Soundstream EGA700.2x up in smoke

[bwilliams]

Hello,
New to forum, green with amplifier repair but know enough to get myself into trouble. Have amplifier which customer hooked up backwards (reverse polarity) and now unit seems to power up but no sound. Reverse polarity diodes test okay, power transistors I suspect from ohmmeter readings are smoked and found area of the PC board deteriorated from corrosion. Does anyone have a schematic for this unit and is there anything else that I should be checking for on this unit that anyone can think of? Thanks in advance.

[Perry Babin]

Is the power supply operating and producing rail voltage?

Did the reverse polarity diodes fail?

[bwilliams]

The reverse diodes are intact as stated earlier however I did find four of the eight MOSFET transistors shorted. They are IRFZ48V and it looks like it may be a bit of a job to find them in Canada. I'll let you know how I make out.

[Perry Babin]

IF the RP diodes aren't burned and there are no burned traces connecting them to the B+ and ground terminals, the amp wasn't likely damaged by a reversed power connection.

You need to replace all of the FETs in the supply if half of them failed.

CLICK

[bwilliams]

Thank you Perry for the information. I had ordered 10 from a company in Texas and received them last night. Great service. I will be replacing all of the power transistors and was surprised as well not to find the reverse diodes blown. The customer said that it worked up until that point. I question whether the whole story is being told, but I can only go by what I see. Hopefully this will cure the problem. I also have to repair some of the eroded PC board. I'll let you know how it works. Thank you for the link as well. Wished I had gotten it sooner but now I have it for future reference.

[bwilliams]

So I carefully replaced all eight power transistors and put it all back together and when we hooked it up, SNAP, went one of the fuses and now it seems like four of the eight have popped again. Not seeing any direct shorts or reason for them to pop. They are still on the board however and I am only using ohmmeter to test transistors but readings are all over the place. I will remove them one at a time tomorrow and see what is going on. Any other ideas as to why they have popped?

[Perry Babin]

Did you have the FETs clamped tightly to the heatsink?

What size fuse?

Pull all of the FETs in the supply and power up the amp. What is the DC voltage on the gate pads for the FETs?

Confirm that all gate resistors are within tolerance.

All voltages should be about the same so you don't need to post each one unless they are significantly different.

[bwilliams]

I removed all the transistors (FETS) and found only one bad one. The 100 ohm resistors (8) all measure 99 ohms and the four 10 ohm resistors all measure 10 ohms. The short seems to have disappeared between the power and ground. There are two thirty amp fuses protecting this unit. The one closest to the middle of the sub was the one that blew. I have not powered it up yet to check voltages but will shortly. I'm wondering if I inflicted this short. There was a section of the PC board eroded away and I soldered a bridge across it and I wonder if the clearance between the board and the case is insufficient and it shorted out there. I believe it was on the positive side. Anyway, I will check those voltages and get back to you.

[bwilliams]

Okay so I put power to the unit and found the voltages were a little erratic but here is what I found. The two FETS on either side were at 0.890 - 0.912VDC and the four on the inside(indicated by green circle) were all at 0.436 VDC. The FET on the left side (inside of board) was the one that blew. Originally though, the four in the middle were the ones that let go. Any ideas?

[Perry Babin]

What is the DC voltage on pins 9 and 10 of the 494 driver IC?