# Kenwood Amp repair



## JonnyD (Nov 9, 2011)

OK so I have been looking all over the net to find info to get this amp fixed myself. All of the sites I have came across, almost everyone just says its blown. Buy a new or have a professional fix it. Well that's too easy. I would rather do it myself 

Ok so here is where I am. I have opened up the Amp and one of the FET's is blown. When I power up the amp with the input voltage at 12V. It will not start up. The smell of burnt starts to get stronger (when it's powered). All the caps and components appear fine. I assume its shorting somewhere because this blown FET because it is dropping my supply voltage to 10v at the amp input.

I can't remember what exactly caused it to blow. Stupid of me but I had my power and ground wires both black and I may have crossed them in the dark hooking up the amp. Before this it was running great. I think I may have been running the gain slightly high also. I can't really remember how it all happened.

The amp in question is a Kenwood KAC-929. I have downloaded the service manual and inspected the entire board with a light. As far as I can tell only Q12 is blown. Q12 in the manual is a IRFI1010N FET. It appears to be part of the input supply for the amp. I went to school for electronics years ago. Im great at soldering, etc. Just can't tell which type of circuit it is. Or how to go about replacing the component? How can I test this is the only cause of the failure before installing another component? Or do I just go ahead and install and see what happens? lol If you look at the link below and go to the schematic. It's page 7 I believe. You can spot Q12 in the lower right hand side. of the amp.

Edit: looks like I can't add links or images because I am a new member. In google type "kac 929 service manual" and it's the first result.

I know of digikey but what other sites do you recommend for buying parts. I am in Canada.

Thanks guys. I hope someone can chime in with some advice on this. I am ready to go !


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## zoomer (Aug 2, 2009)

ok. here goes. First you have to tell me: does the amp short out even when the remote on/off is powered up? or anytime you connect 12 volts to the inputs? 
What part is smoking? find it first! it is probably hot and will probably also have to be replaced. What happened to Q12? physically blown apart? 

The entire power supply is controlled by IC5. This is the Pulse width modulator supply/regulator Ic that provides square waves to transistors Q11 to Q18 via the driver circuitss ahead of them. Q14/Q18 with Q12/Q16 alternately switch 12 volts to the wiring of transformer L4. Q11/Q15 with Q13/Q17 alternately switch 12 volts to the wiring of transformer L3. Together the provide +/- 45.2 volts for the power amp thru rectifier diodes D41 and D44. 
It may not simply be Q12 that is blown as it is in parallel with Q16. Chances are both are gone. Some of the driver transistors may be blown also. If the driver transistors Q19 to Q22 are blown then they will not switch the power transistors properly.
The IC5 is smart. It can sense over current and over voltage and should shut the power supply transistors before something drastic happens. But you never know. 
Have you tried measureing the power supply output of 45 volts plus the lower voltage of +/-25V? If the power amp is blown it may be drawing lots of current. 
I wish I could say it is a simple matter of changing the transistors, but it is not. 
I have fixed a few car amps and its not fun. You have to remove lots of parts. 
Transistors themselves are cheap enough but for small quantities there is shipping and handling. You may also need a tube of heat sink compound.
May I suggest suppliers such as electrosonic, or active components. 

You may want to look at buying a used one. 
1000w*Kenwood KAC-929*2/1Ch.Car Amplifier*OLD SCHOOL! | eBay

I have 2 RF punch 600A5s. I have repaired on, now the second on needs fixing. I have the parts and I now what transistors are blown. I till take me about 4 hours to do the job. I frankly dont have the time. So consider your options..


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## JonnyD (Nov 9, 2011)

Zoomer, 

Thanks for the reply, fast reply I should say. I will get it on the table tomorrow properly power with a car battery. My power supply is very weak I was using. I will power it up with the remote wire tied to 12V. I'll go ahead and find the points from the schematic to measure the main output of 45V and the lower 25V outputs. I'll also check where the smell or heat is coming from. I know it could be more components that are shot. But for now all I can see is Q12 physically. I will at least give it a go. At least I learn something along the way if the outcome is good or bad right. Thanks again for all the help. Cheers!


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## timwebb (Oct 25, 2011)

check all the transistors in the power supply for a short. u have some in the power supply or in the output section. thats pulling the power. take them out and power it up. and see if it pulls to much current.


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## ATOMICTECH62 (Jan 24, 2009)

You will have to replace all the irfi1010 in the amp along with the drivers and the pwm ic.
Because if one part is bad the thing will blow again and it doesnt sound like you have a scope to check the wave at the gate's of the mosfets.
Also you need to test all the output transistors for shorts because that is the main thing that causes the power supply to go out and if you get the power supply running it blow again without removing the shorted output transistor.


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## zoomer (Aug 2, 2009)

ATOMICTECH62 said:


> You will have to replace all the irfi1010 in the amp along with the drivers and the pwm ic.
> Because if one part is bad the thing will blow again and it doesnt sound like you have a scope to check the wave at the gate's of the mosfets.
> Also you need to test all the output transistors for shorts because that is the main thing that causes the power supply to go out and if you get the power supply running it blow again without removing the shorted output transistor.


Replacing ALL the transisors is a bit much, but OP does not have the diagnostic tools to find the real fault anyway. 

Do consider simply buying a used one. If it is not nice cosmetically you can always swap the case.


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