Rockford BD1500

[seankane]

Have a BD1500 in for repair. The amp does not power up. Nothing looks obvious, none of the outputs or power supply fets seem shorted.

The amp is sending no gate voltage to the power supply fets and it is not producing any rail voltage.

Niether the power led or the protect led light up.

The SG3526N reads as follows:

pin 1: 1.919
pin 2: 0.002
pin 3: 4.98
pin 4: 0.003
pin 5: 13.23
pin 6: 0.003
pin 7: 0.003
pin 8: 13.23
pin 9: 5.01
pin10: 5.01
pin11: 0.000
pin12: 4.24
pin13: 4.68
pin14: 4.99
pin15: 0.000
pin16: 0.000
pin17: 0.008
pin18: 3.703

I believe pins 13 and 16 are my output why the low voltage on 13 and no output on 16?

I have not removed the board yet to see if there is a burned trace underneath just doing some trouble shooting first.

 

[Perry Babin]

Confirm that you have 12v on both ends of D14. That feeds pin 17 which is the supply pin for the internal logic circuits for the IC.

 

[seankane]

I have proper voltage at either end of D14.

 

[Perry Babin]

Confirm that you have a good connection between the cathode of D14 and pin 17 of the 3526.

 

[seankane]

I have to remove some of the stripped screws today before I can get the board out. Someone stripped out 4 6/32 screws that clamp the MESHA down.

I will see if there is a break in the connection.

 

[seankane]

You will have to excuse me Perry I have once again for the second time read my readings backwards on the IC. The Sg3526N reads as follows.

pin 1: 5.01
pin 2: 0.000
pin 3: 4.24
pin 4: 4.68
pin 5: 4.99
pin 6: 0.001
pin 7: 0.001
pin 8: 0.008
pin 9: 3.702
pin10: 1.918
pin11: 0.003
pin12: 4.97
pin13: 0.004
pin14: 13.24
pin15: 0.001
pin:16 0.003
pin17: 13.25
pin18: 5.01

No output on pins 13 and 16. The amp after leaving power hooked up randomly turns on and off for a few seconds when ever it feels like it.

It did this twice then Q2 went up in smoke and full short to my power supply. I clipped Q2 out of the circuit and re-powered the amp.

Did the same thing randomly turning on and off when ever it felt like it but removing Q2 got rid of the short and all the other power supply fets test fine.

What could be causing this!!!!!

 

[Perry Babin]

Post the voltage on the LM339.

 

[seankane]

LM339

pin 1: 0.008
pin 2: 0.008
pin 3: 13.24
pin 4: 2.026
pin 5: 1.236
pin 6: 4.37
pin 7: 5.01
pin 8: 0.232
pin 9: 0.230
pin10: 5.01
pin11: 8.26
pin12: 0.000
pin13: 0.008
pin14: 0.001

 

[Perry Babin]

Lift D20 and D21, one at a time to see if either allows the amp to power up.

 

[seankane]

I lifted D20 and tried to power up amp with no luck.

Leaving D20 out of the circuit I removed D21 and powered the amp and it powered up fine with the exception of a whiney transformer which could have something to do with one of the power supply fets out of the circuit.

With these results what do you think? What is the purpose of D20 and D21?

 

[Perry Babin]

Those are connected to the thermistors. Check the thermistors.

 

[seankane]

I repaced both thermistors and both D20 and D21.

The amp powers up and idles fine and produces audio with no issues. As soon as I completely tightened the amp down in the heat sink the transformer stopped whining.

Was the faulty thermistor(s) not allowing the LM339 to produce output?

 

[Perry Babin]

The 339 was producing output but the output was shutting the amp down. At least one of the thermistors was telling the 339 that the amp was too hot so it was being shut down.