SMPS Power Supply Troubleshooting! - diyAudio
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Old 22nd March 2005, 07:32 PM   #1
duck95 is offline duck95  United States
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Default SMPS Power Supply Troubleshooting!

This is Lanzar Amp that I am trying to fix. When I orginally took it apart, I found that resistor 162 (arrow #1) was essentially open. I changed it, but upon powering up the amp, it started to smoke the resistor I just replaced! So that is the problem. I do not know why it is doing that. All my diodes and resistors are ok, so I'm at a loss. I replaced Q58 with the NTE device, but no change. I have 28 VDC at arrow #1 and -22 VDC at arrow #2. Why the difference? I know why it is negative, but why the big discrepancy? Any help is much appreciated and I am pretty good with electronics so feel free to write it tech terms. Thanks for the help!

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Old 30th March 2005, 05:11 PM   #2
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Default Troubleshooting

Replace the resistor again but this time wire a 100W light bulb in series with the 120V AC line to limit the current. Now you should be able to get in there with a meter and troubleshoot.

Remember, resistors only burn out if dsomething made it burn out. You likely have a shorted transistor somewhere.
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Old 1st April 2005, 11:13 PM   #3
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it is common for car amplifiers to have different rail voltages. It's odd, but I have seen it several times.

It does sound like you have a short somewhere because hard current is being pulled thru R162. Check all the transistors. Look up the datasheets and see if the collector>emitter or drain>source is a high resistance. Also check to make sure the base or gate isn't shorted to the other terminals. Check the resistance between the input and output terminals. You said you checked diodes and resistors - does that include the large power diodes in the power supply? If you still haven't got it, remove all the output transistors and then power the amp up. If it's ok, the problem is probably in the amplifier section. If R162 still burns up the problem is likely in the power supply. Check the housekeeping regulators and it's output on any op amps or the SMPS chip. Make sure the big resevior caps aren't shorted. Check that the secondary and primary of the transformer are isolated. Good luck!
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Old 4th April 2005, 07:03 AM   #4
Pierre is offline Pierre  France
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If it is of any help, keep in mind that most of the times, mosfets fail with their three pins shorted, other times they fail shorting drain with source. Have you checked that your mosfets are not shorted?

If they aren't try isolating the amplifier, as Evan suggested. That can be relatively easy done by removing the big resistors near the amp transistors (they are usually connected in series with them), they should be much easier to remove than the transistors themselves.
Another possibility is to remove the psu coil, although that isolates the out caps and perhaps affects feedback. Try the first possibility first and measure current consumption.

Best regards
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Old 10th April 2005, 03:23 AM   #5
duck95 is offline duck95  United States
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I did some research and replaced all my mosfets. The amp does not burn up that resistor any more, but there is now a new problem. The amp pulls 1.01A at idle, which isn't bad, but not good. The problem is that I have 12VDC at my speaker outputs intermittently. The amp will play music, but then you get that popping sound and voltage is at 12VDC. Why? Any help is much appreciated.
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Old 11th April 2005, 05:09 PM   #6
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Default Light Bulb

Sorry. DIdn't realize this was a car amp. Use a regular car headlamp or two in parallel in series for testing.
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Old 12th April 2005, 09:31 AM   #7
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Hi,

Check for cracked/ cold solder joints, can be hell to spot, good suspects are around fat n heavy components like those caps and coils.

If you suspect the output mosfets were causing problems, then also suspect their drivers, and replace them as well. It isn't uncommon for a blown mosfet to take the driver along with it since it usually shorts the drain/gate.

Good luck,
Chris
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Old 15th June 2005, 10:57 PM   #8
duck95 is offline duck95  United States
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