Amplifier burning resistor.

[argonrepublic]

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]

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

 

[argonrepublic]

I just replaced all 5200s

 

[tauro0221]

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]

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]

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]

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]

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]

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

 

[thaumaturge]

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

 

[tauro0221]

Hi,
Also Mouser have them for $1.30.

 

[argonrepublic]

I have for lack of confidence in my testing skills, replaced all resistors and transistors again. I would rather do things right then my current component swapping method. I am about to power up amplifier. Glass is half empty and I am expecting a little mushroom cloud. Board is mounted, transistors are mounted to heatsinks. I have powered the amp without fuses and all is well. Can I substitute a lower transformer voltage to look for anomalies or should I hit switch and watch for sparks?

 

[thaumaturge]

If you have a variac you can bring the amp up slow.... assuming no power on delay relay. Next trick is to replace amp fuses with much lower test values. Myself I sub in a current limited supply and see that all is copasetic before bringing things up on the mains.
Doc

 

[argonrepublic]

I don't have a variac. First attapt, I blew fuses. Last time, fuses didn't blow but a resitor in series with output trans burned. Transformer puts 56-0-56 in to each channel. What if I were to feed 25-0-25 and look for heat. I am just guessing here. I like the lower fuse idea. Is current dependent on how hard it is being driven? If so, I could put in some 2a and idle it. The resitor that burned was to the supply of the output transistor.
Thanks

 

[argonrepublic]

Maybe It's getting worse or I made headway. I just flipped switch and all four 5 amp fuses burned instantly. +73v -73v left and +73v -73v right

 

[tauro0221]

Hi,
Try to build light bulb tester like this. It will let power up the amplifier.

 

[tauro0221]

Hi,
To blow all the fuses you must have something very wrong. Leave the fuses out and check the fuses load side to ground for each fuses. See if you a have a short in each one.

 

[gmphadte]

By any chance have u shorted the transistors to the heatsink.

 

[argonrepublic]

YES!!! Some of the transistors had shims behind them!!
I failed to reinstall!

 

[argonrepublic]

Only a few of them were insulated. Is it safe to insulate all of them?