Blown power supply MOSFET's mrv1507

[EnzoG]

I am working on repairing an alpine mrv-1507. The power supply transistors were replaced (irfz48n) at some point on one side. When I try powering up the amp on a 15 amp fuse it makes a sizzle/crackle which sounds like it is on the opposite side of the power supply board from the replaced MOSFET's. The fuse will blow after about 2 seconds. I have the power supply out of the amp and am just trying to get my +/- 50 volts. I have 4 of these amps to compare this one with and a service manual. I am wondering if anyone has any ideas on what could be shorting out and what to start pulling off the board and testing.

 

[EnzoG]

When testing my functional amp I measure 8.19mohms through both sides of the power supply. On the broken one I get 14.47kohms.

 

[EnzoG]

I replaced the power supply transistors and the amp started up and ran. I had to replace the 3 led indicators because they blew at some point and after changing them the amp started and one of the power supply mosfets blew after 2 seconds.

 

[EnzoG]

When testing my functional amp I measure 8.19mohms through both sides of the power supply. On the broken one I get 14.47kohms.

The difference in resistance was because of shorted mosfets. After replacing the power supply fets the amp worked but had no led indicators. After replacing the led's the amp ran for 2 seconds and shorted a fet in the power supply.

 

[Perry Babin]

Was it the same FET that shorted twice?

 

[EnzoG]

I know that it was in the same group of 3 as before but I can't remember if it was the same place. Should I just pull the resistors for that fet and check them or is it likely the driver?

 

[Perry Babin]

Measure from ground to pins 9 and 10 on both driver ICs. Do you read approximately 100 ohms?

Are all of the gate resistors within tolerance? The FETs that are not shorted can remain in the circuit when checking the resistors. Remove the shorted ones.

 

[EnzoG]

On IC601 both pin 9 and 10 are at 74.9 ohms and on IC602 pin 9 is at 74.9 ohms and pin 10 is at 80.6 ohms. After testing the gate resistors I found:
R623,R624: 80.5 and 74.7 ohms
R627,R628: 65.1 and 61.9 ohms
R633,R641,R649,R632,R640,R646: 10.6 ohms
R637,R645,R653: 106.5 ohms
R636,R644: 123.5 ohms
And for R652 I got .688 kohms in and out of the board.
I can't really tell from the blurry service manual but it looks like the resistors are supposed to be 681 ohms?

 

[Perry Babin]

Double-check the solder connections at the junction of R652, R648 and Q615..

 

[EnzoG]

When I pulled all of the solder off the pad it was completely loose. I guess the trace to R648 is no longer attached. How would I fix that without running a new wire?

 

[Perry Babin]

You can use the long lead from a replacement 680 ohm resistor (R652) to reconnect to the gate of Q615. You can use ribbon cable to reconnect R648 to R652 and the gate of Q615. Examples below. Ignore the green arrows.

 

[EnzoG]

I put a wire to the solder pad and ordered some 680 ohm resistors.

 

[Perry Babin]

Don't power it up without the 680 ohm resistor.

 

[EnzoG]

I cleaned up the board and some solder joints while waiting for the resistor to show up. I got the amp back together and powered it up and everything is working perfectly. The amp draws 1.65 amps at idle and led indicators are working as they should. I loaded the amp down at 4 ohms mono until it was drawing ~30 amps and let it run for a couple hours. I would just like to say thank you so much for taking the time to help me with this amp. I'm not sure if I would ever have found the broken trace. It takes an amazing amount of experience and understanding to diagnose a problem so quickly without even seeing the amplifier.

 

[Perry Babin]

To be honest, this is something that any tech who does this for a living could have done. Analyzing the resistance readings told the whole story.