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Old 9th December 2013, 02:03 AM   #1
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Default SS noob questions

So my knowledge isn't up to speed with today's solid state devices. I have an understanding of transistors themselves but not so much experience working with them. I had a friend with a hum coming from one of his speakers in his car with no audio. Each speaker has it's own amp powering it right at the speaker. He dropped by for me to look at it hoping something was obvious but I didn't see anything blown up. The amp has 12vdc in and + - audio in and + - out to speaker. I hooked it up to a 9v battery and it pulls 40mA. It has 4 mosfets which we will call Q1-Q4. The source of Q1 and the drain of Q2 have continuity to the + speaker out. The source of Q3 and the drain of Q4 have continuity to the - speaker out. The drains of Q1 and Q3 got to a big capacitor and the 12vdc in. I haven't traced the audio in but it looks like first it sees a cap first and off to some opamps.

I guess my noob question is where the heck do I start to troubleshoot this thing? This amp is on a rather small 4" x 4" PCB. He said he can get a new for $150 but he was hoping I could fix it with some radioshack parts and maybe I can learn something about transistor amps.
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Old 9th December 2013, 02:42 AM   #2
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Join Date: Nov 2007
Location: Dallas
Nothing active you describe there sounds like it could hum.
Have you listened to the unamplified signal for hum?
Tried a separate 12V battery to see if the hum was in the power?
Maybe an electrolytic cap has gone bad, any measure short?
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