IM7 Amplifier Board Repair

[PacMan]

Hey everybody!

Anyway, I added a subwoofer output to my Altec Lansing IM7 and also added a potentiometer to the output because I stupidly tapped into the speaker level output connected to the built in sub. Anyway, the way I wired the potentiometer allowed the subwoofer amp to short and stop working.

At first the thing would just shut itself off after a split second of playing music so i turned it on and was testing voltages across various points on the amp board (If I sound like I don't know what I'm doing then you guessed right) and one of the subwoofer amps HEXFETs caught on fire and then the rest of the unit started working again. I didn't want to throw it away because they aren't that cheap and 85% of the thing is still working. The only thing that doesn't work is the subwoofer amplifier, everything else works perfectly.

I replaced the part that caught on fire (one of the two IRF7379 on that part of the amp) and it sort of works again, but the subwoofer makes a lot of cracking noises when turned up loud enough to hear it, at low levels it sounds fine. Some other parts around it also look burned but I have no idea what they do. One of them says "KA2" and the other says "Z 2".



Here is a shot of the 7379 that I replaced, the soldering is so ugly because the solder pads under the burned area fell off.



And could it be the other IRF7379 HEXFET causing the problems? I bought several of them in case I messed up in the soldering process. Any input is appreciated, Thanks!.

[Ryukurai]

The KA2 is a switching diode with the anode on pin 1 and cathode on pin 3.

3
[-----]
1 2

It's an MMBD4148, which is the SMD equivalent of a 1N4148 (75V, 300mA, 4nS).

The Z2 is probably a 5.2V Zener. I can't be positive because that particular family covers a lot of voltages (the Z2 should be 5.2V, but, YMMV). It's the same pinout as the MMDB4148.

Hope that helps in the future