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spooney 16th May 2009 05:17 AM

rockford fosgate 200x2
when i received this amp it had blown ps fets and blown irf540's in one of the output channels.It would not even attempt to power on.I replaced the ps fets with huf75337p3's and now it powers on normally at first with a small bump in amperage and then after a few seconds maxes out my 25 amp power supply.I have removed all of what I thought were the bad output fets and the ones in the other channel are good (or so i think) so i guess i'm just wondering where to go from here.Is there something i may have missed in the known bad channel that could cause this symptom?also curious to know if 68.7 ohms is too far out of spec for a 68 ohm gate resistor . All the rest sit at 68.1 . could this be my issue?

Perry Babin 16th May 2009 06:31 AM

68.7 ohms is withing tolerance.

Are the bias pots fully counter-clockwise?

spooney 16th May 2009 03:25 PM

no they are in between a quarter and an eighth of a turn from being fully clockwise

I Am An Idiot 16th May 2009 11:18 PM

Try turning the bias controls fully counterclockwise.

spooney 17th May 2009 03:48 PM

ok the amp turns on with the pots fully counter clockwise and stays on without drawing a ton of current.Looks to be idling right around 1 amp.The amp was obviously opened by someone before me.Do you guys think they tinkered with the pots and turned them up way too high?maybe causing the amp to blow?

I Am An Idiot 17th May 2009 05:21 PM

That could have been the case, also the amp could have went out for some other reason. When you changed the outputs, the bias needs to be readjusted. You should readjust the pots. Perry can tell you how to set the bias properly, but until then the procedure that will get you in the ballpark is as follows, slowly turn the pot up until the amp begins to pull more current. Then back it off until the current goes back to where the current draw was before it went up. Repeat for the other channel.

spooney 17th May 2009 06:05 PM

i haven't actually replaced the outputs yet.Just trying to find out everything that might be wrong with this amp before I order up some parts.I have some irf540n's just need to get those source resistors perry spoke of in the other rf 200x2 post and maybe those other resistors in the output section if they prove to be bad.

Perry Babin 17th May 2009 08:50 PM

Using the current draw will work well enough.

If you want to adjust it a different way, connect your multimeter (set to DC volts) across one of the 0.1 ohm source resistors and slowly turn the pot clockwise until you read ~0.001v across the resistor. Do this with no audio and no load. Let the amp idle for a few minutes and re-check it. The power transistors must be clamped to the heatsink when you set the bias current. If they're not clamped, they can fail within seconds.

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