Metrik MAB 901

[barber02]

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. I’m not sure if this causing my current problem.

I now have a curious problem I’m 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

 

[Perry Babin]

Positive/negative rail bank?

Does the pulse width for the PS drive decrease as the rail voltage drops?

 

[barber02]

The pulse is present on pin 9 and 10 of the TL494 for about 2 seconds and then stops completely. That’s when the rail voltage begins to sink down.

 

[Perry Babin]

OK. What changes on the various inputs to the 494?

 

[barber02]

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

 

[Perry Babin]

Pin 16 is likely the input where the protection signal is coming in.

 

[barber02]

Ok. I’m 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, you’d never know the amp wasn’t 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

 

[Perry Babin]

Measure the rail voltage with the black probe on the secondary ground and the red probe on the output terminals of the rectifiers.

 

[barber02]

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

 

[Perry Babin]

Do you read the same, no matter whether you use the primary or secondary ground?

 

[barber02]

I’m getting +51.6 and -51.7 on the using the primary ground.

 

[Perry Babin]

OK. I was concerned that you may have had an intermittent primary to secondary transformer short. You may, but it could be intermittent. 50/62 is a huge difference and hard to have due to a mistake if you were measuring on the main rails.

 

[barber02]

The voltages I stated earlier were taken from the center pins of the output mosfets grounded to the primary of the transformer.

 

[barber02]

When I measure the center pin of the output mosfets grounded to the secondary side of the transformer the voltages are almost equal at about 56 vdc + and -

 

[barber02]

Ok. So I pulled the rectifiers and the amp still comes on briefly and the stops oscillation and and never trips the protection LED. Then I decided to bring it up on low voltage and the power supply comes up happy and stays in oscillation. I started at 10 volts and slowly increased the volt and when the supply voltage hits about 12.6 volts the power supply stops oscillating. What gives?

David

 

[Perry Babin]

It's hard to say without a diagram. You need to start at the control IC. See which pin changes in voltage as it shuts down and follow the circuit back to see what's causing it.

 

[barber02]

Bummer. This amp is eating my lunch. But, on the bright side, I’m learning.

David

 

[barber02]

Ok. This is the drive wave coming off of pin 9 and 10 of the TL494 with 12.3 volts of input voltage. If I go higher than 12.5 volts then the chip shuts down. This drive wave doesn’t look healthy. It’s not turning on and off fast enough in my opinion. Pin 3 goes high when I raise the input voltage and shuts the chip down.

With the chip running at 12.3 volts pin 1 is sitting at 1.48 volts and pin 2 is at 3.43 volts. Pin three sitting at .077 volts.

When I raise the voltage above 12.5 volts then pin three goes high at 4.7 volts and pin 1 is at 1.77 volts and pin 2 is at 3.44 volts.

 

[barber02]

This is the area around the TL494 chip.

 

[Perry Babin]

Switch to DC coupling. AC coupling should only be used under the rarest tests.

Are you using your probe set to 1x or to 10x?