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Old 7th June 2010, 08:38 PM   #1
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Default Quantum QSA 1500D

Im working on this amp. The amp powers up and produces output but very low.

I turned the gain pot up and the amp will stop producing audio. So i sprayed some contact cleaner in it and turned it back and fourth 100 times.

Still have the same problem the amp will work until i turn the gain up and it will stop producing audio. When i turn it back and fourth when the amp is up and running the test speaker will flutter back and fourth .

So to me im thinking i have a bad pot. Would this be correct?
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Old 7th June 2010, 09:00 PM   #2
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Location: Louisiana
It would seem that you have a bad pot but there are other possible problems. I'd suggest checking the output voltage of the op-amps to see if any have more than a fraction of a volt of DC.
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Old 7th June 2010, 09:08 PM   #3
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Perry,

Want me to post the voltages on all 4 of the op-amps?

The problem is still there with a differnt pot in circuit
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Old 7th June 2010, 09:11 PM   #4
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Yes. All of the op-amps.
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Old 7th June 2010, 09:31 PM   #5
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All of the op-amps are 4558's

1:12.56
2:12.56
3:0
4:-14.66
5:-.01
6:0
7:0
8:1.84


The other 3 read the same.
1:0
2:0
3:0
4:-14.66
5:0
6:0
7:0
8:1.84
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Old 7th June 2010, 10:02 PM   #6
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Location: Louisiana
There's definitely a problem. One op-amp has positive voltage on its outputs but has no positive supply voltage. I would suspect that something is shorting to the output of the amp (solder bridge, etc.) if the voltages you posted are correct.

None of the others have positive supply voltage either. You need to trace this problem back to the 15v regulator to determine where the problem is.
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Old 7th June 2010, 10:15 PM   #7
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Ok if i have have theese right they are using C2073 and A940 as the regulators?

C2073
1:1.00
2:49.0
3:0.40

A940
1:-0.94
2:-49.0
3:-0.34

I dont see any solder bridges at all on the amp. Myabye a shorted output?
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Old 7th June 2010, 10:20 PM   #8
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Those appear to be the outputs.

It's not likely to be a shorted output.

If you can't find the regulators, post a photo of the inside of the amp.
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Old 7th June 2010, 10:26 PM   #9
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Pin 1 and 2 connect to a C2383 and thoose voltages are

1:14.73
2:5.38
3:2.46


I will get the camera out right now and take a pic of the amp
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Old 7th June 2010, 10:38 PM   #10
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Heres the pics of the board.
Attached Images
File Type: jpg Picture 001.jpg (120.3 KB, 39 views)
File Type: jpg Picture 002.jpg (143.6 KB, 39 views)
File Type: jpg Picture 003.jpg (112.7 KB, 39 views)
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