Well here we go. I don't have a whole lot of amp experience. I helped my friend to figure out the stereo situation in his new (used) car. The previous owner had upgraded the stereo but something wasn't quite right. There were two amps in the back, one was connected to the four cabin speakers, the other was connected to the two 10" subs. What prompted the examination was the fact there was NO BASS. Come to find out, the amp for the subs was not even turning on. So I took it out and took it apart. All I know that is wrong with it is what I can see visually. There are 18 FETs places around the perimeter of the main board, four of which are obviously blown to bits. What could have caused that? Will replacing the FETs fix the problem or could there be something else wrong? The blown FETs are STP60NE's. Someone please help! 
What brand and model of amplifier?
Most likely causes are:
1) A shorted speaker voice coil or wiring to speaker.
2) An overloaded amplifier = Less than recommended speaker impedance.
Since the amp won't turn-on, the power supply MOSFETs are suspect. These should be near a toroid transformer and the battery terminals. STP60NE is a partial part number - I assume the rest is 06 for STP60NE06 = 60-Amp 60Volt SGS Thompson MOSFET.
Things to check:
Remove burnt MOSFETs, Check gate resistors, replace resistors as needed. Leave FETs out for now.
Using an oscilloscope and low-current (I mean 1 - 2 regulated Amps) 12-volt supply, check for a square wave or sawtooth pulse signal at ALL pads for MOSFET gates. The signal should fall to near zero and no higher than the power supply voltage. If you get a flat line, or it doesn't swing between + voltage and near 0V, there are problem(s) in the PWM circuit. Suspect driver transistors for PS MOSFETs or the TL494/594 PWM IC (or SG35XX IC).
Test for shorts in the output devices using an ohm-meter between all 3 pins. Replace as needed and, if shorted output devices are found replace all for that channel (yes I mean P & N types) and check driver devices and emitter resistors.
When you have a correct PWM signal to PS FETs and no shorted output devices, install new PS MOSFETs, heatsnik compound, and re-fit amp board to heatsink.
Using your low current supply again, test-fire amp with an ammeter in series with power terminal and supply. Use 10A scale to start. The reading should be less than 1A once the PS resivoir capacitors have charged = 1-2 seconds. This should be done with no input signal and no speakers connected. If the ammeter reads more than 1A at idle, the amplifier still has problems.
Check for DC voltage at the output terminals. If any more than 10 or 20mV is present at the output terminals, there's still a problem in the amplifier section(s).
If everything above checks-out, connect signal source and turn-on amp, still using low-current supply. Use the O-scope to verify the amp has output for both channels. Then test with a 'junk' speaker.
If all goes well, run amp from high-current 12V supply with input signal and speakers at high level. Before installing in car check for shorted speakers and wiring.
Before installing into car, check to see the the speaker impedance is correct for the amplifier (usually 2-ohms in stereo or 4-ohms bridged unless it's a high-current amplifier)
Good Luck! Please note, I left out any & all soldering and electricity safety info
Most larger car amplifiers will have more than 35V on each rail..... Minimum +/- 35 = 70V = a nice shock! Some car amps will carry more than 120V across the + / - rails = LETHAL!
-Jim
Most likely causes are:
1) A shorted speaker voice coil or wiring to speaker.
2) An overloaded amplifier = Less than recommended speaker impedance.
Since the amp won't turn-on, the power supply MOSFETs are suspect. These should be near a toroid transformer and the battery terminals. STP60NE is a partial part number - I assume the rest is 06 for STP60NE06 = 60-Amp 60Volt SGS Thompson MOSFET.
Things to check:
Remove burnt MOSFETs, Check gate resistors, replace resistors as needed. Leave FETs out for now.
Using an oscilloscope and low-current (I mean 1 - 2 regulated Amps) 12-volt supply, check for a square wave or sawtooth pulse signal at ALL pads for MOSFET gates. The signal should fall to near zero and no higher than the power supply voltage. If you get a flat line, or it doesn't swing between + voltage and near 0V, there are problem(s) in the PWM circuit. Suspect driver transistors for PS MOSFETs or the TL494/594 PWM IC (or SG35XX IC).
Test for shorts in the output devices using an ohm-meter between all 3 pins. Replace as needed and, if shorted output devices are found replace all for that channel (yes I mean P & N types) and check driver devices and emitter resistors.
When you have a correct PWM signal to PS FETs and no shorted output devices, install new PS MOSFETs, heatsnik compound, and re-fit amp board to heatsink.
Using your low current supply again, test-fire amp with an ammeter in series with power terminal and supply. Use 10A scale to start. The reading should be less than 1A once the PS resivoir capacitors have charged = 1-2 seconds. This should be done with no input signal and no speakers connected. If the ammeter reads more than 1A at idle, the amplifier still has problems.
