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Old 12th June 2008, 06:35 AM   #21
thoronx is offline thoronx  Yugoslavia
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another view of the PSU
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Old 12th June 2008, 06:39 AM   #22
thoronx is offline thoronx  Yugoslavia
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the pcb of the amp... via is connected from the bottom of the pcb because it was a bit close to the heatsink so I decided to connect from down, the wire was in insulated tube (removed after for pcb debugging)
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Old 12th June 2008, 06:43 AM   #23
thoronx is offline thoronx  Yugoslavia
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the (via) wire is visible on this picture ... it is about 5mm from the copper wires of the pcb, far enough to make any short circuit :-) ... and it was in insulating tube.
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Old 12th June 2008, 07:26 AM   #24
timpert is offline timpert  Netherlands
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Hi,

I'd start with rewiring your power supply, because if you really built it the way you describe, it can't possibly work. Either disconnect the transformer's secondaries from each other (independent windings) and connect each winding to its own bridge, or connect them together to form a center tapped winding (like you have now) and use only one bridge.

As it is now, with the upper leg of the transformer being positive and the lower leg being negative, all seems fine, but you're charging the buffer caps in series. Any discrepancy between the current draw from the positive supply and the negative supply will make the supply voltage asymmetrical. It is not unimaginable that a situation arises in which your LM3886 gets such weird voltages on its supply that it decides to vent its smoke of life.

It gets worse when the upper leg is negative and the lower leg is positive. In this case, the diodes parallel to C1 and to C8 are in series and with forward bias, and almost present a dead short to the transformer's secondary! You perhaps fried your rectifier as well! Replace it, along with the 3886's. And don't apply power until the supply is corrected.

Good luck!
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Old 12th June 2008, 07:52 AM   #25
thoronx is offline thoronx  Yugoslavia
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thx timpert, I will try once again this weird psu, trying with load eg. adding 2 x 10 ohm/10W on the output that will drive some current (3A) to mesure voltage drops, and current on the resistor.
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Old 12th June 2008, 08:10 AM   #26
timpert is offline timpert  Netherlands
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Before connecting anything critical, try your rebuilt supply with no load for a while (half an hour or so) while monitoring it closely, looking for signs of distress. Nothing should get warm or hum excessively and the voltages should remain correct. Then, load only one output (V+ or V-) with the rated current (3A should be fine, mind the power dissipation in the resistors though, it will be in the order of 80-100 Watts), leave the other voltage unloaded and check voltages again. The load shouldn't affect the voltage on the unloaded branch by more than one or two volts. Things may get mildly warm now (except for the load resistors, which will get really hot), but still not too hot to touch. Repeat this test for the other voltage. Then load both voltages (take care that you stay within your transformer rating) and check if nothing overheats/blows up and voltages stay where they should. Only when your PSU survives this test, connect the amp.

Good luck!
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Old 12th June 2008, 10:33 AM   #27
AndrewT is offline AndrewT  Scotland
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Hi,
I agree that a dual rectifier cannot be used with a centre tapped transformer.

Either use a dual secondary transformer with dual rectifiers
or
use a centre tapped transformer with a single rectifier.

And build up a light bulb tester. And use it every time you power up a modified or new project.
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regards Andrew T.
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Old 12th June 2008, 12:04 PM   #28
timpert is offline timpert  Netherlands
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If at all possible, I complete and test the entire power supply of a project before starting work on the rest of the electronics. This includes all regulators and stuff on a PCB. Resistors first and ICs last is a good way to build a kit with no clue of what's going on. Having the power supply ready enables me to test my makings step-by-step. I start with the low power stuff and then I work my way up to higher power. As soon as things connect to the raw power supply, I use a variac and current meter to increase the voltage from zero and see if everything follows nicely and nothing strange happens. This allows me more control than a lightbulb, but if you don't have a variac (or are too impatient to frequently check your work), then the lightbulb is indeed mandatory. Working step-by-step seems slow at first, but it saves so much grief that I think in the end I'm quicker than when I just build the whole thing and only debug when everything is in place.
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