Rockford Fosgate BD500

[seankane]

Hello everyone.

I have a problem with this Rockford BD500.

The amp is stuck in protect. I have did a few checks and this is what I found.

There is no dc on speaker terminal.

The amp is producing rail voltage of 71.4 vdc on the red and black wires.

All op-amps have supply voltage.

Pin 1 of U11 has a nice square wave form.

Pin 7 of U11 has a nice triangle wave form.

Q16 reads as follows: Q15 and Q16 are getting very hot.

leg 1: -8.53 vdc
leg 2: -17.98 vdc
leg 3: -9.78 vdc

U21 reads as follows:

pin 1: -8.53 vdc
pin 2: 7.90 vdc
pin 3: -8.98 vdc
pin 4: -9.78 vdc
pin 5: 4.99 vdc
pin 6: -7.55 vdc
pin 7: 8.45 vdc
pin 8: 9.80 vdc

All output fets have between +9.76 and +9.66 vdc on the gate legs.

Q111: 0.000 vdc drain
Q112: 0.000 vdc drain

Q104: -75.5 vdc drain
Q105: -75.5 vdc drain

Q108: 0.000 vdc drain
Q109: 0.000 vdc drain

Q101: -76.0 vdc drain
Q102: -76.0 vdc drain

Q101: last leg to + speaker terminal is: -0.002 ohms

Power supply fets have 7.82 vdc on all the gate legs but viewing the gate signal with the scope of the p/s fets it looks kinda funny. Very big squares deflecting only upward about 4 divisions.

Any help would be greatly appreciated. Thanks very much.

[seankane]

I suspected U21 and changed it out with a new one and the amp works great.

[Perry Babin]

If the DC voltage on pin 6 is every below 0v, there is no doubt. The IC has failed.