Rockford Fosgate Punch 600a4

[objext]

I got another 600a4 that needs repair. It "powers on" but only to illuminate the LED - it only draws 0.05A (same as when only remote is given power) and doesn't fully power up. No shorted FETs and no visible damage.

TL494 reads...

Pin 1: 40.4mV
Pin 2: 4.55V
Pin 3: 58.9mV
Pin 4: 99.5mV
Pin 5: 1.487V
Pin 6: 3.367V
Pin 7: 0.9mV
Pin 8: 12.65V
Pin 9: 4.92V
Pin 10: 4.92V
Pin 11: 12.66V
Pin 12: 12.66V
Pin 13: 4.96V
Pin 14: 4.96V
Pin 15: 4.96V
Pin 16: 0.5mV

Any pointers on what to look at? Only getting millivolts at the power supply FET gate legs.

 

[Perry Babin]

It would appear that you have a blown driver section if the voltage from pins 9 and 10 of the 494 isn't reaching the gate resistors. I'm assuming that you checked the gate resistors.

 

[objext]

Finally revisiting this after shelving it for a bit. With just B- and remote I get 5.18V at all power supply mosfet gate legs, but if I try to power the amp normally those readings drop to 0.273v on Q5/Q7 and 0.417v on Q6/Q8.

With only remote and B- I get 0.04A draw.
Normally hooked up I get a small increase to 0.05A.

 

[objext]

Pins 9 & 10 are also dropping to 0.038v

 

[Perry Babin]

What did you do since post #1?

 

[objext]

Nothing to this amp, other than I didn't realize the voltages were different depending on B-/remote vs fully connected.

 

[Perry Babin]

What pins have a different voltage than in post 1 with the same B+ and remote connections?

 

[objext]

With only remote and B- I get 5.198v at all power supply gate legs, which also seems to be decreasing very slowly...5.194v by the time I wrote this. The 494 readings are essentially the same as the first post.

However, with B+ connected, things change to...
Q5/Q7/Q12 gates go to 0.245v
Q6/Q8/Q13 gates go to 0.408v

TL494 reads...

Pin 1: 0.375v
Pin 2: 4.257V
Pin 3: 59mV
Pin 4: 2.565V
Pin 5: 1.487V
Pin 6: 3.354V
Pin 7: 0v
Pin 8: 12.7V
Pin 9: 0.038V
Pin 10: 0.04V
Pin 11: 12.7V
Pin 12: 12.7V
Pin 13: 4.962V
Pin 14: 4.962V
Pin 15: 4.962V
Pin 16: 0V

 

[Perry Babin]

Post the DC voltage on all pins of the LM339.

 

[objext]

With B+ & remote connected, LM339 reads...

Pin 1: -0.602
Pin 2: 3.135 fluctuating slightly
Pin 3: 12.7
Pin 4: 0.323
Pin 5: 2.493
Pin 6: 4.962
Pin 7: 3.561
Pin 8: 4.202
Pin 9: 4.962
Pin 10: 4.962
Pin 11: 7.93
Pin 12: 0
Pin 13: 3.135 fluctuating slightly
Pin 14: 3.135 fluctuating slightly

 

[Perry Babin]

Remove D15. Does the amp power up normally?

 

[objext]

I will try next chance I have. Thanks Perry!

 

[objext]

Removed D15. Amp remains the same, only drawing 0.05A, and same low voltages at the power supply mosfet gates.

 

[Perry Babin]

LM339, pin 7? If it's still the same, check C5 and R17.

 

[objext]

LM339 Pin 7: 9.82V

 

[Perry Babin]

It would appear that the LM339 has a leaky output.

You could try reinstalling D15, lifting terminals 2, 13 and 14 to remove the 339 from the circuit to see if, with those out of the circuit, pin 7 charges.

 

[objext]

Doing that I get 6.043V on pin 7.

 

[objext]

And amperage draw goes up to 0.28A

 

[Perry Babin]

Replace the 339.

 

[objext]

Replaced the 339 with a new one. Amp is back to 0.05A draw and Pin 7 is back to 3.549V.