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shawnk 29th January 2013 03:49 PM

Rockford Fosgate Power 650
I've got another of these 650's here. I'm having issues with the bias/feedback circuit for the the "low frequency" channels (orange/black & yellow brown)

This amp has previously been worked on before and D41D8 + D40D8 have been replaced with 2n6491 + 2n6488 (installed by shifting the transistor over one slot and using a jumper for the emitter)

There were a lot of issues with these two channels, and I have already replaced the majority of the components for them. ie, output fets, drivers biasing pots, several resistors, both op amps, and all zeners.:(

I'm getting ~ -1.5v dc on the outputs for these two channels and I just can't seem to track it down. This is with the bias pot at minimum (fully counter clockwise), but I can actually get rid of the dc voltage on the outputs by adjusting the bias :confused: The problem with this however, is that the output fets are heating up excessively. I'm obviously missing something, but like I've said I just can't track it down. I'm wondering if the 2n6491 + 2n6488 are causing more problems than not?

Thanks in advance

Perry Babin 29th January 2013 05:13 PM

Post the voltage on the op-amp for those channels.

shawnk 29th January 2013 05:39 PM

Hi Perry,

These are the two op-amps for the feedback circuit (I hope that's what you're looking for?)

Op-amp 1 (blue/white & Orange/black)

1) .029
2) -.066
3) 0
4) -17.6
5) 0
6) -.055
7) -.061
8) 18.2

Op-amp 2 (gray/green & yellow brown)

1) .012
2) -.068
3) 0
4) -17.8
5) 0
6) 0
7) -3.4
8) 18.4

Please note that currently there are no output fets installed for one of the low side channels (yellow/brown)


Perry Babin 29th January 2013 05:44 PM

It's strange that pin 7 and pin 6 on op-amp 1 are both negative. Is that one of the channels with excessive offset?

shawnk 29th January 2013 05:47 PM

I noticed that as well.. Yes, this is one of the channels with dc present (orange/black)

Perry Babin 29th January 2013 06:22 PM

It would appear that the op-amp is defective. If you haven't done so, replace it.

shawnk 29th January 2013 06:32 PM

I have swapped them out a couple of times. I just swapped them again and I'm getting the same results


Perry Babin 29th January 2013 06:38 PM

If you clamp everything down and drive that channel to full power (4 ohm load good enough), Does it produce clean audio all the way up to the point where it clips?

shawnk 29th January 2013 06:43 PM

I will try and let you know.... Should I adjust the bias to the point where offset is minimum, or leave the bias pot where it is (lowest setting)?

Perry Babin 29th January 2013 06:49 PM

The bias shouldn't matter but since it only takes a minute to check both ways, do so.

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