JL 500/5 Sub Channel low and distorted - diyAudio
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Old 25th October 2013, 10:34 PM   #1
deocder is offline deocder  United States
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Default JL 500/5 Sub Channel low and distorted

Hello,

I picked up a used JL500/5 and found 4 of the 5 channels working. The 5th channel, the sub section, when hooked up to a speaker has a low distorted full-range sound coming from it, with little effect from the input sensitivity. The speaker has been ruled out, and the external input signal has been ruled out.

One of the internal cards has 1 (the lower one) of the 3 LED's illuminated. I checked the resistance across almost every resistor on this card, particularly the 47k's that Perry mentions to check in this thread: JL Audio 500/5 channels not working. Please Help

The resistors are in spec. I did resolder the 47k next to the upper LED's as described in the referenced thread but that did not solve the problem. I will try again to solder, desolder, and add new solder now that the amp is on my bench.

DC voltage is 0 across the left LED (380 mV across the resistor to its left) and 43V across the right one (49V across the 47k resistor to its right). Volts, across the right, not mV, just to confirm since its high for an LED. I double checked both measurements.

Further inspection and resistance measurements of the amp revealed a loose resistor towards the top left section of the amp (R915 popped off when probing it). In fact R910-R915 all look like they have taken a thermal beating but are in spec. In addition, it looks like R800-R805 have been touched up with solder. (pics attached).

The two side-by-side transformers have sticky residue on them (pic attached)

I have not done any additional testing at this point, but I did want to get a thread started for assistance. Thanks in advance.
Attached Images
File Type: jpg IMAG3498[1].jpg (738.8 KB, 118 views)
File Type: jpg IMAG3500[1].jpg (617.3 KB, 114 views)
File Type: jpg IMAG3502[1].jpg (587.6 KB, 107 views)
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Old 26th October 2013, 12:26 AM   #2
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The 47k resistors are for the full range channels. Those have nothing to do with the sub channel.

Have the 900 series transistors failed?

If they haven't failed, the resistors were probably damaged when they failed previously.
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Old 26th October 2013, 03:59 AM   #3
rigtec is offline rigtec  United States
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Quote:
Originally Posted by deocder View Post
Hello,

I picked up a used JL500/5 and found 4 of the 5 channels working. The 5th channel, the sub section, when hooked up to a speaker has a low distorted full-range sound coming from it, with little effect from the input sensitivity. The speaker has been ruled out, and the external input signal has been ruled out..."

Thanks in advance.
Hi deocder

I know that you've checked pretty much all external signal sq, and the sub-woofer driver itself. But don't forget to check the speaker cable for shorting e.g., pos(+) to neg(-) continuity. Even just one small strand of wire making contact with the opposite pole will cause the amp to go into "Fault protection" mode, causing distortion, anemic sq, amp overheating, amplifier thermal protection tripping, warning led glow, etc... it happened to me so maybe, just check.

rigtec, cheers!

Last edited by rigtec; 26th October 2013 at 04:14 AM.
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Old 28th October 2013, 05:27 AM   #4
deocder is offline deocder  United States
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I was able to pull 3 of the 900 series transistors off the board before my iron gave out on me. I've attached the forms for the readings. It looks like these were replaced before. These were labeled IRFZ44R. I believe they all need to be replaced.

I checked the other FET types out of curiosity and it was interesting to see that Q202,Q203,Q102,Q104,Q402,Q403,Q302,Q303,Q406,Q405, Q422,and Q408 are all IRF504

All 900 series FET's are SUP60N06

R910-915 look pretty burned so I'm going to have to replace those.
R800-805 were either replaced or resoldered as there is excess solder and flux residue. (Pic attached)

I'll get a new iron tomorrow so I can keep working.

I have ruled out the bad speaker wire, but thanks for the input.
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Old 28th October 2013, 09:04 PM   #5
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IRF540. I don't know if you read the number wrong or it's a typo.
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Old 28th October 2013, 11:08 PM   #6
deocder is offline deocder  United States
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Argh, yes that was a typo.

Are the ones I measured from the Q900 bank power supply transistors?
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Old 28th October 2013, 11:51 PM   #7
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The 900 series FETs are the power supply transistors for the sub channel. Q405, 406, 422 and 408 are the output transistors for the sub channel.
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Old 29th October 2013, 12:27 AM   #8
deocder is offline deocder  United States
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Ok, so next step is to check those output transistors and replace the 900 series resistors and transistors. Are there any other things I can check at this point?

Would bad supply transistors cause the two LED's on the card to not illuminate?
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Old 29th October 2013, 12:46 AM   #9
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You need to confirm that the drive circuit for the sub FETs is OK. You'll need to clamp all of the other FETs to the heatsink to protect them while testing the drive circuit.

You shouldn't have 3 cards with LEDs lit and one without. Are the two adjacent LEDs out?

Did you re-solder the connections on the 47k resistor for those LEDs?
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Old 29th October 2013, 04:15 AM   #10
deocder is offline deocder  United States
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When I test the drive circuit, does that assume I have removed all 800 series transistors? Do I need to send I signal through the input to test the circuit? Or am I just going off of the LED's on the cards?

I went ahead and re-clamped the remaining transistors to the sink. Powered the amp and found the same LED lights on and off as stated in the original post (two adjacent LED's out on one card). I had previously re-soldered the 47K resistor. The LED's look a bit burnt as well. I know they can't take 43 volts....they may never light up again. I've attached a pic of both the ones that look burnt and ones that do light up.
Attached Images
File Type: jpg IMAG3576[1].jpg (413.5 KB, 69 views)
File Type: jpg IMAG3575[1].jpg (434.4 KB, 67 views)

Last edited by deocder; 29th October 2013 at 04:43 AM. Reason: Adding pictures and additional text
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