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Old 20th June 2010, 10:09 PM   #1
shawnk is offline shawnk  United States
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Default Help with Rockford Fosgate Power 650

Having some trouble trying to fix this amp.

The amp is drawing excessive current when powered up. I've found if I remove one of the voltage regulators (MJE2955T) the amp seems to power up fine. I'm currently using a bulb on the 12V lead for current limiting. The resistor and diode associated with the regulator appear to be fine as they read the same voltage as the other side of the amplifier. This lead me to believe, although I'm not sure how accurate, that there was possibly a faulty op amp so I removed them yet there's still high current draw. I'm not sure if I'm even going in the right direction here. Any help would be greatly appreciated!

Shawn

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Old 20th June 2010, 11:15 PM   #2
timwebb is offline timwebb  United States
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are the drivers shorted?
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Old 20th June 2010, 11:32 PM   #3
shawnk is offline shawnk  United States
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Hey.. thanks for the reply.

I've checked all of the remaining drivers that are still on the board (you may have noticed I've removed all of the drivers from one side) and none of them appear to be shorted.
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Old 20th June 2010, 11:35 PM   #4
timwebb is offline timwebb  United States
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u can take a pic of the whole board?
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Old 20th June 2010, 11:52 PM   #5
shawnk is offline shawnk  United States
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Here ya go. You may see where I have already removed a few of the op amps.

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Old 21st June 2010, 01:06 AM   #6
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If you remove all of the fuses, does it draw excessive current?

What's the DC voltage on the 3 pads for the 2955 (2955 out of the circuit)?

Is the 2955 defective?
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Old 21st June 2010, 03:15 AM   #7
shawnk is offline shawnk  United States
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Hi Perry. Thanks for the response!

With the fuses removed there's still excessive current.

Here's the voltage on the pads with the 2955 removed

Base: -42.5

Collector: -42.5

Emitter: 8.9

These readings were taken with a 2.9ohm resistor inline with B+

I checked the 2955 using the technique in the tutorial and it looks like the transistor is fine.

Thanks for the help!
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Old 21st June 2010, 04:04 AM   #8
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The Zener diode connected to the base of the 2955 is open.

If the Zener opened, the op-amps in the audio section of the amp have either failed or will fail. All should be replaced. These boards are very easily damaged. I'd suggest cutting the legs and removing them one by one (Tech Tips 6, item #8).

Does it draw excessive without the 2955?
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Old 21st June 2010, 04:39 AM   #9
shawnk is offline shawnk  United States
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OK. Thanks Perry.

I'll have to order up some op-amps and replace the diode.

The amp powers up fine with the 2955 removed.
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Old 13th July 2010, 12:40 AM   #10
shawnk is offline shawnk  United States
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Things are looking a little better for this amp!

I've replaced all of the op-amps (even used sockets :-) ). I also replaced the zener diode for the voltage reg. and I'm having better results.

Three channels appear to work well with undistorted output.

One channel has distortion. This is one of two channels where I had replaced all of the output fets and drivers. I havn't pulled any of the new components just yet for individule testing, but I did notice that the bias pot has "no" effect for this bad channel. However, the pot by itself still tests fine......

Lastly, the amp draws approximately 1.5 amps at idle. I don't think this is excessive, but honestly I'm not sure if this is exceptable for this particular amp and if it needs further investigation?

Thanks
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