Equivalent Transistors for JL 500/1 - Page 3 - diyAudio
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Old 14th November 2012, 02:05 PM   #21
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Thanks, it was a blue pot located on the main board slightly above where the large capacitors are mounted. It didn't have any glue on it like I have seen on most of the Rockford amps that I have repaired in the past.
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Old 15th November 2012, 02:17 AM   #22
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Okay, got back into the amp and wired in the pots with the correct resistance dialed in. Now the amp will pulse on and off and cause my headlamp (current limiting resistor) to brighten before it shuts down and tries to power back up. If I eliminate the current limiting resistor it will power up but transistor Q507 gets hot immediately. I am thinking I may have got the values of the pre drivers on the left side of the audio section backwards. Can anyone confirm that Q508 is a C3207 and that Q511 is an A1070? These look like a mirror configuration of the pre drivers on the right of the amp but I did not jot down the values before removing these. I am certain that Q502 is a C3209 and Q500 is a A1070.
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Old 15th November 2012, 03:40 AM   #23
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You have the drivers in the right locations.

Do you have a scope?
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Old 15th November 2012, 03:55 AM   #24
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Yeah, it looks like the audio side power transistors are starting to produce a square wave before the amp power cycles. I took the rectifier back out and it powers up fine, I definitely have issues on the audio side. One thing I noticed was that when I have the multimeter set to diode mode and probe the outer legs of Q507 and Q505 I get around .58 volts but when I reverse them it's doesn't go to OL, it climbs to about .68 volts and stops. The other 2 power transistors will go to .59 volts and then OL when reversed. Also, after trying to power the amp, it is Q507 that gets so hot, and about 2 or 3 minutes before it cools down, a pair of the legs appear shorted. This one is being a difficult patient. I assume the pre driver section on the left side of the amp (with the Q508 and Q511 components must be driving the power transistors Q507 and Q505.
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Old 15th November 2012, 05:20 AM   #25
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Apply remote voltage then disconnect it after about 1 second. That will allow the low voltage/driver supply to power up. It will remain on for a few seconds. During that time, the drive signal should be visible on the output transistors. Do you see a clean square wave on the gate legs of all of the audio output FETs?
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Old 15th November 2012, 06:16 AM   #26
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Here is what I found from that last test. Output transistors 507 and 504 have a huge square wave pattern on the source and gate legs. (it goes off the scope when set at 5 volts/div). The drain is reaching about 60 volts DC. Output transistors 505 and 506 have a similar off scale large square wave on their drain, their gate has a smaller square wave that reaches about 15 volts peak to peak. Their source is around 0 volts.
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Old 15th November 2012, 06:21 AM   #27
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Did you do this as suggested? If so, there shouldn't be any rail voltage.
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Old 15th November 2012, 06:26 AM   #28
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I cheated a bit and took multimeter voltage readings on legs that were not showing a square wave pattern. I have the 507 transistor (which is the only one really getting hot) rigged to where it is making good contact with the heat sink. All the square waves slowly decay when I remove remote power and look clean.
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Old 15th November 2012, 06:34 AM   #29
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If the drive signals are clean and square, particularly on the falling side of the waveform, it would appear that Q506 is leaky or shorted. Pull 506 and 507 and confirm that there is no leakage.
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Old 15th November 2012, 01:56 PM   #30
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I stayed up till 3:30 A.M. and I fear my point of diminishing returns was reach a couple hours earlier. I pulled the to transistors and confirmed with the diode test that they were not leaky. I did find a place where it looked like the gate resistor trace was not making contact with 507. I jumped it back over and was hoping this would be the problem, but no joy. The same symptoms but now one of the other transistors is starting to get very warm. As I was going back and forth troubleshooting I accidently shorted the bottom of the board against the a metal screwdriver bit that was laying on the bench. It didn't arc too bad and I can't really tell if it did any damage since I decided to shut the bench down for the night after that accident. I guess I need to take a step back and correctly try and map out the shematic of what this audio amplifier section looks like and make sure every thing is correct and work my way back. Would it hurt anything to pull the transistors that are getting warm and try to power the amp with only one or 2 transistors in the audio section just to see if I can get it to power up without starting to go into overcurrent and power cycling? I may also need to see about getting the service guide on this amp. I have been doing well with my repairs so far, batting 9 for 9 but I fear I have meet my match in this JL audio amp, not giving up yet just starting to feel a little defeated, LOL. Again thanks Perry for all the help up to this point.

Kelvin
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