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Re: Climbing DC offset then protection mode
Firstly, check for conductive hold down glue about the output stages. This can cause the problem you have. Or there may be a failing component somewhere in the output stage of that channel. Check for leaky transistors,diodes an capacitors. You will need to remove semiconductors to check for leakage, If you dont have alot of experience faultfinding amplifier power stages, I suggest you take it to a NAD authorised service center.
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Check for shorts in the speaker lines.The easiest way to do this is to disconnect all the speaker lines from the amp. Now, turn it on and see if it goes into protect. If it does,that tells you that the problem is in the amp. If it doesn't go into protect, reconnect the speaker lines one at a time until you find one(or more) that shuts the amp down. That would be your short.
you don't want to do this( bypass the protection0
that d.c. output will burn the coils in your speakers with out making a sound!! you have a biasing problem
or like you said a d.c. offset problem it is best to work on this problem with out a load on the amp, this will allow you to trace back where the trouble is starting at. caps will cause this sort of problem but leaky transistors can also do this, even diodes.
this will require an experienced trouble-shooter.
THIS IS "DC OFFSET". MOST AMPLIFIER PROTECTION CIRCUITS REACT TO THIS CONDITION BY SHUTTING THE AMPLIFIER DOWN. THIS IS A CONDITION, NORMALLY BROUGHT ON BY SHORTED OUTPUT TRANSISTORS, THAT ARE TRYING TO PASS THE POWER SUPPLY DC CURRENT TO THE SPEAKERS, WHICH IN A MATTER OF SECONDS, WOULD BURN THE VOICE-COILS IN THE SPEAKERS. MOST TIMES THIS WOULD BE CHARACTERIZED AS HOOKING THE SPEAKERS UP AND OBSERVING THEM SIMPLY POKING OUT OR IN CONSTANTLY WHICH CREATES MUCH EXCESS HEAT. THIS IS WHY THE AMPLIFIER WILL SHUT DOWN TO PROTECT THE SPEAKERS. YOU CAN TRY TO DISCONNECT THE SPEAKERS AND THE RCA INPUTS THEN POWER THE AMPLIFIER UP. IF THE DC OFFSET LIGHT STILL ILLUMINATES, THEN YOU WILL HAVE TO HAVE THIS UNIT REPAIRED. ONCE REPAIRED, BEFORE REINSTALLING IT, MAKE SURE TO CHECK YOUR SPEAKERS FOR DAMAGED VOICE-COILS, WHICH COULD HAVE BEEN THE PRIMARY CAUSE OF FAILURE. OK. LET ME KNOW IF YOU NEED FURTHER ASSISTANCE....V
I discovered that there is a compatiblilty problem between the Bose receiver and the Sony amplifier. If I disconnect the receiver from the amp then the amp starts up normally. Apparently the receiver sends out a dc offset or a "thump" that the amplifier doesn't like and goes into protection mode. I solved the problem by putting a 1 second delay circuit between the receiver startup and the amplifier startup. A simple RC delay on the remote input to the amplifier solved the problem.
You may have loosened a solder joint on one of the components inside with your air blast. A repair tech would monitor the outputs DC offset voltage and he will probably see one that is not zero volts and while wiggling various things he should see a sudden change in the DC offset voltage on the bad channel when he touches the bad connection. Then it's just a matter of resoldering the connection. Good luck on this one.