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Old 29th February 2004, 05:06 AM   #1
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Default Kenwood KAC 820 Amp went Boom!

I have a Kenwood KAC 820 Amp and the D1062 Transistors went out quite destructively. I changed them out with the NTE377 part, and guess what. Same thing.

Now I need a schematic or service manual for this amp to track it further. Does anyone out there have access to a schematic for this amp and can you send me a copy of the service manual or schematic?

Also, does anyone know where I should start looking to fix this problem? Is it a common problem or something someone has seen before?

I thank anyone who can help me out.

Regards,

Sid

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Old 4th March 2004, 03:14 AM   #2
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Default OK - More Focused Question

I have determined that the collectors of the D1062 NPN transistors are pulling down one of 2 sides of a transformer. This is obviously the DC to DC power supply for the amp. The transformer appears to be a +12V center tapped unit with either end tied to a pair of NPN transistors. Transistors are in parallel for more juice I suspect. There is no resistance between the Transistors and the transformer or the transistors and ground (Collector and Emitter circuits).

Now that I am here, I checked the transformer Center tap to each end resistance and the end to end resistance to see what might be going on in the transformer. From what I can see it is very (near zero) resistance in the transformer coil. I can see this if the coil is short and of heavy guage wire. I have no data on the transformer to determine this.

But with 2 x 10A transistors on either end of the transformer, this is likely a high current coil.

Anyway, does anyone know if the transformer coil resistance should be very low? Does the impedance of the coil (there are also some chokes ahead of the center tap) is the only load for the collector of the transistors? Or is it shorted maybe?

Another thing. All the transistors (4 of them) measure as shorted. I will replace them, but would one transistor in this circuit take out all 4 of them? I measured the drive transistor and it is good. Further, with the transistor bases removed, I can see that the base to the 4 D1062 units is getting a nice square wave drive, so it looks like the output is OK from the driver circuit.

I cannot figure out why the transistors blew in the first place (very destructively as I said) and whether the transformer might have a problem (although it looks perfect, no heating evident etc.)

Thanks for your help.
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Old 4th March 2004, 07:01 AM   #3
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Interesting that they used bipolars for the transformer drive, must be old.
The transformer will show a short on the meter since its measuring DC. If you had a way to measure impedence at say 20khz, thats what the transistors would see. Most likely its fine
If they blow again, check theres nothing on the secondary side shorting to ground or drawing excessive current. If the transformer saturates it will draw an large amount of current and kill the transistors.
A computer power supply should stop the transistors blowing again if something goes wrong. Most yumcha 250w PSU will shutdown at 10-15 amps.
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Old 4th March 2004, 03:15 PM   #4
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Default I'll try that

Good idea on the PC power supply.

Have you seen any examples where the transistors all go out at the same time as it did here?

Maybe one failed due to old age and took out the rest????

Any comment?

Sid
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Old 4th March 2004, 10:49 PM   #5
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Default Comments galore

Ahh, yes. Old Kenwood amps.

Replace all of the transistors in the power supply. You would not believe how many problems car amp SMPS have to deal with to stay alive - mostly due to poor installation. It is VERY common for one transistor to short B-E then there is a significant current/thermal strain on the remaining units.
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Old 16th March 2004, 06:25 AM   #6
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Default Got It.

Well. I got it. Thanks a bunch everyone who had some advice.

This is something I was not expecting but the board at the bridge circuit was shorting between the traces. All black. This put a very low resistance short across the Xfmr output and caused the transistors to overload and blow. Low current source let me see the smoke in the area without taking out the transistors due to the short.

Cut the board with a knife. Coated with conformal spray. Tunes once again.

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Old 17th March 2004, 02:54 AM   #7
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Default Great!

That type of troubleshooting was very clever and smart. Current limited sources are a must when servicing car amplifiers and your eyes and nose (!) are great tools when searching for problems.
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Old 2nd December 2004, 06:08 AM   #8
horn is offline horn  New Zealand
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Hey, i was wondering if you could tell me how to test the D1062 transistors to see if theyre screwed, and also where abouts I would be able to get replacements from? Thanks
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Old 3rd December 2004, 05:58 AM   #9
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I usually use the transistor tester thingie that RadioShack sells,pretty handy gizmo.. You can also use a VOM/DMM.

I usually first disconnect the SMPS from the amplifier (unsolder a couple of connections on the PSU diodes.) and test the SMPS by itself,perhaps with a small load on it. (100ohm resistor,or something handy on the bench) to make sure it is functioning correctly,if it is,the amp is probably fried somewhere.Sometimes the SMPS will be toast too..
Using an external adjustable +/- supply would be good for testing the amp,seperate from the SMPS

I have an old beatup Majestic 200x2/400x1 amp that I got from a friend to try to fix for him..Looked bad inside,couple ground traces were smoked away..SMPS FET's all toast....Told him it looked grim but I'd see what I could do.
I tinkered with it for a while,and got the amplifier working on a seperate supply..then finally fixed the SMPS..new set of BUZ71A's and a few other parts,and replacing of the smoked traces with some tinned wire..It's been powering my subwoofer for a couple years now.. Granted,I think I replaced almost every semiconductor in it..But I learned alot along the way.

I also have a Kenwood KAC-645 here that a friend gave me.It was working fine in his uncles truck,untill one day,Nothin..it just didn't turn on anymore.So he gave it to me since he knows I'm into electronic junk.
I opened it up,and found that the solder connection on one of the SMPS rectifier diodes had come loose.. 5-minute fix,and I had a new amp
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