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Old 29th January 2009, 02:24 PM   #1
fongman is offline fongman  United States
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Default old coustic 360 amp

I'm still new at this. I just love to do this kind of work. I'm also learning at the same time. I don't have much of test tool but i do have a Fluke 87 Multimeter, 12DC converter from Radio shack, and a Soldering gun.
Okay to make the story short, I got an old coustic 360 amp. When I first got it, and tested out the power, the fuse blew( this is with the power and ground connected, and as soon as i hook up the blue remote to power). The fuse pop when I leave the blue remote unconnected. The fuse does not pop. Only pop on me twice. The green light still turn on.
I replace it and its okay now but, I can hear a wine or hasssing coming out of the amp. I then open the amp and ckeck all the transistors,resistors,capacitors, and transformers. Everything seem fine. No burn, but I can see that capacitors are melting on the bottom but they are still ok and on the board. So i try touching the amp and its getting hot(only one side with the transformer is on). the transformer and 2 capacitors are hot, sounded like the noise is from the transformer. The out put transitors are hot also.
I now measure the voltage without the remote connectd of 14v, with the remote connected is around 9v. and heats up in 3 mins. I know its got a short and sounds like the transformer is leaking, something is draining the voltage. But i have very little experience. can someone help me. I have check the thread but have not found somthing like this? Help is much appreciated.
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Old 29th January 2009, 04:12 PM   #2
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If the outputs are heating up, the problem is in the audio section.

Measure the DC voltage across the large emitter resistors. They should have near 0.000v DC across them with no audio signal and no load. If one or both channels read higher, reduce the bias current (small potentiometer near the resistors) until the voltage drops to ~0.001.
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Old 29th January 2009, 09:23 PM   #3
fongman is offline fongman  United States
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thanks for the help perrty. I know i said outputs are heating, but i think i could be wrong. They are the ones on the side of the power supply. There are also two more on the other side a total of 4 transitors one side.
I measure the large emitter resistors(the larger one) and is about .5v and 1v. on the other side its -3v and -2v.
I try to adjust the 2 small potentiometer but nothing happens. its still hot. the transformer and 2 capacitors are still hot.
Do you think the power supply is short? why is ther a hassing sound, almost like leaking coming from the power supply? How can i tell if the power supply is good?
thanks for you help.
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Old 29th January 2009, 09:31 PM   #4
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If you have more than a few millivolts across the emitter resistors (one meter probe on each end of the resistor), the outputs are passing too much current and may be shorted.
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Old 30th January 2009, 01:02 AM   #5
fongman is offline fongman  United States
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I will pull them out and test them. Why are they negetive on the other one side?
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Old 30th January 2009, 01:03 AM   #6
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Reversing the probes on the resistor terminals would have given positive voltage.
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Old 30th January 2009, 01:04 AM   #7
fongman is offline fongman  United States
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sorry about the name spelling perry.
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Old 30th January 2009, 01:06 AM   #8
fongman is offline fongman  United States
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I will try to pull them out tonight and post up the # and hopfully find some sub for it.
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Old 31st January 2009, 12:33 AM   #9
fongman is offline fongman  United States
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I took out 4 IRF540 IR9025 2M 8K and measure them. Before i took them out, i measure them and with out the blue remote it was 14v on the base and o on the collector, and emitter. When the remote is on the base was drain to 9v and collector is also the same. Afther i took them out when i measure the 3 pin.
with negative on the base and positive measure 2 ohms. and zero is i had them backward. no ther read if i move the negative and positive on the emitter. Are these bad or did i took out the wrong transistors. I have not pull the big transistors.
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Old 31st January 2009, 01:12 AM   #10
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Do this...

Measure the resistance from leg 1 to either other leg. You should read infinite resistance. If you read anything less than infinity, the FET is defective.

Short/connect leg 1 to leg 3. Measure the resistance from leg 2 to leg 3. It should read infinity in one direction. If you read anything less than thousands of ohms, the FET is defective.

The IRF540s are the power supply FETs. The TIP142/147 are the output transistors.
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