RF 325.2 blows fuses - diyAudio
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Old 29th August 2011, 01:55 AM   #1
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Default RF 325.2 blows fuses

Circa 2006 325.2
I bought this amp not working, like everyone I buy. This one had the supply fets blown and by the looks of it one of the outputs was "repaired" as well. I replaced the supply fets and gate resistors. I don't see anything wrong with the outputs, transistors and resistors measure fine.

If I switch power on for a split second I see +-rail voltage along with +-15V supplies. If I keep it on longer it will blow the fuse I have in-line with B+.

Wondering if I should pull the outputs to verify the supply is operational; is there an easier way? I did apply just remote power and seen the outputs from the 494.

cory
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Old 29th August 2011, 02:09 AM   #2
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What i do with this type of problem is remove output transistors and feed VAS output back into LTP to close the loop. I can then check for dc offset and bias without blowing output mosfets.
If there is still a fault with outputs removed I start pulling all the transistors and diodes and check them out of circuit. While the transistors are out I check all resistors and check capacitors for shorts.
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Old 29th August 2011, 02:12 AM   #3
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Set the bias pots fully counter clockwise.
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Old 29th August 2011, 02:25 AM   #4
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Bias pots are set CCW, should have mentioned that.
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Old 29th August 2011, 02:34 AM   #5
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Power it up through a current limiter and measure the DC voltage across all of the 0.1 ohm source resistors. Do all read 0.000v DC?

If not, which ones read greater than 0.000v DC?
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Old 29th August 2011, 02:42 AM   #6
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The channel that was previously "repaired" is reading ~28mVDC across each one. The other channel is reading 0.
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Old 29th August 2011, 02:49 AM   #7
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Which 0.1 ohm resistors, specifically, have voltage across them?
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Old 29th August 2011, 02:55 AM   #8
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It would be R280 and R281, but the problem is J200 was cut from the board. It looked like a solder connection, but it had none.

I need to look more closely at the previous repairs, most of the time that is the issue.

Does measuring across those resistors show signs of leakage?
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Old 29th August 2011, 03:07 AM   #9
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Measuring the voltage tells you where the current is flowing to narrow down the channel with the problem or to determine whether the excess current is due to a problem in the output stage or elsewhere.
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