2000 and almost every year or two he will send me a free updated one. So now every amp I repaired with his help I will give him like 10-15% profit after I sold the amp. I was an aeronautical engineer with two masters from ivy league school and I still can not compare at all to him in amp repair.
Perry is so humble and helpful to anyone and make time for people who needs help. I think he only sleep like 5 hrs a night. 
I think I enjoy helping others from his influence. Excellent role model.
I have gotten to know him a bit in a personal level last couple years. 
Just a wonderful guy.


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## lowrider2962 (Feb 26, 2014)

thanks for the replies guys. sooo interesting update. i decided to reinstall the amp and see how long it took for it to get hot. i was going to use my infared thermometer and pinpoint where the heat was originating from. while i was installing the motherboard back into the body i decided to thoroughly clean the old thermal paste and apply new. now with it installed and running alittle harder than i usually set it, it just gets a tad warm. previously it would get so hot that u couldnt touch the body without nearly burning yourself. so its like something has changed in the year it spent just sitting around. i let it play some music with good bass for about 30 mins and it didnt seem to get unsually warm. previously in that amount of time even with the radio not turned up it would get blistering hot. i dont think new thermal paste could make such a difference, but then i could be wrong. im gonna leave it installed for a few days and just monitor it closely to see what it does.

id be interested to hear your thoughts on this, and thank you for all the help and advice.


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