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Old 18th July 2012, 09:25 PM   #1
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Default MTX TA 81001

This amp powers up but doesnt produce audio..

Reading some other threads i got some info on the amp..


Heres what i have on the OP-AMP closest to the OPTO_COUPLERS

Pin 1:-13.91
Pin 2:0.23
Pin 3:0.08
Pin 4:-15.50
Pin 5:-13.62
Pin 6:0
Pin 7:14.23
Pin 8:15.05

I have Positive 81 volts of rail and negative 82.2 volts of rail..

I also have + - 15 volts of regulated voltages on the op-amps..

I also have audio on the 2 large SMD Caps near the RCA Jacks C306 and C307

The other thing i noticed is theres no voltage on the gate legs of the power supply fets ..

Any suggestions ?

Last edited by S Merlington; 18th July 2012 at 09:41 PM.
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Old 18th July 2012, 11:13 PM   #2
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This power supply drive the FETs differently. You can't use voltage to troubleshoot it. If you have the tutorial, read the 'MTX Power Supplies vs Other Amplifiers'.

The op-amp appears to be defective but the input is very near the supply voltage so the output may be inverted from that and may not actually be defective.
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Old 18th July 2012, 11:18 PM   #3
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So try replacing the op-amp to start with?
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Old 18th July 2012, 11:23 PM   #4
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Yes. It's difficult to tell for certain (in this instance) if it's defective from the voltages on the terminals.
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Old 18th July 2012, 11:42 PM   #5
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Ok replaced the op-amp with a brand new one ..

Heres what i have now..

Pin 1:-13.64
Pin 2:0.14
Pin 3:0
Pin 4:-15.44
Pin 5:-12.49
Pin 6:-11.44
Pin 7:-12.58
Pin 8:14.99

Still no output..

Is Q405 (C1URA) a muting transistor) if so can i pull it to see if the amp has output or leave it alone ?

Last edited by S Merlington; 18th July 2012 at 11:53 PM.
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Old 18th July 2012, 11:57 PM   #6
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Post the DC voltage on the other op-amp in that area.
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Old 19th July 2012, 12:03 AM   #7
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NE5532

Pin 1:0
Pin 2:0
Pin 3:0
Pin 4:-15.48
Pin 5:0
Pin 6:0
Pin 7:0
Pin 8:15.43
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Old 19th July 2012, 01:01 AM   #8
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Double-check your numbers. This op-amp feeds the other op-amp so the input to the second op-amp shouldn't be significantly different than the voltage on the output pins of this op-amp.

I missed the question about the muting transistor. Don't remove it.
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Old 19th July 2012, 02:52 AM   #9
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trippled checked the voltages and i come up with the same thing as posted..
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Old 19th July 2012, 07:22 PM   #10
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What's the DC voltage measured directly across R402?

What's the DC voltage measured directly across R404?
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