Check for DC voltage at the output terminals. If any more than 10 or 20mV is present at the output terminals, there's still a problem in the amplifier section(s).
If everything above checks-out, connect signal source and turn-on amp, still using low-current supply. Use the O-scope to verify the amp has output for both channels. Then test with a 'junk' speaker.
If all goes well, run amp from high-current 12V supply with input signal and speakers at high level. Before installing in car check for shorted speakers and wiring.
Before installing into car, check to see the the speaker impedance is correct for the amplifier (usually 2-ohms in stereo or 4-ohms bridged unless it's a high-current amplifier)
Good Luck! Please note, I left out any & all soldering and electricity safety info
Most larger car amplifiers will have more than 35V on each rail..... Minimum +/- 35 = 70V = a nice shock! Some car amps will carry more than 120V across the + / - rails = LETHAL! -Jim
Car amplifiers blow all the time believe it or not ...due to abuse.
99% of the failing car amplifier is the power transistors mounted
on the heatsinks, replacing the bad ones should bring the
amp back to life. Ideally, it's best to remove each one and
measure the transistors individually, typically they short out
so all you need is a DMM to check it. The output stage,
if bipolar, check those with the diode check on the DMM
after you removed them, but if you are saavy enough, you
might get away with doing a check without removing them
provided that you are able to interpret the readings in circuit.
After repair, verify that the amplifier is working properly
and double check the wiring in the car - to make sure
there is nothing funny going on that might have caused
the amplifier to toast.
99% of the failing car amplifier is the power transistors mounted
on the heatsinks, replacing the bad ones should bring the
amp back to life. Ideally, it's best to remove each one and
measure the transistors individually, typically they short out
so all you need is a DMM to check it. The output stage,
if bipolar, check those with the diode check on the DMM
after you removed them, but if you are saavy enough, you
might get away with doing a check without removing them
provided that you are able to interpret the readings in circuit.
After repair, verify that the amplifier is working properly
and double check the wiring in the car - to make sure
there is nothing funny going on that might have caused
the amplifier to toast.
Refer to post #2 
MTX probably uses a small transformer to drive the PS FETs (I'm not familiar with "HO" - most do). This means the PWM circuit is probably OK. Without an O-scope, you can't tell what the pulse looks like though.
Were ALL PS MOSFETs replaced?
Was there no load on the speaker terminals?
Did you check the output MOSFETs (IRF540 / 9540) for shorted source to drain?
Was it mounted to the heatsink?
Did it fail with only battery + and ground connected or when the remote turn-on power was connected?
What you experienced is exactly why I always use a current-regulated power supply to do the initial test.
Go through the amp refering to my 1st post and the above. Aslo, I may be able to help with MOSFETs - mine are less expensive than retail prices
MTX probably uses a small transformer to drive the PS FETs (I'm not familiar with "HO" - most do). This means the PWM circuit is probably OK. Without an O-scope, you can't tell what the pulse looks like though.
Were ALL PS MOSFETs replaced?
Was there no load on the speaker terminals?
Did you check the output MOSFETs (IRF540 / 9540) for shorted source to drain?
Was it mounted to the heatsink?
Did it fail with only battery + and ground connected or when the remote turn-on power was connected?
What you experienced is exactly why I always use a current-regulated power supply to do the initial test.
Go through the amp refering to my 1st post and the above. Aslo, I may be able to help with MOSFETs - mine are less expensive than retail prices
lionfire17 said:
hmmm... ...them darn HO's. The HO's use a really weird output stage. Both Plus and Minus rails use an IRF540 transistor... ....really odd to not use complementary pairs. They are supposed to be a good amp... ...just need some repairs once in a while.
I've repaired several of them. They use an IRF540 for a audio output device, both + and - Rails. These output fets will go, causing the power supply fets to go. Any one of these go bad, and you'll blow power supply fets.
My advice is to replace all of the fets in the amplfier. I'll probably cost you about $15...
You may also want to check for one other thing (assuming all of the amplifier FETS are good. There is a variable resistor, one for each of the ch. It's a small pot, and you adjust it to set the bias current of the output stage. Sometimes as these amps age, the bias current will change, causing high quiescent current draw, blowing the switching fets. My best advice is to turn it all the way counter clock wise, then watch the output for cross over distortion (Use a 10Khz Sine wave at a good amplituce, with a 4 or 8 ohm load) You'll have lots of diso with the pot all the way counter clock wise, slowly increase it until that just dissappears.
DO THIS TEST WITH A 100W 12Volt LIGHT BULB IN SERIES WITH THE +12 BATTERY LEAD!!! (It will may save you on the cost of several sets of powe supply fets!)
