The other day I turned on the faithful Aleph 4 that my brother's Dad built and it instantly popped it's internal fuse.
There is no soft start on this amp and it has 1000VA of transformers (2 x 500VA) in it. It's got 4 x CLCC supplies, 12 x 33000uF caps in total.
It is about 8 years old now.
Could this just be a lazy fuse that has failed or might it be something more sinister ?
There is no soft start on this amp and it has 1000VA of transformers (2 x 500VA) in it. It's got 4 x CLCC supplies, 12 x 33000uF caps in total.
It is about 8 years old now.
Could this just be a lazy fuse that has failed or might it be something more sinister ?
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I would check the value of the mains fuse, it should probably be around 13 amps for that amount of storage.
The 5A mains fuse is intact, the internal fuse might have been smaller than that. The beast weighs over 50Kg so I haven't been able to drag it out and open it up yet. I'll try to drag it out on Wednesday and just change the internal fuse.
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If it's not a fuse, check your PSU caps for short. I had one of my PSU caps short a while ago, it drove rail voltage onto the ground and blew the fuse.
I finally got around to digging the beast out to have a look. Sadly I replaced the 10A internal fuse and it failed immediately.
I'll need to dig deeper to find out what has failed.
It's a dual mono-block in one chassis so at least I can separate the two amp's easily enough.
I might just try a soft-start to see if that works.
I'll need to dig deeper to find out what has failed.
It's a dual mono-block in one chassis so at least I can separate the two amp's easily enough.
I might just try a soft-start to see if that works.
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A variac or dim bulb tester will work as an adequate soft start before you dig into changing internal wiring.
I would suggest checking caps for short and then checking your output mosfets for short. Use your DMM and test each leg of each mosfet for continuity with the sink. Any non "open loop" reading is a problem for output devices. My money is on a bad power supply cap...
I would suggest checking caps for short and then checking your output mosfets for short. Use your DMM and test each leg of each mosfet for continuity with the sink. Any non "open loop" reading is a problem for output devices. My money is on a bad power supply cap...
The mains fuses are slow blow type. It's been working fine for over 5 years. I'll check the caps. Nothing has happened to stress the output MOS-FETs.
I was thinking about testing the caps with a 470R 5W resistor in series with the existing transformer and diode. I'll check the diodes and the transformer beforehand.
I'd expect good caps to charge to about 35V (70% of Vcc) in about 15 seconds.
Is there a better way of testing them ??
I'd expect good caps to charge to about 35V (70% of Vcc) in about 15 seconds.
Is there a better way of testing them ??
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Caps will have to be removed from the circuit for testing. Your proposed method of charging them should work well. What might be revealing is to see how long they hold the charge you give them. Charge them up, let them sit for a while (few hours?), then measure their voltage to see where they are. I suspect you'll have one or more that will be completely discharged in relatively short order. The faster they discharge, the worse they are.
True; but fast discharge would not blow a fuse.
A fuse-blowing fault would be *instant* discharge; or inability to get a charge at all.
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True. I was trying to think of a good way to identify a bad cap with minimal equipment. Probably the best way I can think of is to test caps one at a time, directly connected to the power supply as usual, with a "dim bulb tester" in series with the transofrmer primary. When the light doesn't go out, you've found the bad cap. If one is bad and they are all from the same lot, the others are likely to expire soon as well.
Attached is a link to a video of how to test a capacitor ESR with a scope and a function generator.
YouTube
at 10:15 how shows a bad capacitor
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Maybe disconnect the psus from the amps and see if the short is in the psus or amp boards. Might shorten the search.
Having ascertained that the caps have been cooked inside the hot amplifier chassis I am considering using an external PSU.
All the expensive caps can then be kept cool in the external enclosure.
What local decoupling would then be necessary inside the main amplifier enclosure ?
All the expensive caps can then be kept cool in the external enclosure.
What local decoupling would then be necessary inside the main amplifier enclosure ?
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I don't have a definitive answer for you. I am aware that nearly everyone who advocates for a separate power supply will also advocate for some caps local to the circuit due to the resistance of the umbilical cord. I don't know how necessary this REALLY is... I'm not sure what I would attempt to measure or observe one way or the other to reach a conclusion.
If you keep your PSU close by with short cables (0.5m) and use heavy wire (12g), maybe it won't make that much of a difference?
If you keep your PSU close by with short cables (0.5m) and use heavy wire (12g), maybe it won't make that much of a difference?