Rockford Fosgate Power 650

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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
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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)

Ok, using a 4ohm load the amp produces a clean sinwave all the way up to clipping (~27v) Adjusting the bias pot produced a nearly identical clipping point.

I adjusted the bias using the tip in the tutorial (by viewing my current meter inline with B+).

The interesting thing is by adjusting the bias to the point of minimal current increase I was able to achieve only ~ -.055v of offset for this channel. I know this isn't perfect, but as I had said before I'm at a loss trying to get it any better than this. Is this possibly due to the 2n6488/2n6491 substitutes?

Is there anything else I should look for? I still have another channel to go after this.
Make sure to check the caps and zeners in that channel.

Is the amp blowing the fuses for that channel?

Like Perry has told me before you cant rely on DC with no outputs installed.

You are sure all transistors and drivers are installed in the correct locations?

Thanks for replying,

I've checked just about everything for this channel. Hell, I've even replaced practically everything in this channel as well. That's what's so fustrating :eek:

She's producing clean output up to clipping now... I guess the next test is for "long term" stability.
I have no problem with that, I dont need that many but want to get them because I am sure we will be repairing these amps for years to come and soon they just wont be available anymore.

There are subs but I like retaining the factory part and look after a repair ya know?
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.