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Old 27th June 2002, 10:52 PM   #1
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Exclamation Leach amp blowing NPN output transistor!

I have built a Leach amp one year ago and it works fine without any problem. But a few days ago it blew negative power supply fuse on right channel. I changed fuse and it blew another time. I found that two transistors MJ15004 were blown. I changed them and amp have worked fine for one day and then it happened again. The same transistors are short-circuited from colector to emitor. The other channel works fine.

What the hell is wrong with him?

Supernet
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Old 27th June 2002, 11:21 PM   #2
swede is offline swede  Switzerland
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The first thing that come to my mind is: "counterfeith transistors".

The second one is: "oscillation"

Where did you get your transistors? Local store? Direct from OnSemi? That guy with the trenchcoat at the corner of your street?

My first attempt to build a Leach Amp ended in disaster and some blown up transistors and some capacitors. This was due to some fake transistors (cheap copies) I bought in a local shop.

Did you connect a Zobel network between the speaker binding posts?

Are any other transistors damaged on the same side? How about Q15, Q17? Did you use thoose "hard to get" 2n5415 for Q15? How about the biasing? Did you measure the idle current to 100mV for each rail?

I think we need some more background information before we can help you. I would go for pure OnSemi devices if you are unsure of the quality of your output devices.

Best regards,
//magnus
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Old 27th June 2002, 11:35 PM   #3
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I used original transistors from Motorola, which are now replaced by ON semiconductor also I got all transistors from Leach page at local electronic shop. Everything you mentioned is fine,but I will once again check driver transistors.

Thanks fo help!!

Supernet
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Old 29th June 2002, 11:54 AM   #4
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Check your bias diodes for cracks. If you crack one, you'll unexpectedly blow power supply fuses... and if your output transistors aren't burly, you'll blow those too.

Will
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