Rockford BD1500

[seankane]

Hello got this amp in for repair. Original complaint was it would not power up. I hooked the amp up and it powers up fine and produces audio.

The amp draws about 7 amps of current which is a bit much. I let it sit on the bench and play audio for about an hour. The amp got extremely hot on the heatsink but did not shut down. It plays audio fine but gets warm and draws a bit much current.

I see a previous repair on 8 of the output fets next to the speaker terminals. The inside of the amp looks like it has been very hot, and there is alot of black soot on everything. Almost looks like it caught on fire at one point the bottom cover is burned and rusted.

The board looks ok just wondering what could cause the current draw and heating problem.

[Perry Babin]

It sounds like it was in a car or house fire.

When you power it up from a cold start, does the power supply side or the audio output side get hot first (at idle)?

[PRR]

Idle current adjustment?

[seankane]

PRR: There is no idle current adjustment on this amp, assuming you are refering to the bias settings?

I will fire it up later and let you know Perry, I have been re-soldering cold joints from the heat that I see as a potential problem in general probably not specific to what I am dealing with.

[seankane]

Hey Perry I cleaned up the board a little and re-soldered all cold joints I had found. The amp "seems" to be drawing less current now. The heating problem is still present.

Upon firing up the amp I have determined that it is the power supply side generating the heat or most of it anyway. The output fets are getting warm but not hot. I did not have it clamped in the sink so I think its normal for the outputs to get warm.

On one side of the power supply 2 fets especially are getting really warm. I attatched a pic of the board. The power supply fets are 75N06's. It looks as if someone has changed all the gate resistors. They are using 10 ohm which I think is a little low. 1 power supply fet is using a 51 ohm gate. Not sure why but maybe this is a problem.

The pink circle is the output section thats getting kinda warm, the red circle is the ps section that getting warm, the green circle is the 2 power supply fets that are getting really warm, and the yellow circle is where someone installed a 51 ohm gate resistor.

Do you think the low value gates and the mis-matched one could cause the heat problem?

[Perry Babin]

The gate resistors are supposed to be 10 ohms.

Compare the gate drive signal on the emitters of the drivers for each group of power supply FETs.

[josesony68]

Sorry for my intrusion, I saw the photo, and I am intrigued by the electrolytic capacitor U14 above, I can not help, ask: why does this capacitor?

[seankane]

I dont know why the junper wire and capacitor are there. I have seen this mod on many Rockford BD amps. I asked Perry previously and he had not seen it before either. This is probably the 10th amp I have seen with this mod. I am not sure what it is for.

[seankane]

I have a strange finding. Q5 had a gate resistor of 51 ohms and Q7 had a gate resistor of 100 ohms. I switched them back to the 10 ohm as they should be and the amp will not power up it has a dead short.

Testing with my multimeter I found Q5 to be shorted. Do you think someone installing the different gate drive resistors allowed the amp to power up even with a faulty power supply fet?

I clipped the fet out of the circuit and powered the amp up again and it powered up fine but one of the torroids whined until the protect light went off. The amp idles normal now and draws about 2.5 amps of current which is normal.

What do you think of all this Perry?

[Perry Babin]

I think there was likely a broken connection between the drive circuit and the gate leg. When you powered it up, the gate charged, the FET turned fully on and failed.