Help with JL 500/1 2nd one - diyAudio
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Old 24th February 2008, 04:26 AM   #1
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Default Help with JL 500/1 2nd one

I didn't want to stomp on the other guys post so I created my own thread.

I'm working on a 500/1. Power supply kicks on for a brief second then shuts off. It keeps trying to start the supply then dies.

I dont have a schematic for this amp.

All power supply transistors have been replaced. All Audio transistors 540s have been replaced. Light turns green and stays green. Will even play audio for a brief second but then shuts down.

Any ideas?
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Old 24th February 2008, 04:35 AM   #2
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Will it shut down if you drive signal into it but don't have a speaker connected to it?

With a speaker connected but no signal, does it remain on?

Does it produce a pop through the speaker when it turns on?

Does the green light go off when it shuts down?

Will it come back on after it shuts down or do you have to disconnect and reconnect the remote turn-on?
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Old 24th February 2008, 04:55 AM   #3
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Doesn't matter if a speaker is connected or not.

Power supply start up then immediately shuts down. Then starts back up again then shuts down. It just repeats this.

I have the remote tied to B+ so it just continues this cycle on its own. Audio will play for the brief second its turned on. Even without the speaker it does this.

I do have a scope so I can see whats going on. Just not sure what is shutting the supply off.

Both soft start and compensation are immediately pulled low on the PWM.

U601 gets warm to the touch and so does Q612 but without a schematic I dont know what these do.
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Old 24th February 2008, 05:10 AM   #4
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These amps are very sensitive to low voltage. Are you sure that the supply voltage isn't dropping as the amp powers up?

U601 is the 5v regulator for the logic IC (most are between the top and bottom boards).

Q612 drives the yellow transformer which generates the low voltage for the preamp level ICs. I think it may also be used to produce drive voltage for the high-side outputs.

Neither Q612 or U601 should get hot.
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Old 24th February 2008, 05:20 AM   #5
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I have the amp current limited to 3 amps.

Amp turns on sinks 500mA. I imagine this is before it "actually" turns on. Few seconds later the amps tries to really turn on. It then sinks the entire 3 amps and immediately shuts down.

I dont have the amp in the heat sink which is why I current limit. But I have never ran across an amp that cant turn on without a load and 3 amps of current.

I'm thinking of pulling the soft start leg up just to see what happens. If it would stay on long enough for something to get "hot" this would be easier.
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Old 24th February 2008, 05:47 AM   #6
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The amp may not have a problem. These amps are the worst when it comes to sensitivity to voltage drop.

Let it try to power up for a minute or so. If none of the heatsink mounted devices get hot (out of the sink), I'd suggest mounting it back to the sink and removing the limiting. If you have a large supply (rated at more than 10 amps continuous), install a 10 amp fuse.

For non-class D amps, it's often OK to defeat the protection circuit but on class D amps, it can often lead to instantaneous destruction of the power semis.
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Old 24th February 2008, 06:14 AM   #7
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Cranked the current limit up to 25amps. There is definetly shutting this amp off. There must be a second PWM that goes to T601 because I see somewhat of a square wave on that.
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Old 24th February 2008, 06:30 AM   #8
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There is a small driver IC that drives Q612 which drives the yellow transformer. I don't know of anything on that circuit that can shut down the amp (but that doesn't mean it's possible).

Did any of the outputs or power supply transistors get hot when you powered it up out of the sink?

If you connect your meter to the the speaker terminals (black lead on negative, red lead on positive), is there any DC voltage?

How much current did it draw when you removed the limiter?

What rev. amp do you have?

If it continues to go into protection/shutdown, pull the outputs. That will allow you to check most of the rest of the amp for proper operation.
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Old 24th February 2008, 06:56 AM   #9
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No DC output on the output. The output seems to be ok when it is on.

The supply transistors dont even get warm. I have a really nice Tek supply PS2520G for low current but anything higher then 3 amps I have to switch to a piece of crap 25 amp switching supply so I have no way of measuring current with it. So no clue on the amount of current its pulling. But I dont think it gets a chance to pull any significant current because it powers up and shuts off instantly. Basically the instant it turn on the instant it turns off.

Do you know what type of signals I should be seeing on T601? Each side of the transformer. I'm really tempted to bypass the protection and let her rip. What's the output out of the supply?
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Old 24th February 2008, 07:16 AM   #10
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This photo is the signal on the tab of Q612. The timebase is set to 5us. The vertical amp is set to 10v/div.

Click the image to open in full size.

What revision is your amp? There were modifications to this drive circuit.
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