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Old 1st December 2006, 11:51 PM   #1
spooney is offline spooney  United States
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Default amp blows fuses at turn on where should I start looking?

I have an old 4 channel majestic amp (don't laugh I know it sucks) laying around the garage. A buddy gave it to me years ago because it did nothing but blow fuses and said If I fix it its mine.Well five years later I'm finally getting around to messing with it. All wiring to the amp was good so the blown fuses are being caused by something internal.I originally suspected the power supply section but everything seems in order.Where do I go from here?I'm a newb at this and even though the amp is cheap it would make a decent addition to my audio system.I don't have a lot of test equipment on hand....just my DMM. Will the DMM alone be enough to help me fix this thing?
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Old 2nd December 2006, 12:18 AM   #2
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It will provide a good start ,the DMM that is.If the fuse(s) are main fuse(s) check the power supply diodes and capacitors for a short. If the fuse(s) are rail fuses check the output transistors for shorts .
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Old 3rd December 2006, 04:28 PM   #3
spooney is offline spooney  United States
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all the diodes and capacitors check fine.I'm a little confused on the difference between a main fuse and a rail fuse.By main fuse do you mean the inline fuse on the amps power cable or the fuse mounted to the amps chassis?Other than the fuse mounted to the amps chassis there are no other fuses in the amp.The one mounted to the amp blows at turn on.I checked out the output transistors and I believe one to be bad.I 'm getting continuity between the middle leg and the left leg(sorry I don't know the proper lingo) on the one particular transistor and none of the others test that way.If this is indeed a bad transistor did something else down the line cause the transistor to fail or did it just give up on its own?when the transistor went did it take something else with it?any help is greatly appreciated
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Old 3rd December 2006, 05:08 PM   #4
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Chances are that the one transistor that showed a short may be your whole problem.What is the identification number on the device?Most amps have a fuse on the primary side"AC" of the transformer, also power supply fuses for the + and - PS rails "DC".
To accurately check the suspect transistor it needs to be remove or isolated from the circuit . Look up the data sheet for that transistor and check for a short between the collector "C" and the emitter"E"
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Old 3rd December 2006, 07:43 PM   #5
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well i have removed the transistor from the pc board and it still checks with a short between the collector and the emitter. I don't see any more burnt and/or cracked parts so I'm hoping this part is the only culprit. I was thinking of hooking the amplifier up now without the transistor in it to see if it still pops that fuse.From the looks of it that transistor only affects one of the four channels in the amp.Is it a bad idea to try this?Should I just wait until I get the replacement transistor in there to power it up?I'd hate to get it all back together and just be right back where I started.
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Old 3rd December 2006, 08:21 PM   #6
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Rather than powering up without the transistor. Just check for continuity from the collectors from remaining transistors in the circuit to ground should show open or very high resistance. This will tell you that you have removed the short -to-ground that was popping fuses. By the way what is the part number on the transistor?
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Old 3rd December 2006, 09:08 PM   #7
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the transistor has a capital K at the top of it.Directly below that it says D998.From what I could scrape up online the actual part number is KTD998 but they omit the T on the actual part for some reason.I'm having a hell of a time finding a place that will sell me just a few of them though
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Old 3rd December 2006, 09:32 PM   #8
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I'll do a little looking also.
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Old 3rd December 2006, 10:50 PM   #9
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You are certainly right about availability. Another option is to replace all the outputs with more readily available suitable substitutions.
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Old 3rd December 2006, 11:45 PM   #10
spooney is offline spooney  United States
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How do I go about finding a good substitution? Is there some kind of cross reference online that could tell me what would replace what I have now? Each channel appears to have two different transistors assigned to it.One KTD998 and one KTB778 per channel. Will I have to replace the KTB778's with a new substitution as well or will they be ok if I just swap out all of the KTD998's?
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