JL 500/1: no power to PWM IC?

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Got a cheap amp to play with, all the FET’s were blown as usual along with the PSU gate resistors. Cooindicentally, don’t you love SMD gate resistors? Virtually guaranteed to burn the board when they go, they should have used metal film through hole components there instead and left them spaced above the board a bit.

Anyway, when I apply power with no FETs installed I get square waves at the output stage 4427 IC (yes one side is garbled because I haven’t changed it yet), but absolutely nothing for gate drive from the main power supply. I believe it’s a UCxxxx series chip. There’s no power that ever gets there that I can tell, so my question is how is that circuit receiving power from the remote wire? How can the output stage wake up and not the power supply?
 
It’s never turning on, question is why? PWM IC never gets power, it’s a remote turn on thing. If I jumped 12v to R400, I can get square waves at the mosfet gates like normal. So which transistors on this preamp card are responsible for waking up the main power supply because I think that’s what’s wrong
 
Post the DC voltage on the 10 pin connector using the numbering scheme in the attached image. Also post the DC voltage on the LM324. Copy and paste the following into your reply.


Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:


Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
 

Attachments

  • jlaudio500slashone10pinconnector01.jpg
    jlaudio500slashone10pinconnector01.jpg
    155.8 KB · Views: 61
Pin 1: 0
Pin 2: 0
Pin 3: 11.7
Pin 4: 10
Pin 5: 12.5
Pin 6: 12.5
Pin 7: 0
Pin 8: 0
Pin 9: 0
Pin 10: 0


Pin 1: 12.2
Pin 2: 1.14
Pin 3: 6.66
Pin 4: 12.5
Pin 5: .672
Pin 6: 8.3
Pin 7: 0
Pin 8: .28
Pin 9: 9.8
Pin 10: 8
Pin 11: 0
Pin 12: 6.4
Pin 13: .17
Pin 14: 12.2
[/QUOTE]
 
JL guards their documents, jealously. I have repaired a lot of them and made a lot of notes and partial diagrams. If you want a diagram of a JL amp, it's likely that you will have to make your own.

If you have to use a Zener with wire leads, use something like ribbon cable to connect it to the board to prevent stressing the pads. If that gets the amp working, order the correct SMD zener.
 
Found a 5v zener on an old flat panel TV power supply, swapped it in and it’s running now. Thanks!

What I mean is most of the amps I see don’t have much more than a transistor based remote trigger that sends voltage to the power supply, why an amplifier needs a zener voltage reference and a quad OPAMP to do the same thing is beyond me. Is it tied into the protection circuits or something?
 
Virtually all amps have some sort of circuit that eliminates a turn-on pop. This one powers up a low voltage circuit before the main supply. That involves a delay circuit but that's the main difference. Many amps use a a quad comparator or multiple dual op-amps or comparators that can prevent the power supply from starting. Most of those use the 5v reference from the driver IC but the driver IC isn't powered initially so that's not possible here.

The 324 in this amp does see input from various protection circuits but the comparators and op-amps in other amps do the same. Attached is a 25 year old design that uses a reference and a quad comparator.

I'm not saying that this isn't a complex amp, it is, but the turn-on circuit isn't overly complex.
 

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  • rockford - punch40i-0606.pdf
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