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Perry Babin 1st October 2020 05:05 AM

What, specifically, do they think is wrong?

objext 1st October 2020 06:22 AM

One person thinks the regulators should be running at exactly equal & opposite voltages. Another person thinks one is being loaded higher than the other, and that there is a problem with the "imbalance of voltage of nearly 2 volts.". One of them I believe is fairly knowledgeable/respected, the other I don't know as well. Neither seem to have much time to go back and forth about it, which I understand.

Perry Babin 1st October 2020 07:10 AM

They should have explained how the regulators work.

As with op-amps or comparators, they use a reference voltage and adjust according to that reference. For these regulators, they are designed to have a 1.25v difference between the output and adjust terminals. They do all they can to maintain that 1.25v difference. Older regulators or lower quality regs will vary more than the 1.25v. The precise offset isn't very important unless you have a circuit where voltage is critical (it absolutely is not for this application).

The two 'programming' resistors are also a factor. The resistors have a tolerance and sometimes drift in value. These make a difference in the way they divide the voltage and therefore affect the final voltage.

If you have the repair tutorial, item #25 on the Audio TroubleShooting page goes into more detail as well as gives a calculator to see how the various components affect the output voltage.

objext 5th October 2020 06:48 AM


Originally Posted by Perry Babin (
Are the waveforms for the power supply gate drive identical on both banks?

You can check without B+ connected.

The wine could simply be the regulator for the rails. If so, it's not a problem.

Dies reducing the bias reduce the drop across the Rx70/Rx71 resistors?

I went ahead with swapping the power supply caps and both regulators for piece of mind (aka glutton for punishment).

Turning the bias pots fully ccw made no change to the voltage drop across any of the Rx70/Rx71 resistors.

Even with the bias fully ccw, the regulators heat the same - 221F for the LM317, 209F for the LM337, 72F ambient temperature.

Reset the bias via current draw method, starting at .98A idle and turning up bias pots individually for .05A rise, ending at 1.18A idle. Regulators maintaining same temperature.

Perry Babin 5th October 2020 09:54 AM

Do you still believe that there is a problem?

objext 5th October 2020 06:46 PM

A fault, maybe not, but I still think it's a problem to get hot enough to damage the PCB. I will reach out and see if anyone else has the same model and can check the temps.

Perry Babin 5th October 2020 07:31 PM

I've seen regulator damage to the boards on home receivers that cost $2k. You saw it on another rockford amp that was in a much cooler environment.

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