JL Audio 500/1 Amplifier Repair Help

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Audioguy564

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Joined 2009
#1
I blew my JL 500/1, It had a blown power supply and output devices. I replaced the power supply and have a perfect 35Khz pulse driving the power supply Fets. +/-15 regulated voltage is present on the op-amps. Replaced the output IRF540's and now I have 80 volts on the drain of all four fets. It's like the output is no longer switching. My questions to you guys are.

If there is a 5volt regulated voltage on this amp, where can I probe to verify its present?

Have any of you ever run across this problem and what was your solution. Is there a typical part that will fail causing the channel not to operate correctly?

the voltage on the output connector referenced to ground is 80volts on both the negative and positive terminal with 0volt DC difference between the two.

Any help will be appreciated! Thanks in advance.
 
#2
Check the connections on all of the busbars. With the amp on, use a non-conductive probe to move them. If they have broken connections, you should see a spark where they go into the board.

If none are broken, post the DC voltage on all 3 legs of all of the output FETs.

Q504
Pin 1:
Pin 2:
Pin 3:


Q505
Pin 1:
Pin 2:
Pin 3:


Q506
Pin 1:
Pin 2:
Pin 3:


Q507
Pin 1:
Pin 2:
Pin 3:
 
#3
all the rails look good touched them up anyways. Here are the voltages you asked for. Thanks for your help.


Q504
Pin 1:86.67
Pin 2:82.16
Pin 3:82.15


Q505
Pin 1:0.13
Pin 2:82.15
Pin 3:0.00


Q506
Pin 1:2.33
Pin 2:82.18
Pin 3:0.00


Q507
Pin 1:85.79
Pin 2:82.15
Pin 3:82.19
 
#8
I replaced the IC and now the amplifier is drawing excessive current. I measured the fets with the power off and they measure right, before the IC they were a bit off. I have my power supply limited at 10 amps and it's current limiting. Is there any tricks to troubleshoot the channel without the output devices? so that I can eliminate the excessive current draw?
 
#9
Did you clean up the solder connections and examine them with a lighted magnifying glass to see if the connections were good and there were no bridges on the IC?

Is the IC getting hot?

Did you check all of the low value resistors near the outputs (along the edge of the board and all of the ones inside of the rail caps - near the IC) to confirm that none were out of tolerance?

Is any one of the 4 output transistors getting hotter than the others?

I'm not sure what will happen if you pull the outputs and power it up (don't know if you would get any sort of switching drive signal) but you can confirm that only one FET in each pair is getting drive voltage. For each pair, you will likely have one FET with well over 3.5v (pin1-3). The other FET should have essentially 0 drive voltage (near 0v pin1-3). Is that what you have?
 
#10
I checked all the resistors and they measure right on target. The high current problem started when I changed the IC. I checked the IC and the solder is good. I did measure that all four fets are being turned on by the channel. Here are some measurements I took measuring from gate to source (pin1-3)

Q504
4.8v

Q505
7.0v

Q506
6.8v

Q507
4.6v

What would couse this condition? Also I was wondering where do buy the transistors used for switchnig the output fets. C3207 and A1070?? I was able to find the 2N393906, I have a second amplifier that needs them. The channel is also blown and that amplifier! Thanks in advance!
 
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