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Metrik MAB 901
Metrik MAB 901
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Old 10th May 2019, 05:31 PM   #1
barber02 is offline barber02
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Default Metrik MAB 901

The original problem with the amp was the negative rail bank of power supply FETs were shorted and one dead mosfet on the positive rail bank along with one burned gate resistor on the positive rail power supply mosfet that was shorted. There was also one shorted mosfet in the output section on the positive rail. I replaced the positive rail mosfet pair in the output section with exact replacement components. I then turned my attention to the power supply.

I replaced the burned open gate resistor in the power suapply on the positive rail. The power supply mosfets were Samwin 50n06 and I have FQP 50n06 mosfets in stock but only had enough to completely replace all of the shorted power supply mosfets on the negative rail but could not replace all of the power supply mosfets on the positive rail so I just replaced the one shorted mosfet on the positive side of the power supply. So the the positive side has sw50n06 and one FQP50n06. Iím not sure if this causing my current problem.

I now have a curious problem Iím trying to work through on a Merik 1900 watt mono amp. With 12 volt positive, ground, remote, voltage applied and nothing else connected to the amp the rail voltage starts out at 45 volts positive and negative on either side and slowly drops down and settles out at 4.5 volts and just sits there. This takes about 1 minute. The amp does not go into protection. The board is installed in the heat sink. If I connect RCAs and a speaker, The amp will play music through it to a connected speaker until the rail voltage drops too low.

Does anyone have an idea as to what I am over looking?

Thanks for the help,

David
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Old 10th May 2019, 05:37 PM   #2
Perry Babin is offline Perry Babin  United States
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Positive/negative rail bank?

Does the pulse width for the PS drive decrease as the rail voltage drops?
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Old 10th May 2019, 06:49 PM   #3
barber02 is offline barber02
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The pulse is present on pin 9 and 10 of the TL494 for about 2 seconds and then stops completely. Thatís when the rail voltage begins to sink down.
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Old 10th May 2019, 07:09 PM   #4
Perry Babin is offline Perry Babin  United States
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OK. What changes on the various inputs to the 494?
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Old 10th May 2019, 08:04 PM   #5
barber02 is offline barber02
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Pin 1 is 1.8 volts
Pin 2 is 3.6 volts
Pin 3 is 4.7 volts ( it’s being driven high)
Pin 4 is 0.3 volts
Pin 5 has a saw tooth wave
Pin 7 has 0.0 volts (ground)
Pin 8 has 14.2 volts
Pin 9 and 10 has no oscillation wave
Pin 11 has 14.2 volts
Pin 12 is 13.4 volts
Pin 13 is 5 volts
Pin 14 is 5 volts
Pin 15 is 5 volts
Pin 16 is 8.8 volts

Last edited by barber02; 10th May 2019 at 08:23 PM. Reason: I had pin 16 labeled as pin 15. Corrected
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Old 10th May 2019, 10:49 PM   #6
Perry Babin is offline Perry Babin  United States
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Pin 16 is likely the input where the protection signal is coming in.
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Old 13th June 2019, 06:39 PM   #7
barber02 is offline barber02
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Ok. Iím back on this amp again. My positive rail goes to about 65 vdc and my negative rail only gets to just above -50 vdc before the amp stops oscillating and the rail voltage on both sides begins to sink back down.

The protection LED never gets triggered and the green LED stays lit. Other than the music fading down as the rail voltage sinks back down, youíd never know the amp wasnít working properly. Any ideas on what is causing the mismatch on the rail voltage?

Is this what is triggering the power supply to stop oscillating!

Throughout the amp on different points that I have checked voltages even with the power supply not oscillating and the rail voltages down below 5 volts I am reading 2 to 3 volts higher on the positive rail versus the negative rail.

David
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Old 13th June 2019, 06:44 PM   #8
Perry Babin is offline Perry Babin  United States
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Measure the rail voltage with the black probe on the secondary ground and the red probe on the output terminals of the rectifiers.
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Old 13th June 2019, 07:15 PM   #9
barber02 is offline barber02
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Ok. Well straighten out my reading discrepancies. I’m getting-56 and +55.9. So close enough.

I still don’t understand what is shutting the power supply down without causing the amp to trigger the protection LED.

Thanks for the help Perry
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Old 13th June 2019, 07:20 PM   #10
Perry Babin is offline Perry Babin  United States
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Do you read the same, no matter whether you use the primary or secondary ground?
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