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Old 21st September 2011, 08:01 PM   #1
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Default Amplifier burning resistor.

I recently repaired this amplifier. It was shutting down and blowing fuses. I replaced all output transistors, fuses and a lot of resistors which were recommend changes by Manufacturer. Could you please hep me to determine why R2-24 is still causing a problem? I powered amplifier without speakers or input. R2-24 burned in 1 second. I turned of amp immediately.
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Old 21st September 2011, 09:13 PM   #2
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Check component SC5200 for shorted transistor Q2_8. That would definately smoke R2-24.
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Old 22nd September 2011, 03:38 AM   #3
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I just replaced all 5200s
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Old 22nd September 2011, 04:19 AM   #4
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Hi,
Check resistor R2_14. It is possible is open. That will make the base of Q2_8 positive. I think it should be negative to disable the Q2_8. To check it remove R2_24 and check the voltage at the base of Q2_8.
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Old 22nd September 2011, 06:47 PM   #5
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R2-14 is definitely open. It was a new resistor yesterday. Thanks for your expertise. The traces are very tiny so I will look for excessive solder and study the schematic to the best of my ability. I would sure like to know what caused R2-14 to burn as well.
What are the possibilities?
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Old 22nd September 2011, 07:14 PM   #6
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Trust me on this, new or not if R2_14 is open then SC5200 is bad. Shorted Q2_8 puts 73 volts right across R2_14. No other path does that can be seen on schematic as posted. Measure ohms between base of Q2_8 and ground to check out the SC5200. Bet you find a dead short.
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Old 22nd September 2011, 07:17 PM   #7
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Thanks so much. Is it possible that I have oversoldered? Otherwise what would cause SC5200 to short? Thanks so very very much. You are really helping me to understand.
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Old 22nd September 2011, 08:17 PM   #8
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From what I can see R2-14 is parallel to Q2-2 (cutoff schematic) I am 90% certain that Q2-2 is bad. I am going to pull Q2-2 and Q2-8 and check them both.
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Old 22nd September 2011, 08:20 PM   #9
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What is a common alternative to MJE15033 (8 amp PNP)
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Old 22nd September 2011, 08:37 PM   #10
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Why? Digikey lists them currently for $1.48 ea.
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