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daveoxide Unregistered guest | I purchased a non-working Kenwood KM-205 power amplifier for about $15. I figured I could use a little project to work on and learn at the same time. Well, what a PITA this is... The condition I got it in was that it kept blowing its internal fuse. Fine, so I take a look and find 3 or 4 resistors on one channel totally smoked. I replace them and the fuse blows when powered up. Next. I start digging around and notice one of the huge transistors has a hole burnt in it. I replace that one, one other huge one on that channel, and a smaller transistor that is also attached to the main heat sink. Try to power up, and poof, blown fuse. I find a few other small transistors that are internally shorted and replace them. I make some continuity/resistance measurements to compare the bad channel to the "good" channel, and everything seems to be equal. I also replace the fuse with the proper one (I was using a smaller rating fuse, I figured the power on current peak may have been blowing the fuse prematurely). I plug it in, power it up, and yikes, resistors catch fire, transistors come flying apart! All hell broke loose! The 4 resistors fried big time, plus an extra one, and one of the little transistors blew apart! The large transistors are still ok, but all of that other stuff went up in smoke! I am new to this amplifier repair thing, so its a bit frustrating being so unsuccessful. Are some other transistors still shorted out? Is there a stuck relay somewhere? I tested the one that is visible, and it isn't stuck on or leaking. There is this funny thing stuck under a heatsink that I can't get to or see exactly what it is, its about 3/4" square and both power leads go to it and it eventually connects to the huge caps. It seems that there is a direct short somewhere on that one channel, on the other channel nothing is affected. I haven't purchased the repair manual for it yet, but I think that may be my next purchase. Any help or ideas would be greatly appreciated! |
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Gold Member Username: Jan_b_vigneDallas, TX Post Number: 6880 Registered: May-04 | It is impossible to simply do visual checks and repair the problem you are having. There is a reason the resistors and transistors continually fry and you have to find that before you can proceed. After the problem is located and the parts replaced you should be bringing the amplifier up to full power slowly with the ability to watch what is happening in the amplifier. When it begins to draw too much current, you need to shut down everything quickly or else you will continue to replace parts. The problem is most likely in the power supply of the amplifier but without the proper test gear and information you will continue to have a "project" on your hands. |
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daveoxide Unregistered guest | I agree with you fully. Unfortunately my test equipment for this is limited to a semi-decent DVOM. I have read elsewhere about using a light bulb inline with the power leads to help reduce power or to show a significant fault. Could I use a dimmer type switch to do a similar thing? If I had a variac I would use that, but I don't, so I can't. Any other ideas for limiting power? |
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Gold Member Username: Jan_b_vigneDallas, TX Post Number: 6885 Registered: May-04 | No, you need the proper test equipment or else you are wasting time and money. You either need to invest in some test equipment and learn how to troubleshoot properly or else take the unit to a qualified repair technician. The products that have been sold in the last 25 years are similar to automobiles in that the shade tree mechanic can no longer crawl inside the engine compartment and figure out what is wrong using nothing more than a screwdriver and a light bulb. You sound like you are in over your head. Nothing wrong with that, that's just how products are built today. Stop wasting money and either decide the unit is beyond the ability you currently have to test and repair until you get more equipment or just call it a day on this one. |
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daveoxide Unregistered guest | Lets say I wanted to continue with this project and fix the amplifier. What test equipment would I need to troubleshoot my problem? A variac? A scope? The idea of this repair was for me to learn. I wouldn't mind borrowing or purchasing some test equipment to fix this thing. I am reading as much as I can on troubleshooting and repair of amplifiers. There is a nice site at http://sound.westhost.com/articles.htm. Like I said before, amplifier repair is not something I have ventered into before, so everything is still new to me. I am still learning and want to continue to learn, and at this point, I just need the equipment to continue my lesson. |
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Gold Member Username: Jan_b_vigneDallas, TX Post Number: 6897 Registered: May-04 | I think if you read enough about how to troubleshoot and repair and amplifier, you will know what equipment you should have available. The essentials are a scope, a variac, a signal generator, some way to test capacitors and a load resistor. Beyond that you will need vairous devices as the situation warrants. I would go down to a shop and ask their advice on what to buy and where to buy these items. Some larger cities have electronics "fairs" on a regular basis where equipment can be had for much lower prices. |