Amplifier burning resistor.

argonrepublic · 08:08 PM

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.
Thanks

thaumaturge · 21st September 2011, 09:13 PM

Check component SC5200 for shorted transistor Q2_8. That would definately smoke R2-24.
Doc

argonrepublic · 22nd September 2011, 03:38 AM

I just replaced all 5200s

tauro0221 · 22nd September 2011, 04:19 AM

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.

argonrepublic · 22nd September 2011, 06:47 PM

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?

thaumaturge · 22nd September 2011, 07:14 PM

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.
Doc

argonrepublic · 22nd September 2011, 07:17 PM

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.

argonrepublic · 22nd September 2011, 08:17 PM

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.

argonrepublic · 22nd September 2011, 08:20 PM

What is a common alternative to MJE15033 (8 amp PNP)

thaumaturge · 22nd September 2011, 08:37 PM

Why? Digikey lists them currently for $1.48 ea.
Doc

21st September 2011, 09:13 PM	#2
thaumaturge diyAudio Member Join Date: Aug 2011 Location: Utah	Check component SC5200 for shorted transistor Q2_8. That would definately smoke R2-24. Doc __________________ Ne timeas a facie mulierum ea ignorare

22nd September 2011, 07:14 PM	#6
thaumaturge diyAudio Member Join Date: Aug 2011 Location: Utah	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. Doc __________________ Ne timeas a facie mulierum ea ignorare

22nd September 2011, 08:37 PM	#10
thaumaturge diyAudio Member Join Date: Aug 2011 Location: Utah	Why? Digikey lists them currently for $1.48 ea. Doc __________________ Ne timeas a facie mulierum ea ignorare

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22nd September 2011, 03:38 AM	#3
argonrepublic diyAudio Member Join Date: Apr 2005 Location: Simcoe Ont	I just replaced all 5200s

22nd September 2011, 04:19 AM	#4
tauro0221 diyAudio Member Join Date: Nov 2008 Location: Victoria,TX	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.

22nd September 2011, 06:47 PM	#5
argonrepublic diyAudio Member Join Date: Apr 2005 Location: Simcoe Ont	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?

22nd September 2011, 07:17 PM	#7
argonrepublic diyAudio Member Join Date: Apr 2005 Location: Simcoe Ont	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.

22nd September 2011, 08:17 PM	#8
argonrepublic diyAudio Member Join Date: Apr 2005 Location: Simcoe Ont	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.

22nd September 2011, 08:20 PM	#9
argonrepublic diyAudio Member Join Date: Apr 2005 Location: Simcoe Ont	What is a common alternative to MJE15033 (8 amp PNP)

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