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MMats M2000.2D blown PS Fets (twice)
MMats M2000.2D blown PS Fets (twice)
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Old 14th September 2019, 10:40 PM   #1
Formas is offline Formas  Jamaica
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Default MMats M2000.2D blown PS Fets (twice)

This amp came in all power fets blown, the turn out was I found bad railcaps (80v 5600uf) they were shaking and top bulgy I didnít test them electronically because they already failed the physical parameters.
I changed the fets (3205) and fet driver (4424) all gate resistors were good 10ohms, the caps I got looked Chinese a bit slimmer and a little taller but only one had the value of 5698uf on my tester I gotten 10.76mF ???** I put them in thinking my meter was malfunctioning. It played on my bench the voltage across the caps was 63ish volts, played well and all fets both PS & OP was cool.
The customer brought it back with one cap blown, all PS fets blown, could poor quality railcaps pull down the power supply, or is there something Iím missing?
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Old 14th September 2019, 10:56 PM   #2
Perry Babin is offline Perry Babin  United States
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The cap could have failed and shorted while blowing.

As a very crude way of determine if caps are OK, you can do a couple of things. Touching your meter probes across the cap and seeing how long it takes to charge to a certain reading, then reversing the probes or discharging the caps and repeating.

You can also connect a signal generator across the cap to see what frequency drops the output to about 1/2 of the no load voltage. Then you can compare to a known good cap. You can even roughly calculate the capacitance with the formula C=1/2PiFR (R being the output impedance of the generator).
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Old 16th September 2019, 03:39 PM   #3
Formas is offline Formas  Jamaica
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None of them tested short, I don’t know if it’s possible to be shorted while blowing then opened after the failure (like exploded transistors).
So if physically the caps are bulging and shaking within the package, then it may still be good?
I do not have a frequency or signal generator (well at least not yet), what determines the reading against time and wouldn’t that determination be varied from different values cap?
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Old 16th September 2019, 05:04 PM   #4
Perry Babin is offline Perry Babin  United States
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Capacitors are generally limited in capacitance, for a given voltage, by the size of the can. If the capacitor is not tight in the can, it's likely defective. New or not.

You have an audio source that you can connect in series with a resistor to make a generator to test capacitors. The test isn't ideal and best used to compare known good caps to questionable caps.
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Old 18th September 2019, 02:13 AM   #5
rdiaz is offline rdiaz  United States
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On this amps when the power supply goes bad I recommend you to change SG3525 as well and make sure the resistors next to mic4424 are good, what’s the rail voltage at the moment? Because I think they updated the caps to 100v instead of 80v.
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Old 20th September 2019, 06:26 PM   #6
Formas is offline Formas  Jamaica
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Is there any reason why the rail voltage would be 85.5v when I measure across the rail caps? Everything is apparently normal otherwise. I didn’t put any load (speaker) on it but there isn’t any DC voltage when measured across the terminal.
The caps are original (80v 5600uf) from a used board I’ve also changed various PWM(SG3525) and various fet driver (MC4424) and the volts remain the same.

If it is ok, does capacitors have a tolerable overshoot percentage or should I upgrade the caps to 100v?
The owner says he has the amp over two years working, just the power supply fets and fet driver was destroyed and the capacitors was bulging.
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Old 20th September 2019, 07:27 PM   #7
rdiaz is offline rdiaz  United States
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The rail voltage you seen is normal that’s why they updated the caps. On the speaker terminal you have to get half of this rail voltage measuring from battery ground to (+)(-) of speaker terminal.
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Old 20th September 2019, 10:34 PM   #8
Formas is offline Formas  Jamaica
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The owner has another one of this amp and when he went to check the rail caps was bulging also, his problem is that other amp would play and at higher volumes it would cut out until the volume is lowered, he never had that problem when amp was new.

**the one that’s on this forum is periodically having one bank of PS mosfets been getting hot, I’ve checked all the follow up components and they are good and all resistors are ok.
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Old 21st September 2019, 12:42 AM   #9
rdiaz is offline rdiaz  United States
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The problem we’re the amp is cutting out is probably because the caps are going bad, What is the value of the resistor before the mic4424?
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Old 21st September 2019, 02:47 PM   #10
Formas is offline Formas  Jamaica
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Yes I figured that on that amp but this one, what resistors you are talking about before the fet driver? R180 and R183 are 24.9ohms and all the gate resistors are 10ohms.
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