Question about Pyramid PB779X Car Audio Amplifier

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Parallel connection of power mosfets with two of 5 smoking

In opening this amplifier up, which was supposed to be brand new but had smoke coming out of it from the first time connected to a power source, I found on the one side of output power mosfets a group of 5 IRFZ46N devices that appear to be wired in parallel with a couple exceptions. device #s 3 & 4 in that set of 5 immediately begin burning upon application of a 12V source with a visual of the parts showing a bright red dot approximately in the center of the case area. The wiring, as I mentioned, for all 5 is parallel connections with a 100 ohm resistor into the Gate input also all 5 paralleled with the exception of #3 devices input resistor is not connected to all the other resistor inputs and Gate pins. Thus my ohm meter reads 25 ohms across each of the 4 100 ohm resistors that are parallel connected as expected and the middle one, with no connection on the mosfet side though shorted on the other side with the other 5 reads 100 ohms. Given the identical wiring, would anyone think that those two Mosfets that immediately start burning are just bad components which can be replaced or does anyone think the whole circuit as described is somewhat suspect having all those devices paralleled in the first place? Further, should that middle 100 ohm resistor actually have a connection on the Mosfet side to the other 4 so that the group as a whole are connected the same? I jumpered that connection as an first try experiment which made the resistors measure only 20 ohms, what would be the case with 5 100 ohm resistors in parallel, but the result was still Fets 3 & 4 immediately started burning through their case. No short circuit protection was in evidence based on the fact that the burning continued until power was removed. Further, the amp should not have even turned on as far as I know since no signal at all was ever connected to or applied to the remote input of the amp. My other amp only works when a signal is applied to it's remote input, and I would think they all work similarly, maybe I'm wrong. Any suggestions? Anyone have a decent schematic?

Posted by David Scott on

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The IRFZ46N are n channel high speed switches . They basicly drive the Mosfets. Sounds like you have some problem there my friend, and unless you paid alot for this amp. Its going to need probobly all the n channel switches as well as every mosfet + the bias resistors repalced. Before plugging in the amp jump out the fuse with a 100 watt light bulb.. If the bulb glows brightly when you apply power there is still a short in the unit and could be in the switching power supply section of the amp. If it glows bright and dims, The shorts are most likly gone. The bulb will minimize the amount of current the unit can draw and most of the time wont let the new parts blow out. Check out this service number and see if they can help you. Good Luck Main support number 978-823-0300

Posted on Jul 26, 2006

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