Picked up a cheap 250/1 from a pawn shop that was dead. Powered it up and the main power supply would soft start, then start drawing a bunch of current. Traced that problem back to the audio output stage, which wasn’t receiving any square waves from the driver IC. Replaced the driver chip (4427) and that woke up the output stage, now the amp idles at around 1.2A and I have clean audio. Trouble is if I ask for any volume from it, the main power supply drops offline and I momentarily lose audio. If I leave it powered on the power supply will come back on it’s own, but I have to reduce volume to keep it from tripping repeatedly. This will occur at only 3A input from my bench supply, so it’s definitely not running out of juice from the bench.
Scoping the power supply MOSFET gates reveals there’s apparently no PWM going on in relation to current consumption from the audio stage. Meaning that I would expect to see increasing on time for increased volumes, but instead what I’m seeing is the (vertical) height of the gate drive waveform shrinking until it fully collapses- bouncing in time with the music. This leads me to believe the supply isn’t regulating properly. Next I scoped the inverting and non inverting inputs to the error amp on the 3525 chip, and I don’t see either input doing much of anything which again is counterintuitive. I would expect to see one of them moving around, telling the IC to increase on time as demand from the audio stage increases.
I assume this is a voltage mode supply like they usually are, and if that’s the case does anyone know which error amp input is being used so I can figure out where the fault lies?
Scoping the power supply MOSFET gates reveals there’s apparently no PWM going on in relation to current consumption from the audio stage. Meaning that I would expect to see increasing on time for increased volumes, but instead what I’m seeing is the (vertical) height of the gate drive waveform shrinking until it fully collapses- bouncing in time with the music. This leads me to believe the supply isn’t regulating properly. Next I scoped the inverting and non inverting inputs to the error amp on the 3525 chip, and I don’t see either input doing much of anything which again is counterintuitive. I would expect to see one of them moving around, telling the IC to increase on time as demand from the audio stage increases.
I assume this is a voltage mode supply like they usually are, and if that’s the case does anyone know which error amp input is being used so I can figure out where the fault lies?
May have been the bench supply after all, put a car battery in parallel and now it’s not dropping offline but there’s a different issue. If I advance the gain control, it sounds like it’s breaking up. Scope confirms flat topped appearance to the audio output waveforms, and I can hear the output inductors making some noise too. They also run a little warm, that normal or do they fail from time to time?
Same symptom on the power supply, I’m not really seeing any PWM action but the 60v rail seems to be pretty solid.
Same symptom on the power supply, I’m not really seeing any PWM action but the 60v rail seems to be pretty solid.
Aaaand.... I’m a dumbass.
Not quite sure what I did, but there’s apparently no problem now. I think there was actually 2 things going on here:
1. These amps are apparently current hogs, I had to use a car battery and a boost charger to get input voltage stable enough to keep it from dropping offline. Guess I just didn’t see it on the other amps I’ve been working on, or I simply didn’t push them hard enough to cause this particular problem. That, or these JL class D amps are very sensitive to their input power requirements.
2. Apparently if you use a load higher than 4 ohms (I’ve been testing using 8-ohm house speakers), it runs out of steam real fast. In all likelyhood, what I was seeing on the scope was it hitting the rails, or whatever the class D version of that is. I hooked it up to a 4 ohm sub, and no more problems.
In the future I won’t waste the forums time, I’m making a 2 and 4 ohm test sub box tomorrow so I make sure I’m always testing under normal loads. I’ll also complete my fused 1kw 13.8v power supply, to make sure I don’t run out of testing juice.
All is well, my lesson is learned. Good night all.
Not quite sure what I did, but there’s apparently no problem now. I think there was actually 2 things going on here:
1. These amps are apparently current hogs, I had to use a car battery and a boost charger to get input voltage stable enough to keep it from dropping offline. Guess I just didn’t see it on the other amps I’ve been working on, or I simply didn’t push them hard enough to cause this particular problem. That, or these JL class D amps are very sensitive to their input power requirements.
2. Apparently if you use a load higher than 4 ohms (I’ve been testing using 8-ohm house speakers), it runs out of steam real fast. In all likelyhood, what I was seeing on the scope was it hitting the rails, or whatever the class D version of that is. I hooked it up to a 4 ohm sub, and no more problems.
In the future I won’t waste the forums time, I’m making a 2 and 4 ohm test sub box tomorrow so I make sure I’m always testing under normal loads. I’ll also complete my fused 1kw 13.8v power supply, to make sure I don’t run out of testing juice.
All is well, my lesson is learned. Good night all.
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