JL Audio 500/1 Rev. 3 Two with same problem

RJohns777 · 16th November 2011, 06:31 PM

Hi. I'm sure someone has seen this problem before since I've got two doing the same thing. Maybe someone can help.

Both are JL Audio 500/1 Rev. 3 amps. Power supply and outputs are fine. Nothing burned. The PWM circuits are doing the same thing. When I power the units, the PWM voltage slowly climbs to 3.2 V and 30.5 KH. The duty cycle is about 20 percent. This is with no inverters in the power supply. With the inverters in the power supply and everything running, the duty cycle is still about 20 percent. The output is noisy and very low power. When I crank the signal input I get more distortion than anything else. Output is noisy even with no signal into the preamp. Nothing is running hot. The controls on the amp are set properly. Signal is from a B & K signal generator and amp is hooked up to a Sencore PA81. I have changed the PWM driver board with a known good one.

Any help would be greatly appreciated. Thanks.

Perry Babin · 16th November 2011, 09:41 PM

What is the rail voltage on the center leg of the rectifier?

Do you have both positive and negative regulated voltage on all of the audio op-amps?

Do you have rail to rail oscillation on both sets of output transistors?

RJohns777 · 16th November 2011, 11:46 PM

What is the rail voltage on the center leg of the rectifier? 83V

Do you have both positive and negative regulated voltage on all of the audio op-amps?
Yes. Actually -13.6 and 14.4

Do you have rail to rail oscillation on both sets of output transistors?Sorry, I'm not sure what you're asking. The outputs are turned on at about 5.5V and I do have a square wave on the outputs of about 58kh. If this isn't what you need to know just explain how to test and I will get you the info.

The output is very noisy even with no signal and with full signal the output is less than 1/2 watt and very distorted.

Thank you for your time on this.

Perry Babin · 17th November 2011, 12:49 AM

I'd expect it to be closer to 80kHz but I don't think that's a problem.

Do you have ~40v DC on all speaker terminals (black probe on the amp ground)?

Do you have a signal like the one in the photo on the center legs (or tabs) of one output transistor in each group?

RJohns777 · 17th November 2011, 12:44 PM

Do you have ~40v DC on all speaker terminals (black probe on the amp ground)? Yes

Do you have a signal like the one in the photo on the center legs (or tabs) of one output transistor in each group? Yes

Perry Babin · 17th November 2011, 08:41 PM

Do you have clean audio on the end of R587 that is NOT directly connected to pin 2 of the NJM2068?

You'll either have to solder a jumper wire to the resistor or find a trace on the bottom of the board that's directly connected to that resistor to check the audio.

RJohns777 · 17th November 2011, 10:11 PM

Do you have clean audio on the end of R587 that is NOT directly connected to pin 2 of the NJM2068? No. Signal looks like the signal you would see from the ringing of a flyback transformer

Perry Babin · 17th November 2011, 11:23 PM

Have you tried operating all controls (pots and switches) through their entire range to see if the audio ever played through cleanly?

Is your signal source ground connected to the ground of the 12v power supply?

RJohns777 · 18th November 2011, 02:11 PM

Have you tried operating all controls (pots and switches) through their entire range to see if the audio ever played through cleanly? Yes

Is your signal source ground connected to the ground of the 12v power supply?Yes. I've worked on 4 of these units this week. Two with power supply and output problems. Both repaired and tested. Worked great. The two with the problem we are discussing have been tested on the same equipment set up. I was suspecting the PWM circuit because of the low duty cycle and all the noise riding on the PWM signal. I (assumed) it was continuing on through to the output somehow. But I could be wrong.

I still have one of the repaired units. I think I will swap the preamp boards and see what happens. That would at least narrow it down. Don't know why I didn't think of that before. I'll let you know.

Thanks for your help so far.

RJohns777 · 18th November 2011, 07:12 PM

I swapped the preamp board with one from a working amp. Problem is still in the amp. Problem is not on the preamp board.

Are we out of ideas?

