S-892 power supply issues - diyAudio
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Old 29th September 2012, 12:51 PM   #1
Nite is offline Nite  Europe
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Default S-892 power supply issues

I have this Chinese amp with PS problems. As soon as the power is applied, the amp starts drawing too much current. The LED is flashing red and green. I have tried to fix it myself, but without any success. I have removed the rectifiers and mosfets, disconnected the secondary winding of the transformer, but the issue is still there. Soldered a socket for the 494 chip and replaced it with another one, but nothing. When the chip is out, the LED is green. So, it must be related to the drive circuit. I get intermittent 22kHz at the gate of the mosfets. When I install the mosfets, they instantly get hot. Could it be because the signal driving them is too wide? Do you have any ideas what to check? Thanks.
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Old 29th September 2012, 03:11 PM   #2
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Did you try twisting the transformer when the FETs were in the circuit to see if there is an intermittent short?

Do you have a clean square wave on the gates of all of the power supply FETs?

If you're not sure that the driver components are working properly, load it by connecting a 1k resistor from the gate to drain and then from the gate to source for each FET location. The drive signal should remain square and shouldn't change in amplitude more than about 1/2v from loaded to unloaded (with the 1k resistor).
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Old 29th September 2012, 05:54 PM   #3
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I have an old scope, but it's good for audio range only and I can't see 22kHz clearly, but it seems to be square wave. The fequency is present for 1 second, then the protection kicks in, and after that the entire process repeats. The transformer can't be the case here, because I removed it, only to find that the problem was still there. All gate pads have intermittent drive signal on them.
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Old 29th September 2012, 06:00 PM   #4
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Do you read 0 ohms between pins 13 and 14 of the driver IC?
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Old 29th September 2012, 06:18 PM   #5
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Yes, its 0 ohm.

Last edited by Nite; 29th September 2012 at 06:27 PM.
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Old 29th September 2012, 06:44 PM   #6
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Then the pulse width isn't going to be excessive unless the driver circuit is defective.

When you stated that the problem was still there with the transformer out of the circuit, what problem were you referring to (excessive current draw or amp going into protect)?
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Old 29th September 2012, 06:51 PM   #7
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When the transformer was out of the circuit, the amp would still go into protect (the LED flashes red and green at 1 sec interval).
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Old 29th September 2012, 10:04 PM   #8
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To clarify... there are two problems. One problem causes the amp to go into protect and the other is excessive current draw, even when the rectifiers are out of the circuit. Is this correct?
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Old 30th September 2012, 08:44 AM   #9
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It's been a while since I last worked on this amp, and I wrote the opening post from my memory. It's one of the tough cases I haven't been able so sort out. I installed one mosfet at each PS bank, but nothing happens. Currently, the primary winding of the transformer is connected, two PS mosfets, and 494 that is supposed to be driving them. The rectifiers and output transistors are not in circuit. The gate pad of the mosfets is pulsing between 0.1 and 0.4V. Does this suggest that the drive or protection circuit is not working properly?
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Old 30th September 2012, 09:39 PM   #10
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Is there any way to get your scope working properly?
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