Rockford Fosgate 1500BD issues - diyAudio
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Old 30th September 2010, 11:45 PM   #1
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Default Rockford Fosgate 1500BD issues

This fosgate is throwing me in the ringer here....

It had a bad output FET Q25, whilst the rest of them were fine, no shorts.

So before i throw in the money and replace all 16 output FETs, i wanted to at least check to see if the other circuitry is ok.

I removed the shorted FET out of the circuit, and the protect LED keeps kicking on. amplifier chirps at startup. I removed Q120 to disable the protection circuit for testing reasons, and the amplifier is not stable at all.... speaker is dummy loaded at 8ohms and is sitting roughly -60 to -80VDC on the output. and amplifier is making strange screeching sounds, like the amplifier is not stable and out of control...

So i decided to put in an audio signal and scope the output, the output sinewave is present and no distortion/clipping, but there is lots of noise modulated within that signal. and as the amp sits powered up for a few seconds, the signal goes away and the scope fills with streight noise and oscillations, and the amplifier transformers start to scream bloody murder.

I then hooked up my current limited power supply which is capable of only supplying 2amps, and it stays pegged at 2 amp draw, at idle. somethings going on.

So, whats the safest way to test the rest of the circuitry, Remove all of the outputs? or.....

Any ideas? thanks
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Old 1st October 2010, 12:27 AM   #2
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I just found D19 shorted, and a trace off of D19 burned open which appears to go to ground.

What is the function of D19?
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Old 1st October 2010, 12:43 AM   #3
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replaced diode, fixed trace, same issue. speaker terminal starts off at 62v and starts rising towards 70V when the amp pinns the power supply at its max capacity.
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Old 1st October 2010, 01:02 AM   #4
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That diode was likely connected between the shield and primary grounds. It fails when 12v contacts the RCA shield.

What's the DC voltage on all 8 pins of U17?

U17
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:

What's the DC voltage on the speaker output terminal?

Polarity is important. If the voltage is negative, make a note of it.
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Old 1st October 2010, 01:38 AM   #5
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I figured out which speaker terminal is negative/ground. so i used the black probe there, retested it again. its positive. 60v. and it creeps up to almost 90V as the amp gets "louder/more noisey" and more unstable and not safe to operate in favor of the output FETs.

i just so happened to scope the drives to the gates to the fets. the side closest to the speaker terminals the drive pulse looks like a clean square wave, these are dead-key readings with nothing hooked to RCA inputs.

the other side, closest to RCA jacks, the signal goes off the scope range (5v per div) and looks like a HEAVILY distorted square pulse. This is on the opposite driver from which i found the shorted output.

I will get you those readings when i get back into the shop tomorrow. As far as the diode, and RCA shield being shorted to 12V at one time, its very possible, and could likely explain the failure of this amplifier.

I checked the +/-10V and +/-9V and they are on target at all of the ICs.

Last edited by mbates14; 1st October 2010 at 01:43 AM.
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Old 9th June 2011, 07:48 PM   #6
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I still have not been able to figure out this amplifier. pulled it back off the shelf, does the same thing.

Here is the reading from U17:

U17
Pin 1: 0v
Pin 2: 9.1v falls to 5.2v when amp screams somtimes, somtimes stays at 9.1v
Pin 3: 3.2mV
Pin 4: -9.6v
Pin 5: 0v
Pin 6: 3v, falls to less than 1v slowly when amp screams, cant get stable voltages, sometimes starts at 8V, falls to 3v and slowly rises to 5V
Pin 7: 9.6v
Pin 8: 0.5mV
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Old 9th June 2011, 08:20 PM   #7
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U10 pin3, and pin4 is 100 to 80VDC. Something wrong here.
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Old 9th June 2011, 08:41 PM   #8
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U17 appears to be defective.

U10 floats with the speaker output voltage. If you have 100v on U10, you likely also have 100v on the speaker terminals.
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Old 9th June 2011, 08:47 PM   #9
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the +vcc and -vcc pins 3 and 4 float with speaker output voltage?

Because U6 is 9V and -9V like its supposed to be.
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Old 9th June 2011, 09:15 PM   #10
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I am going to put an order in for the LM6171 i guess and see what happens.
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