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mike49504 22nd November 2008 12:25 AM

rockford fosgate 1500 bd
Im working on a fosgate power 1500 bd. 6 of the outputs are blown. they are mtp75n05hd wondering what a good cross is to theese? also there is a total of 12 do i need to replace all 12 or ust the 6? and 1 more question do i need to check anything else in the amp before im done?

mike49504 22nd November 2008 12:55 AM

i found the irf3205z will cross. Is this a good match? also do i need to check anything else in the amp?

mike49504 22nd November 2008 05:44 AM

well i over looked it before but someone had fun with this amp.

in the output section they took out 11 of the surface mount resistors.

wondering if anyone has a schematic for this amp ?

not sure fosgate would like me asking them to tell me what are 11 resistors values are

Perry Babin 22nd November 2008 06:50 AM

Are you referring to the power supply FETs?

If so, the 3205 is a good sub.

The gate resistors are 10 ohms.

You also need to check the MPSA56 driver transistors. There are 2 sets of drivers. One set is near the center of the board. The other set is in a group of capacitors near the transformer.

If you have 11 burned gate resistors, ALL of the power supply FETs need to be replaced. The gate resistors won't burn unless the FET has failed.

Rockford will provide schematics for amplifiers that are out of production (as long as the circuit isn't still in use in the new amps -- that was the case for the BD amps, anyway).

mike49504 3rd December 2008 05:31 PM

I checked the 10 ohm gate resistors. 5 of them read crazy numbers like .682 and 126.5 . Im testing the resistors off the board. So im assuming theese are bad. Should i replace all of the gate resistors in the amp ? the other resistors read fine but since 5 read bad. wondering should i replace all of them?

Perry Babin 3rd December 2008 05:47 PM

I would replace all of the gate resistors for the power supply FETs (especially since you've already removed them).

mike49504 11th December 2008 09:59 PM

ok i replaced all power supply fets and the gate resistors.

Figured out and replaced all resistors that wer missing in the output side.

went to power the amp amp and the transformer makes a loud screeching/hissing noise wondering what i can do to fix this problem or what else do i need to check?

Perry Babin 11th December 2008 11:39 PM

Does the amp work other than the hissing noise?

Does the protect LED go off?

Is it drawing excessive current?

Is there any DC on the speaker terminals?

mike49504 12th December 2008 04:20 AM

the protect light doesnt go off. and when i was about to measure to see if there was any dc on the speaker terminals the amp blew 4 power supply fets . i used the irf1018e for the power supply? wondering if the fosgate amps like theese or should i switch to something else?

Perry Babin 12th December 2008 04:40 AM

I've never used those FETs but it looks like they should work. I generally use IRF3205s.

It's possible that they failed from any of the following:

* Bad connections between the drivers, gate resistors or FETs
* Defective driver transistors
* Defective FETs (damaged during installation)
* Excessive current draw from the audio section (you didn't say how much current it was drawing).

Before you power it up again, disconnect the red/black twisted wires (from the power supply side of the amp). This will disconnect the power supply from the audio section and you'll be able to test the basic operation of the supply to determine if it has any other problems. Before desoldering them, measure the voltage directly across the wires. If there is more than a few volts, discharge the caps (by shorting across the red/black wires with a large low value resistor - a dummy load works well).

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