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Old 23rd August 2012, 10:38 PM   #1
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Join Date: Jun 2008
Default MTX 251D power source problem

hello friends.

I have problems with MTX RT251D

The last week the sound was dirty, I measure all components and I changed Q110 (was leak) and the sound was clean again.

But after a few hours of use the sound is like shorted.

All the components in the amplifier appear be fine, but the voltage in the center legs of the rectifiers are just + - 9v

there are the voltages on TL494

1 0.09
2 4.6
3 0.06
4 3.9
5 1.5
6 3.4
7 0.02
8 11.7
9 0.007
10 0.007
11 11.8
12 11.8
13 5
14 5
15 5
16 0.008
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Old 23rd August 2012, 11:40 PM   #2
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Join Date: Nov 2003
Location: Louisiana
It appears that it's in protect mode. Post the voltage on the pins of the LM339.
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Old 30th August 2012, 10:30 PM   #3
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12.2 v applied in the positive and remote, no audio and no speakers connected.


1 7.3
2 10.8
3 11.6
4 0.78
5 3.8
6 1.28
7 3.8
8 3.6
9 5.3
10 5.3
11 7.5
12 0.00
13 7.4
14 7.4
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Old 3rd September 2012, 05:15 PM   #4
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Any idea Perry?
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Old 4th September 2012, 05:35 AM   #5
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Join Date: Nov 2003
Location: Louisiana
I don't see a problem with the input voltages to that IC. Is there a PNP transistor with its collector connected to pin 4 of the TL494?

If so, lift it and see if the amp will power up. Be careful because this will defeat the protection circuit. The remote on/off may not work so be prepared to disconnect B+ in case there is a problem.

Does the amp power up and produce clean audio?
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