# Amp repair help,suggestions ?



## jbreddawg (Dec 28, 2009)

I have a Zed built planet Audio amp. The amp looks great overall except for some rust on the bottom plate. The board looks mint ,nothing burnt anywhere. 

When I put power to it the power light and the protection light comes on ,then the power light slowly fades out. 

My questions are what does the protection circuit look for ? I'm pretty handy with a meter but would like to know where to start ? Of course I'm hoping for something simple lol 
It does this without the speakers being hooked up so I know it's not an impedance issue or a short. And it cant be a thermal issue since it does it right away.

Any advice where to start looking ?


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## envisionelec (Dec 14, 2005)

jbreddawg said:


> I have a Zed built planet Audio amp. The amp looks great overall except for some rust on the bottom plate. The board looks mint ,nothing burnt anywhere.
> 
> When I put power to it the power light and the protection light comes on ,then the power light slowly fades out.
> 
> ...


It is something which is triggering the protection to engage. This can be thermal overload (broken thermistor if the amp isn't actually hot), excessive DC offset caused by a shorted output transistor, predriver or opamp or perceived undervoltage.

Most of the time, it's an issue in the output stage.


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## minbari (Mar 3, 2011)

as a tech, these are the hardest kind of failure to fix. (not audio specific, but in general) how do you troubleshoot something that wont come on? lol.

all I can say is good luck. without a scematic, this is gonna be tough.


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## envisionelec (Dec 14, 2005)

minbari said:


> as a tech, these are the hardest kind of failure to fix. (not audio specific, but in general) how do you troubleshoot something that wont come on? lol.
> 
> all I can say is good luck. without a scematic, this is gonna be tough.


95% of the amps I see don't "come on" and it takes about 15 minutes to troubleshoot.


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## ATOMICTECH62 (Jan 24, 2009)

I think you will find a shorted output transistor or mosfet.Im 95% sure.


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## jbreddawg (Dec 28, 2009)

Thanks, that gives me a place to start !


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## spooney (Nov 5, 2010)

I had a zed amp on my bench recently that wouldn't power on without going into protect. Somewhere along the line the shield ground in the amp had been destroyed and when you tried to power on the amp it would cause a huge dc spike on the speaker outputs and shut the amp down. I found the break on the board and repaired it and the amp would power on normally after that without any crazy dc spike. Not saying thats what you have but its worth a look. Do you have continuity between the rca shields of the amp and the non bridging speaker terminals?


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