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Old 1st April 2014, 08:56 PM   #21
x-pro is offline x-pro  United Kingdom
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Quote:
Originally Posted by jomor View Post
I have ordered STP40NF10L which are the closest equivalents to the factory mosfets (HUFA76639P3) from Mouser.com and I hope I will have them in my hands in a few days.

I would prefer to try a solution like this first, because i will have both channels identical, if this doesn't work as expected then I will have to try your modification advice

Another option would be to risk again and buy HUFA76639P3 from ebay but I doubt if they are originals.
There are many counterfeit HUF76639 transistors around (usually these are not even logic level) and probably no originals left. I would use IRL540N as a replacement, at least you can easily get a genuine article and they are more suitable than almost anything else available at the moment. STP40NF10L also a possible option.

Cheers

Alex

P.S. - don't forget to put the bias adjustment pot to the minimum bias before replacing the MOSFETs, otherwise you may not have a chance to adjust it, they would just blow if the bias is too high.

Last edited by x-pro; 1st April 2014 at 09:06 PM.
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Old 1st April 2014, 09:05 PM   #22
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Not hindered by any knowledge but if BUK555 logic MOSFETs can help you let me know.
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Old 2nd April 2014, 08:00 AM   #23
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by jomor View Post
I have ordered STP40NF10L which are the closest equivalents to the factory mosfets (HUFA76639P3) from Mouser.com and I hope I will have them in my hands in a few days.

I would prefer to try a solution like this first, because i will have both channels identical, if this doesn't work as expected then I will have to try your modification advice

Another option would be to risk again and buy HUFA76639P3 from ebay but I doubt if they are originals.
I can understand wanting both channels the same, I would be doing that too. Ebay... I hear far to many horror stories regarding semiconductors so best to avoid imo. You would always be wondering.
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Old 2nd April 2014, 05:38 PM   #24
sbrads is offline sbrads  United Kingdom
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re "I also spotted a flatulent smd capacitor....."

High value SMD ceramic caps (>=47n approx) tend to fail under power after they've cracked internally due to physical stresses. Often happens if the board is flexed, or the cap is right next to a mounting bolt that crushes the board a bit, possibly even if it's next to a hot component, it's not the heat as such, it's the thermal expansion at work. Seen hundreds fail like this on new production 1st version equipment being burnt in, it's the type of thing that gets sorted on later versions.

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Old 2nd April 2014, 07:30 PM   #25
jomor is offline jomor  Greece
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Quote:
Originally Posted by sbrads View Post
High value SMD ceramic caps (>=47n approx) tend to fail under power after they've cracked internally due to physical stresses. Often happens if the board is flexed, or the cap is right next to a mounting bolt that crushes the board a bit, possibly even if it's next to a hot component, it's not the heat as such, it's the thermal expansion at work. Seen hundreds fail like this on new production 1st version equipment being burnt in, it's the type of thing that gets sorted on later versions.
interesting.. ok I could try replacing it anyway, but with what type exactly? C20 on the 5350's schematic is 100nF bypass capacitor on the TL074's power supply. Is the type of dielectric important? I have 100nF x7r.

The basic question remains: could this capacitor be the cause of the oscillation? or let me rephrase the question, looking at the schematic which part of the circuit or component is responsible for stability? What can i do to work on tracing the oscillation's cause? I will replace the mosfets and adjust the bias but what if it oscillates again? Knowing that it oscillated twice, how will i be relaxed enjoying the music instead of sticking my eyes on the amp for smoke?
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Old 3rd April 2014, 06:18 AM   #26
Mooly is online now Mooly  United Kingdom
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Its extremely unlikely C20 would cause any major issue even if it were open circuit. The IC is used as a DC servo to maintain a low DC offset at the speaker terminals. The type of cap and value isn't critical. If you have any doubts then just replace it. Even a small electroyitic would be perfect.

If you suspect oscillation somewhere then you need a scope to investigate. There is no one component responsible for the amps stability, its a combination of the whole design. All the caps in the pf range are there for stability reasons.
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Old 3rd April 2014, 07:10 AM   #27
jomor is offline jomor  Greece
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I will have an oscilloscope available in a few days. But oscillation happens suddenly and gets uncontrolled in seconds so I guess I am dealing with an unknown enemy waiting in ambush.
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Old 3rd April 2014, 07:35 AM   #28
jomor is offline jomor  Greece
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Another thing is that the sophisticated protection circuits (there is a micro controller responsible for monitoring dc offsets, currents and voltages at various points) failed to protect the amplifier. Looking from a non technical customer's point of view, I ve got a ~ 2K dollars amplifier (Destiny 2 costs €2350 in my country) with hi-tech protection systems and I have barely managed to listen music more than 20 hours and most of them where in anxiety of smoke that might appear in sudden. The microcontroller prevents me from throwing away the final stage and replacing it with two holton modules for example (in case the final stage cannot be permanently repaired), which makes the whole amplifier useless.

Last edited by jomor; 3rd April 2014 at 07:37 AM.
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Old 3rd April 2014, 07:42 AM   #29
Mooly is online now Mooly  United Kingdom
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I can understand your frustration... and I've no idea what the reputation of these amps is like reliability wise either.

Are you sure its oscillating rather than just a DC fault that is frying the outout stage ?
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Old 3rd April 2014, 07:54 AM   #30
jomor is offline jomor  Greece
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no of course I can't be sure, but I think (enlighten me please if I am wrong) that DC is easier to detect and protect from, compared to an oscillation and I assumed that the advanced protections would not allow such a simple failure. But everything is possible and I would be happier if this is a DC case if DC is easier to troubleshoot!
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