American Pro 1200w 4 channel VS 1596

[barber02]

I have a question about an American Pro 1200w 4 channel VS 1596 that I’m working on. I seems apparent that this amp has been worked on before. It’s pretty old. The amp immediately goes into protection when powered up. The power supply never fully comes on. There is 12 volts sitting on the center pins of the power supply mosfets. I have check all of the mosfets for shorts in both the power supply and output section. Pin 3 is being driven high (4.8) volts. I took a closer look at the output mosfets and noticed something odd. The right bank of output mosfets do not march up with the left bank of output mosfets. The numbers on the middle mosfets are different on the right bank than on the left bank. The left bank are all D718 mosfets and the right bank has D718 and B688 mosfets. I have a picture attached. My instincts are telling me that the right bank is wrong and it should match the left bank which are all the same, BUT even though the right side bank of mosfets have the same numbers the two in the middle look different than the two outer ones. Does anyone have any idea what the correct configuration should be for these output mosfets. Your input is much appreciated. Thank you for your time.

David

 

[Perry Babin]

Pin 3 of what transistor is being driven high?

 

[barber02]

Pin 3 of a KIA 494AP IC.

 

[Perry Babin]

Post a photo of the area around the 494.

 

[barber02]

This the area around the 494 IC. Maybe it’s a good enough photo.

 

[barber02]

This is a picture of the left side output transistors. The mosfets have matching numbers but they are slightly different indicating a possible swap out at some time in the past.

 

[Perry Babin]

You can't have the same part number in both locations for each channel. The NPN transistor will have positive rail on its center terminal. The PNP transistor will have negative rail on its center terminal.

 

[barber02]

Ok. So the left bank is wrong. That makes sense. I’ll change the left bank tomorrow. It’s pretty late here.

Thanks,
David

 

[barber02]

Ok. I finished up with this amp. Corrected the output mosfets on the left side so that there is a NPN and PNP pair for each channel. Amp booted up and works great. Thanks for the help Perry.

David

 

[Perry Babin]

Something that I forgot about was the solder connections on this board. This is a CEM board without copper vias. However, they use it as it is a standard 2-layer board, soldering on both sides of a lead that passes through the board. This type of design has a high failure rate. Any time you take an amp in that has this type of design, expect to have trouble from those connections. Some of the soundstream (newer) are the worst. You can't just resolder them and repairing them right (not necessary in this amp) can be VERY time consuming.

 

[barber02]

Thanks for the heads up. I did notice copious amounts of solder at all of the through holes on this board. I thought it to be odd. When I changed out the mosfets they drank up a considerable amount of solder. In fact I had to solder some of the holes twice because of small hole pockets around some of the legs.

David