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Should I do it or Not??? Tech question Part 2

I just want to thank everyone who replied to my post and also let them know that I went ahead and replaced the four power transistors myself. A few weeks ago I posted this topic (see below copied from my original post).

"I accidentally shorted out the speaker cables connected to my Creek 5350(not SE version). There are 2 fuses that I replaced(same rating of course) but it keeps on blowing up every time I turn on the power. I contacted Creek Audio and told them about the problem. Here is their reply:

"The fuses inside the 5350 are in series with the power supply. They are there to protect the power supply in the event that there is a fault with the amplifier that would cause it to draw unlimited current. Shorting the output has unfortunately blown the power transistors on that channel. They fail as a short, which is why the fuses have blown. I'm sorry to say you have been unlucky, but will need to have the power transistors replaced."

I replied back telling them that I can replace the power transistor myself(I already bought the parts) and this is their reply.

"Dear Sir,

This is not the type of repair that can be carried out by the public so I would advise that you contact the distributor for help in getting the unit fixed.

Without seeing the amplifier I would not be able to say if it was just the output transistors and even if it was, you will need to be able to power the unit up on a stabilized power supply before adjusting the BIAS current to avoid damaging the amplifier again."

Here is my question to the tech savvy out there. Should I go ahead and replace the power transistor? I don't know what "you will need to be able to power the unit up on a stabilized power supply before adjusting the BIAS current" involves, plus I don't have any of that equipment other than a volt meter. What are the chances that it'll blow up again if I skip those steps and just replace the power transistor? This amp is probably worth $300 now, if I send it in for service, it will probably cost me $200 plus shipping it back and forth, so I'm willing to gamble on it if my chances are good."

royy

Showing 1 response by minorl

minorl
1,014 posts
 
You should check and adjust the bias anytime you make repairs or replace components. That is the correct thing to do and it is very simple to do. Since you changed the output transistors yourself, you already know where he output emitter resistors are. Place a voltmeter across one of the output emitter resistors (with the unit off and unplugged, with the dial on voltage, plug the unit in, turn it on and measure the output voltage in millivolts. Ask the manufacturer what the bias current or voltage should be through or across the output emitter resistor. If they give the bias in current, then you want to check the voltage = IR, where I is the current value they give you times the resistor value. If they give the value in millivolts, then simply read the voltage. The bias potentiometer should be adjusted after the device has sufficiently warmed up.

The output transistors are biased for a reason, to not check and rebias is a mistake.

enjoy
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