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Old 4th June 2010, 03:45 AM   #1
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Join Date: Sep 2006
Default Scosche vr3

Im working on this amp i found 2 bad outputs in it. I removed them from the board and powerd the amp up.

Its producing negative rail voltage. The positive rail voltage starts out at 27 volts then starts decreasing so like i start with 27 volts then it goes to 20 volts then 15 volts all the way down to 3 volts then it starts over agin 27 volts then decreases.

Wondering what might cause this problem and where do i need to check?
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Old 4th June 2010, 05:21 AM   #2
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I did somemore checking and This is what i got for voltages

Power Supply Fets IRFZ44N's
1:0
2:13.92
3:0
No wave form on gates.

TL494
1:1.52
2:3.64
3:4.72
4:4.20
5:1.45
6:3.62
7:0
8:13.92
9:0
10:0
11:13.92
12:13.92
13:5.01
14:5.01
15:0
16:5.01
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Old 4th June 2010, 10:53 AM   #3
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Join Date: Nov 2003
Location: Louisiana
It sounds like the power supply is coming up and is sensing a problem so it shuts down. The voltage drops slowly as the filter caps discharge.

I think you have the voltage on 15 and 16 reversed. Re-check that.

The voltage on pin 4 is too high also.

A photo of the inside of the amp would be helpful. I've never worked on one of these.
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Old 4th June 2010, 05:04 PM   #4
jol50 is offline jol50  United States
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Join Date: Nov 2007
DC offset is one thing, otherwise follow the protection circuits back to see what is shutting it down but that is harder to do I usually fire them up with the meter on different things to catch the obvious such as another bad output.

If you see gate voltage to the PS fets and then it goes away its near always protecting. Does it have a protect LED?
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Old 4th June 2010, 05:05 PM   #5
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Perry,

I had some leaking outputs in this amp with them removed it powers up fine.

The amp is using IRF540's for the outputs. Would ^40's work in this amp also or is that not a good idea?
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Old 4th June 2010, 05:09 PM   #6
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Join Date: Nov 2003
Location: Louisiana
640s?

The 640s are rated for significantly less current. They 'may' be OK for testing but I wouldn't use them as replacements.
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