Rockford Fosgate Power 400a4

[Oppenheimer]

All measurements: 0.1 V/Div 5 uS/Div
Using probe ground on non-bridging speaker terminal as reference.

No treble boost on this amplifier.

Adjusting gain does not change anything.

Low pass and full pass positions do not change anything.

 

[Perry Babin]

Check the bias on ch3.

 

[Oppenheimer]

0.778 mV

 

[Oppenheimer]

I haven't moved the pot positions since adjusting them to 1 mV before bench testing today.

 

[Perry Babin]

Does the bias setting make a change in the noise that you see on ch3?

 

[Oppenheimer]

Yes, it flattened out to resemble Ch 4 above then as soon as the P.S. ammeter bumped up it changed to the image below and stayed that way until I cycled the remote.
1.05 mV across the resistor in image, after I cycled the remote the voltage across the source resistor read 0.04 mV although the pot was not adjusted.

0.1 V/Div and 5 us/Div

 

[Perry Babin]

It appears that the oscillation is what's causing the increase in current draw and it doesn't stop until the supply is shut down.

Which channels did you replace the output transistors in?

 

[Oppenheimer]

I didn't do anything to the power amplifier section on this amplifier, other than the two source resistors I replaced last night in Ch 4.

I replaced the power supply FETs and the 2G Transistors Q2 and Q3 in the power supply.

The output FETs may be original, 540s and 9540s, I really couldn't see any evidence that they were changed before.
I did test them initially for shorts when I first got the amplifier.

 

[Perry Babin]

Are the resistors R378 and R478 near the speaker terminals within tolerance.

 

[Perry Babin]

Pin 1:

 

[Oppenheimer]

They all read a little out of tolerance.
R178: 4.17 ohms
R278: 3.03 ohms
R378: 3.12 ohms
R478: 3.08 ohms

Which component for Pin 1: in post #50?

I will have to get back to it this afternoon when I get back in.

 

[Perry Babin]

I didn't intentionally post #50.

Do you have a capacitance meter to check the capacitors in series with those resistors?

 

[Oppenheimer]

I have a Fluke 187 that will measure capacitance, but I'm not sure how well in circuit.

I also have, if you squint real hard, you might loosely call an esr meter (MESR-100). It is supposed to test esr in circuit and I have actually identified bad capacitors before in circuit. It also seems to work okay when comparing known good capacitors to other capacitors of the same value that are in question.

 

[Perry Babin]

With the resistors out of the circuit, the capacitor is essentially out of the circuit.

 

[Oppenheimer]

So pull Rx32 and Rx78 to check Cx21 and Cx28?

 

[Perry Babin]

Cx28 was the one I was asking about. You only need to desolder one leg of the series resistor.

 

[Oppenheimer]

They are all smd components so unless the resistor would stand a better chance of resisting damage by removing and re-installing I will just remove the capacitors to test. I guess it's six of one and half a dozen of another.
What do you think?

 

[Perry Babin]

Whatever works best for you. The resistors are likely more rugged. Heating them and sliding them off of their pads is quick and easy.

 

[Oppenheimer]

With Rx78 removed.

Meter with open probes reads 0.38 nF

C128: 0.086 uF
C228: 0.077 uF
C328: 0.086 uF
C428: 0.079 uF

I did find R278 out of tolerance, reading 4 ohms and should be 2R7 ohms

 

[Perry Babin]

Is ch3 still the noisiest?

If so, does the removal of R378 make a difference in the amplitude of the noise?