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Old 4th May 2012, 06:24 PM   #1
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Default MTX 801D power source problem

Today I'm working in one MTX 801D.

When is connected, the led don't turn on

The amp maintain drawing 1.6 AMP

the power source only produce 5-11 v + -

I disordering the 4 mosfets 75339p and are OK

No visible damage in all the amplifier

No shorts in the mosfets outputs or driver

the voltages in the TL494c:

1 0.2
2 4.6
3 0.06
4 1.1
5 1.5
6 3.4
7 0.00
8 10.8
9 0.00
10 0.00
11 10.8
12 11.1
13 5
14 5
15 5
16 0.0


the voltages in the LM339M:
1 4.8
2 4.8
3 11.2
4 5.045
5 6.7
6 3.3
7 5.045
8 1.1
9 3.45
10 4.8
11 3.45
12 0.0
13 0.0
14 4.8

May someone help me? Please
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Old 4th May 2012, 06:52 PM   #2
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Are you getting a good drive signal on the gates of the power supply FETs?
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Old 12th May 2012, 01:59 AM   #3
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Sorry by the late response.

Now I can't install the software for mi USB scope in my new computer. (apparently doesn't exist driver for win 7 X64).

ok, lets continue...

I change caps near mosfets, resolder cool solders, and itīs alive again.

+ - 51 V.

but now have another problem.

When amp is running burn the D118.

Click the image to open in full size.

I remove the 4 transistors for measure everyone and are now OK.

But continue burning D118 and R193 its hot too.

I see something rare, R193 its 10K, but when is soldering measure 2 ohms

Any idea where star to search?
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Old 12th May 2012, 02:52 AM   #4
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The transistor would appear to be shorted or leaking.
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Old 24th May 2012, 07:23 PM   #5
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I was working again on this amplifier.

the previous problem wasnīt in the transistor, was in the circuit board, the conductors under the transistor have little metal particles that was shorting, I remove the transistor, the resistor and clean the area, and now it's all fine.

The amplifier doesn't produce sound yet.

I found one operational amplifier with one lost feeding voltage.

Click the image to open in full size.


Where should be producing this voltage?

Last edited by Solosound; 24th May 2012 at 07:31 PM.
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Old 24th May 2012, 07:47 PM   #6
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Is D118 shorted?

Is R193 within tolerance?
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Old 24th May 2012, 08:19 PM   #7
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Of curse!!

D118 was shorted, R193 its 10K same that R194

The operational have now the feeding volages.

And the amplifier have output, but sound distortioned. =(

May you help me plase?
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Old 24th May 2012, 09:21 PM   #8
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Does operating all of the pots and switches through their entire range make a difference?

If not, post a photo of the distorted waveform.
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Old 1st June 2012, 04:26 PM   #9
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My scope isn't ready yet.

In a visual inspection, I found R140 burned, I changed and the noise goes!!

But the audio goes too.

Now is drawing 0.8 ampere without audio.

I see differences in the feeding voltage from operational amplifiers:
Click the image to open in full size.

and the amplifier:
Click the image to open in full size.
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Old 1st June 2012, 06:18 PM   #10
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this is the operational amplifier in which I have doubt
Click the image to open in full size.
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