Oh, yeah. Stick with the ST parts for the switching fets. You can use IRF or fairchild for the audio output fets. You can save a few $$ there. I used fairchild for the last ones I've repaired. Still going strong. (over a year ago now!)
Good Luck!
-Dan
P.S. In my experience, power supply fets ammount to only about 50% of the most common causes of car audio amplifier failure. Often times there is another problem, that causes them to die. As you have probably experienced learned
WOW!
Post 2 was a very good post but he left out one thing that I do and that is to take out the suspect output devices and check both rails for short circuit condition. Go from ground to the rail. If you get a very low ohm reading, chances are, you still have a shorted part in the amp section. This needs to be removed before you can troubleshoot the supply. Do this on the AMP side of the power supply diodes because you don't want to read the transformer secondary coil. (close to 0 Ohms) This will confuse you a lot!!!
The supply.... I Would replace the PWM chip regardless... Chances are, it is damaged too. Change ALL gate drive resistors. This way you KNOW they are all good and very close to the same value. I would replace ALL of the mosfets. Chances are, the others will have been stressed at blow up. I don't know about gate drive transformers in car audio. I have an old lanzar amp that gives 540WRMS/channel and no gate drive transformer. It has 12 mosfets, 6 per side! Mine are IRFZ44's. Check ALL semiconductors for short circuit. I suggest taking out the power parts (TO-220 and larger) most diodes will test in-circuit.
Check your power rectifier doides for short circuit too. From the description, it sounds to be a catistrophic failure so replacing the above parts would not be a "shotgun" approach to repairing the amp.
Good luck and let us know!!!
Chris
Post 2 was a very good post but he left out one thing that I do and that is to take out the suspect output devices and check both rails for short circuit condition. Go from ground to the rail. If you get a very low ohm reading, chances are, you still have a shorted part in the amp section. This needs to be removed before you can troubleshoot the supply. Do this on the AMP side of the power supply diodes because you don't want to read the transformer secondary coil. (close to 0 Ohms) This will confuse you a lot!!!
The supply.... I Would replace the PWM chip regardless... Chances are, it is damaged too. Change ALL gate drive resistors. This way you KNOW they are all good and very close to the same value. I would replace ALL of the mosfets. Chances are, the others will have been stressed at blow up. I don't know about gate drive transformers in car audio. I have an old lanzar amp that gives 540WRMS/channel and no gate drive transformer. It has 12 mosfets, 6 per side! Mine are IRFZ44's. Check ALL semiconductors for short circuit. I suggest taking out the power parts (TO-220 and larger) most diodes will test in-circuit.
Check your power rectifier doides for short circuit too. From the description, it sounds to be a catistrophic failure so replacing the above parts would not be a "shotgun" approach to repairing the amp.
Good luck and let us know!!!
Chris
oh good golly. so much information! 
this is going to take me a little while to figure out. i have electronics knowledge but it is all merely from my own experiences and im only 19! haha so if you guys are willing to step me through this a little slower id be very gracious and im very willing to learn.
this is going to take me a little while to figure out. i have electronics knowledge but it is all merely from my own experiences and im only 19! haha so if you guys are willing to step me through this a little slower id be very gracious and im very willing to learn.
lionfire17 said:oh good golly. so much information!
this is going to take me a little while to figure out. i have electronics knowledge but it is all merely from my own experiences and im only 19! haha so if you guys are willing to step me through this a little slower id be very gracious and im very willing to learn.
We're all willing to help... ...so ask away!
Car audio amplifiers are all very similar in design. Once you understand how they work, you'll find fairly easy to troubleshoot and repair them.
In the case of your amp, the first thing that I'd do would be to remove and test all of the TO-220 or larger devices (same as the fets you just put in). They will all be mosfets with the exception of some diodes...
...I guess the first question is:
Do you know how to test a mosfet and a diode with an ohm meter?
-Dan
Thanks for this, Internet.... I wasn't about to type it for hours
Basic Testing of Semiconductor Devices
P.S. Refer to Richard Torrens' MOSFET test with DMM on Dide check.
1st rule of repair: Patience. Take a break when you feel frustrated.
Feel free to ask-away.
Basic Testing of Semiconductor Devices
P.S. Refer to Richard Torrens' MOSFET test with DMM on Dide check.
1st rule of repair: Patience. Take a break when you feel frustrated.
Feel free to ask-away.
I have an MTX 225HO that has blown MOSFETS in the pwr supply and it toasted the gate resistors as well. Does anyone know what the values of the gate resistors are? THe resistors are toasted black and I cannot read the color code. If anyone even knows the color code that would be helpful becasue I know how to read color code as well.
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