Rockford Fosgate Punch Power 250.2 PC-0767-J

[RiLoWa]

I have an issue going on after replacing the outputs and drivers in both channels, three of the source resistors and Q316 & 317 (left channel)

I am using drawing No. SCH-0767-J rev.D

When the amp is driven to moderate levels, playing music, bridged or right channel only, loaded with a four ohm speaker, the amp will protect briefly. I see pin 1 of the LM339 drop voltage as a result of pin 7 dropping below pin 6. I'm not finding the defective part causing this and it doesn't seem to do it on a 4 ohm resistive load.

What am I missing?

 

[Perry Babin]

Are the voltage on U301/401 approximately the same.

U302/402?

 

[RiLoWa]

Yes, pin for pin within a few mV.

 

[Perry Babin]

To clarify, you did nothing to the right channel. Is that correct?

If you did not, did you carefully check all source resistors in the right channel to confirm that they're within tolerance?

 

[RiLoWa]

No, I replaced outputs and drivers in both channels.

I checked all source resistors, but only replaced three, in the left channel that go to the 540's.

When setting the bias, I measure 1mV across two of each group of three source resistors, but one of each group of three is lower, ~.6mV or so both channels. How sensitive is it toward a mismatch? The 540's are from the same batch and the 9540's are same batch, both channels.

 

[Perry Babin]

Were the FETs clamped to the heatsink when measuring the bias voltage across them?

It's difficult to accurately measure the low value source resistors. Does your meter have a higher resolution than most meters?

 

[RiLoWa]

Yes, FETs clamped to the heat sink.

I was using a Fluke 179 to measure the voltage drop, but its not reliable for that low of resistance.

 

[Perry Babin]

If you're concerned about equal current sharing, drive a load with 5-10v AC and compare both the AC and DC voltage for the resistors for each group of parallel outputs.

 

[RiLoWa]

I'll do that later on, but I'm not convinced the current sharing is the root of the problem since I have similar imbalance in the channel working correctly with the new source resistors also.

What else should I be looking for?

 

[Perry Babin]

The over-current protection current uses the voltage drop across the source resistors to determine when the amp should shut down. You seemed to be concerned about the difference in voltage across the source resistors. That's why I suggested checking the voltage across the source resistors at higher current levels.

Pin 7 of the 339 is driven to ground by one of the other 4 outputs of the 339 via D4. When there is a fault detected (low voltage, thermal, high voltage), the delay cap is discharged and the delay begins again when the fault clears.

 

[RiLoWa]

I'm not positive it is or isn't the over current protection so I will test as you suggested later on today.

I'll also post the voltages of the 339 later on today with it working and then when it's not for your review.

 

[Perry Babin]

I don't think there's any feedback from the over-current circuit to the 339. Make sure that the voltage on the remote and B+ isn't dropping.

 

[RiLoWa]

The B+ and remote lines were staying steady. The amp does mute (pin1 of the 339 goes to 0V), at ~11.2VDC on B+.

I had noise on the left channel that I didn't notice before. I touched up solder on R389 and that fixed that.

Now after doing that, I can't get the amp to mute like before bridged or right channel only. When driving the amp moderately with audio and a 4 ohm speaker, pin 1 of the 339 does drop as low as ~5.7VDC, but it does not mute anymore. Driving a 4 ohm resistor with audio, pin 1 stays steady at ~16.7VDC.

Here are the measurements across the source resistors as of now. First measurement is at idle, no input signal. Second & third measurements are with ~5VAC across the 4ohm dummy load in mVDC and mVAC respectively.

Left Channel 9540's
RM1: 1.1mVDC 17.9mVDC 23.0mVAC
RM2: 1.9mVDC 20.9mVDC 26.9mVAC
RM3: 1.6mVDC 19.0mVDC 25.6mVAC

Left Channel 540's
RM4: 1.2mVDC 20.4mVDC 28.0mVAC
RM5: 1.5mVDC 22.7mVDC 30.6mVAC
RM6: 1.5mVDC 23.4mVDC 31.4mVAC

Right Channel 9540's
RM7: 1.5mVDC 20.0mVDC 25.5mVAC
RM8: 1.7mVDC 20.5mVDC 26.6mVAC
RM9: 0.9mVDC 16.9mVDC 24.0mVAC

Right Channel 540's
RM10: 0.9mVDC 21.0mVDC 29.3mVAC
RM11: 1.1mVDC 22.4mVDC 30.8mVAC
RM12: 1.1mVDC 22.6mVDC 30.8mVAC



LM339 measurements at idle, no signal.
1: 16.720
2: 5.400
3: 17.870
4: 0.276
5: 2.595
6: 4.990
7: 5.770
8: 4.160
9: 4.990
10: 4.990
11: 7.840
12: 0.000
13: 5.400
14: 5.400

I'll have to capture measurements of the 339 with amp driving a speaker later to show the output pins changes.

 

[Perry Babin]

Is the positive regulated voltage dropping significantly.

Is the negative regulated voltage dropping (towards 0v)?

 

[RiLoWa]

Both regulated voltages hold steady while driving audio into the dummy load.

 

[RiLoWa]

I was not getting the amp to mute with my 4 ohm woofer but when I changed it out to a 2 ohm woofer wired only to the right channel it mutes easily. It does not when wired to the left channel.

Here are the measurements on the 339 I captured while it was muting.

1: as low as 0
2: 4.362
3: 17.23
4: 0.209
5: 2.560
6: 4.989
7: as low as 1.6
8: 4.16
9: 4.989
10: 4.989
11: 7.85
12: 0.00
13: 4.365
14: 4.365

The regulated voltages on U301/401 & U302/402 are about +/- 16.5V when normal and about +/- 17.3V when muted. B+ and remote remain stable as the amp mutes.

 

[Perry Babin]

Does the audio on the left channel drop out when the audio on the right channel drops out?

 

[RiLoWa]

Yes, both drop

 

[Perry Babin]

On the 339, how can 7 drop without 11 or 13 dropping?

 

[RiLoWa]

Unless I am doing something wrong, the only thing I suspect is a defective 339. But if it were in fact defective I would expect the amp to mute when driving either channel.

I have checked all resistors around the 339 in circuit and they are measuring ok.

Maybe the voltage drop on pin 13 is happening faster than I can see on my meter?