16th November 2011, 06:31 PM	#1
RJohns777 diyAudio Member Join Date: Mar 2011	JL Audio 500/1 Rev. 3 Two with same problem Hi. I'm sure someone has seen this problem before since I've got two doing the same thing. Maybe someone can help. Both are JL Audio 500/1 Rev. 3 amps. Power supply and outputs are fine. Nothing burned. The PWM circuits are doing the same thing. When I power the units, the PWM voltage slowly climbs to 3.2 V and 30.5 KH. The duty cycle is about 20 percent. This is with no inverters in the power supply. With the inverters in the power supply and everything running, the duty cycle is still about 20 percent. The output is noisy and very low power. When I crank the signal input I get more distortion than anything else. Output is noisy even with no signal into the preamp. Nothing is running hot. The controls on the amp are set properly. Signal is from a B & K signal generator and amp is hooked up to a Sencore PA81. I have changed the PWM driver board with a known good one. Any help would be greatly appreciated. Thanks.

16th November 2011, 09:41 PM	#2
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	What is the rail voltage on the center leg of the rectifier? Do you have both positive and negative regulated voltage on all of the audio op-amps? Do you have rail to rail oscillation on both sets of output transistors? __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

17th November 2011, 08:41 PM	#6
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Do you have clean audio on the end of R587 that is NOT directly connected to pin 2 of the NJM2068? You'll either have to solder a jumper wire to the resistor or find a trace on the bottom of the board that's directly connected to that resistor to check the audio. __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

17th November 2011, 11:23 PM	#8
Perry Babin diyAudio Member Join Date: Nov 2003 Location: Louisiana	Have you tried operating all controls (pots and switches) through their entire range to see if the audio ever played through cleanly? Is your signal source ground connected to the ground of the 12v power supply? __________________ Links >> Basic Car Audio Amp Repair * Basic Car Audio Electronics * New Site * Basic Switching Power Supply Design * Basic Computer Skills << Links

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16th November 2011, 11:46 PM	#3
RJohns777 diyAudio Member Join Date: Mar 2011	What is the rail voltage on the center leg of the rectifier? 83V Do you have both positive and negative regulated voltage on all of the audio op-amps? Yes. Actually -13.6 and 14.4 Do you have rail to rail oscillation on both sets of output transistors?Sorry, I'm not sure what you're asking. The outputs are turned on at about 5.5V and I do have a square wave on the outputs of about 58kh. If this isn't what you need to know just explain how to test and I will get you the info. The output is very noisy even with no signal and with full signal the output is less than 1/2 watt and very distorted. Thank you for your time on this.

17th November 2011, 12:44 PM	#5
RJohns777 diyAudio Member Join Date: Mar 2011	Do you have ~40v DC on all speaker terminals (black probe on the amp ground)? Yes Do you have a signal like the one in the photo on the center legs (or tabs) of one output transistor in each group? Yes

17th November 2011, 10:11 PM	#7
RJohns777 diyAudio Member Join Date: Mar 2011	Do you have clean audio on the end of R587 that is NOT directly connected to pin 2 of the NJM2068? No. Signal looks like the signal you would see from the ringing of a flyback transformer

18th November 2011, 02:11 PM	#9
RJohns777 diyAudio Member Join Date: Mar 2011	Have you tried operating all controls (pots and switches) through their entire range to see if the audio ever played through cleanly? Yes Is your signal source ground connected to the ground of the 12v power supply?Yes. I've worked on 4 of these units this week. Two with power supply and output problems. Both repaired and tested. Worked great. The two with the problem we are discussing have been tested on the same equipment set up. I was suspecting the PWM circuit because of the low duty cycle and all the noise riding on the PWM signal. I (assumed) it was continuing on through to the output somehow. But I could be wrong. I still have one of the repaired units. I think I will swap the preamp boards and see what happens. That would at least narrow it down. Don't know why I didn't think of that before. I'll let you know. Thanks for your help so far.

18th November 2011, 07:12 PM	#10
RJohns777 diyAudio Member Join Date: Mar 2011	I swapped the preamp board with one from a working amp. Problem is still in the amp. Problem is not on the preamp board. Are we out of ideas?

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