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Old 5th May 2010, 07:48 AM   #1
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Question 800a2 Power Supply Heating up

I just got another 800a2 in for repair when you power it up the transform starts squealing and starts heating up and four of the fets heat up as well Q12,Q13,Q14,Q15 They were replaced with IRF3205 the other side Q5,Q6,Q7,Q8 are MPT75N05HD they are staying cool the amp is drawing 3 1/2 amps at idle with the Bias turned all the way down there no shorted outputs the amp plays all the resistors are good.Perry you have any idea what it could be?
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Old 5th May 2010, 09:28 AM   #2
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Did you check or replace the gate resistors and the driver transistors?
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Old 5th May 2010, 04:12 PM   #3
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The gate resistors are 68ohms for the IRF3205 and the same the the MPT75N05HD.I pulled the center legs on al lfour of the IRF3205 on the amp now it idles at 1.26amps
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Old 5th May 2010, 04:38 PM   #4
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I would make sure that both sides of the PS use the same FET. I know Perry a lot of times also suggests going down to a smaller resistor (22 ohm?) when using the IRF3205 assuming the drive circuit can handle the extra current.
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Old 5th May 2010, 04:41 PM   #5
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68 ohms may be a bit high for the 3205s. I'd suggest using 47 ohm resistors.

Measure the DC voltage on the gate leg of each of the power supply FETs (FETs out of the circuit as you have them now). What do you read on gate legs?
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Old 5th May 2010, 06:23 PM   #6
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Gate voltage Q15 5.50V, Q14 5.50V, Q13 5.50V, Q12 5.50V, Q5 5.49V, Q6 5.49V Q7 5.49V Q85.49V
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Old 5th May 2010, 06:49 PM   #7
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Those look OK. The problem is likely the gate resistors.
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Old 5th May 2010, 07:00 PM   #8
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Quote:
Originally Posted by Perry Babin View Post
Those look OK. The problem is likely the gate resistors.
so change the gate resistors from 68ohms to 47ohms ?
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Old 5th May 2010, 07:03 PM   #9
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That would be my suggestion.

You could also try measuring the DC voltage across each of the 0.1 ohm resistors in that channel to confirm that every one has exactly 0.000v DC across it.
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Old 5th May 2010, 07:19 PM   #10
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Quote:
Originally Posted by Perry Babin View Post
That would be my suggestion.

You could also try measuring the DC voltage across each of the 0.1 ohm resistors in that channel to confirm that every one has exactly 0.000v DC across it.
with all the power fets center legs disconnect i am getting 262.5mv on the irf9640 and 118.9mv and going up on the irf640 at the 0.1 ohm resistors
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