Rockford Punch 4040DSM

Dr Zeus · 15th January 2011, 04:01 AM

Working on this little guy for a shop, I first noticed the 4x BUZ11s were toast so I replaced them with IRF3205s, but not before fixing a voltage problem at the gate. I found a shorted surface mount transistor (3B / BC856B) from base to collector so I replaced it with an MMBT3906 as that is all I had which was close to the 3B description wise.

Gate voltage now rides at 6.1vDC whereas before it was hitting ~8vDC. I didnt replace them, but all 4 gate resistors are sitting at 100ohms +- 2%.

CR7, a 39v Zener, was shorted so I clipped it off the PCB but have not yet replaced. CR6 was OK.

I didnt clamp down the transistors yet, but already notice that powering the amp off my 5A supply causes the fets to get warm after about 20 seconds.

The amp has +- 20vDC rail voltage.

With inputting sin wave through RCAs - with the scope I was able to see a good sin wave output at all four speaker terminals. I have yet to hook up a test speaker as I really dont want to fry the new 3205s.

Dr Zeus · 15th January 2011, 04:30 AM

I removed the rectifiers and the power fets still get hot pretty quick.

On the scope, the fets are outputting a clean +25v square wave, however the rise side of the square wave has a straight line pointing up to 45v.

Perry Babin · 15th January 2011, 01:14 PM

Did you change the gate resistors to 47 ohms?

Dr Zeus · 15th January 2011, 02:11 PM

No. I guess thats it then? Wow I feel like a dummy.

chipper · 16th January 2011, 02:57 AM

So if I did not change the gate resistors in punch 500 and I installed the new IRF3205s, would I have problems with transistors heating up?

Perry Babin · 16th January 2011, 02:38 PM

Yes.

Dr Zeus · 17th January 2011, 01:30 PM

Replacing the 100ohm resistors with 47 ohm units solved the heat problem with the 3205s. Now when the amp powers up the channel nearest the endplate with the power terminals seems to be striking in-and-out of over-current protection. I dont have an amp meter but I can tell my bench power supply strains for 2 seconds on, 2 off.

I measured the DC voltage across the defective channel's emitter resistor and it keeps flipping from 0.050v to 0.001v every ~2 seconds. After about 15 seconds of the amp doing this I felt heat coming from transistors on the driver board. The output transistors check OK by the dvm.

What is the trick to testing little transistors like this on verticle driver boards? Its sometimes hard to touch all the transistor legs when they are so close together.

Perry Babin · 17th January 2011, 02:40 PM

Bias current?

Dr Zeus · 17th January 2011, 03:00 PM

Well, the bias pots are all full counter-clockwise on all channels. I usually do that so an amp is not easily fried while working with it. I think one of the surface mount transistors on the driver board has a problem; one or some get too hot to touch.

Perry Babin · 17th January 2011, 03:12 PM

Please post a photo of the inside of the amp?

15th January 2011, 04:01 AM	#1
Dr Zeus diyAudio Member Join Date: Oct 2010	Rockford Punch 4040DSM Working on this little guy for a shop, I first noticed the 4x BUZ11s were toast so I replaced them with IRF3205s, but not before fixing a voltage problem at the gate. I found a shorted surface mount transistor (3B / BC856B) from base to collector so I replaced it with an MMBT3906 as that is all I had which was close to the 3B description wise. Gate voltage now rides at 6.1vDC whereas before it was hitting ~8vDC. I didnt replace them, but all 4 gate resistors are sitting at 100ohms +- 2%. CR7, a 39v Zener, was shorted so I clipped it off the PCB but have not yet replaced. CR6 was OK. I didnt clamp down the transistors yet, but already notice that powering the amp off my 5A supply causes the fets to get warm after about 20 seconds. The amp has +- 20vDC rail voltage. With inputting sin wave through RCAs - with the scope I was able to see a good sin wave output at all four speaker terminals. I have yet to hook up a test speaker as I really dont want to fry the new 3205s.

15th January 2011, 01:14 PM	#3
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Did you change the gate resistors to 47 ohms? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

16th January 2011, 02:57 AM	#5
chipper diyAudio Member Join Date: Oct 2008	So if I did not change the gate resistors in punch 500 and I installed the new IRF3205s, would I have problems with transistors heating up? __________________ Feel the PUNCH

16th January 2011, 02:38 PM	#6
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Yes. __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

17th January 2011, 02:40 PM	#8
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Bias current? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links


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15th January 2011, 04:30 AM	#2
Dr Zeus diyAudio Member Join Date: Oct 2010	I removed the rectifiers and the power fets still get hot pretty quick. On the scope, the fets are outputting a clean +25v square wave, however the rise side of the square wave has a straight line pointing up to 45v.

15th January 2011, 02:11 PM	#4
Dr Zeus diyAudio Member Join Date: Oct 2010	No. I guess thats it then? Wow I feel like a dummy.

17th January 2011, 01:30 PM	#7
Dr Zeus diyAudio Member Join Date: Oct 2010	Replacing the 100ohm resistors with 47 ohm units solved the heat problem with the 3205s. Now when the amp powers up the channel nearest the endplate with the power terminals seems to be striking in-and-out of over-current protection. I dont have an amp meter but I can tell my bench power supply strains for 2 seconds on, 2 off. I measured the DC voltage across the defective channel's emitter resistor and it keeps flipping from 0.050v to 0.001v every ~2 seconds. After about 15 seconds of the amp doing this I felt heat coming from transistors on the driver board. The output transistors check OK by the dvm. What is the trick to testing little transistors like this on verticle driver boards? Its sometimes hard to touch all the transistor legs when they are so close together.

17th January 2011, 03:00 PM	#9
Dr Zeus diyAudio Member Join Date: Oct 2010	Well, the bias pots are all full counter-clockwise on all channels. I usually do that so an amp is not easily fried while working with it. I think one of the surface mount transistors on the driver board has a problem; one or some get too hot to touch.

17th January 2011, 03:12 PM	#10
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Please post a photo of the inside of the amp? __________________ Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links

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