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Hifonics 2400.1 power supply
Hifonics 2400.1 power supply
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Old 22nd July 2019, 04:13 AM   #1
barber02 is offline barber02
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Default Hifonics 2400.1 power supply

Iíve been working on a Hifonics 2400.1 today. All of the power supply mosfets were shorted. All 12 of them and both pnp driver were turned into resistors and you could tell that they got really hot and almost fell out of their holes.

I replaced all of the power supply mosfets. They are 80nf70 mosfets. I replaced both of the pnp drivers. I replaced a few output mosfets (irf640) because some of them were shorted.

The amp powers up, I get a green LED, and I can hear the relay kick in, but the amp draws abosolutely no current. No rail voltage is produced.

When I look for the 13.5 volts on the center leg of the power supply mosfets I get a reading of -0.245 volts. What?

I have the 13.5 volts at the power terminal and remote, I have it past the fuses, and I have at the primary winding of the transformers. But some how between the primary of the transformers and the center leg of the mosfets it dips to -0.245 volts. I have 0 volts on the third leg of the mosfets to ground so itís not being pulled to ground. This is strange. Any ideas?

Thanks,
David
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Old 22nd July 2019, 04:19 AM   #2
Perry Babin is offline Perry Babin  United States
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If you have 12v on the transformer primary windings, the traces may be open at the FETs. You can see where they open in the attached photo.
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Old 22nd July 2019, 04:29 AM   #3
barber02 is offline barber02
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Oh wow. I see what you are pointing out. What would you use to bridge this tiny gap?

Thanks,
David
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Old 22nd July 2019, 04:42 AM   #4
Perry Babin is offline Perry Babin  United States
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I'm sure you noticed but the gate resistors look burned.

I generally wrap a wire around a dental pick placed in the through hole (while FETs are out of the board) and solder the wire to the board. Here, scraping the board is required. It removes most of the mask, fine sandpaper wrapped around the end of a pencil or a fiberglass scratch brush (preferred) cleans it up nicely.

Example below is from a pioneer amp but the process is essentially the same.
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Old 22nd July 2019, 04:57 AM   #5
barber02 is offline barber02
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Thanks for tips Perry.

I did notice the gate resistors. They all checked out fine with an ohm meter with the mosfets out of the board. Do you think I should change them anyway?
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Old 22nd July 2019, 05:09 AM   #6
Perry Babin is offline Perry Babin  United States
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I would change them.
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Old 22nd July 2019, 10:43 PM   #7
barber02 is offline barber02
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Thanks for the help Perry. The amp is up and running again. No issues.

David
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Old 22nd July 2019, 11:04 PM   #8
bnae38 is offline bnae38  United States
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You said you replaced a few of the output mosfets. Ideally, you would replace all of them from the same lot to hopefully get the best matching and current sharing..

Something to think about, although you might get away with it.
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Old 23rd July 2019, 12:14 AM   #9
barber02 is offline barber02
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I appreciate the heads up bnae38. I do know it’s good practice to change them all. I do have a large supply of irf640s on hand and was able to find some very closely matched mosfets to drop in there so I went with that option to keep repair cost down seeing as how the output didn’t go down very hard at all. I decided to roll the dice on this one. I know the owner of the amp pretty well so there’s that as well.

David
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Old 23rd July 2019, 12:51 AM   #10
Perry Babin is offline Perry Babin  United States
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I didn't think that you would have mixed parallel FATs. bnae38 is right. Replace all in any group that had a failure with FETs from the same batch. Replacing all is even better seeing how cheap the 640s are. You don't want to do this all over again for $10 worth of parts.

Even if you match FETs some way by measuring various parameters, that doesn't tell the whole story. As the temperature changes, the parameters may not track and the amp could fail again.
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