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Old 4th July 2011, 04:05 PM   #1
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Default Overheating Class A

I have a Musical Fidelity P270-2, It has been much modified over the years with Black Gates, schottky diodes, star earth etc. I have had no problems for a long time and was quite proud of it considering I am no electronics expert! the amp itself sounds superb! I used it with Musical Fidelity P173, kit valve and lightspeed pre-amps with no problem whatever, having had the amp returned from Portugal by freight recently, it now overheats on the right channel only, in fact the heatsink gets so hot it would cause a burn if touched.The amp is working but I use a gainclone kit instead but just not the same quality. Has anyone had similar problem with large Class A amp? I would be interested and any help would be appreciated.
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Old 4th July 2011, 11:21 PM   #2
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Check the standing current.

Check if output transistor has become loose.
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Old 10th July 2011, 08:45 PM   #3
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Default Fault found

I now know that a resistor is burning out, I have marked this on the drg. I downloaded from this site which was kindly posted for another problem.
Does anybody have any idea why this resistor would keep blowing? The amp still works but I guess because the other two FETs are having to work harder the heatsink on this side gets so hot I feel it could cause further damage over a period of time!
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Old 10th July 2011, 08:48 PM   #4
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Could be a gate to source short on the MOSFET.
I had exactly the same problem with a new amp I was testing.
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Old 10th July 2011, 08:50 PM   #5
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bias too high . do you have more of that schematic to show us ? or a service manual that explains checking and adjusting ?

cheers Woody
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Old 10th July 2011, 08:52 PM   #6
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The fet with the affected resistor have likely failed, replace it and see if that fixes the problem.
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Old 10th July 2011, 09:00 PM   #7
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Default Input section of drg.

Hi, Thanks for the quick replies, I have attached the input side of the amp and I will try and source a replacement FET.
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Old 10th July 2011, 09:04 PM   #8
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Default check the current

Quote:
Originally Posted by Tekko View Post
The fet with the affected resistor have likely failed, replace it and see if that fixes the problem.
hi
Put a dvm in line and check the current draw on that fet,or do the math across the resistor for current,If the resistors are good ,not much left,
good luck
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Old 10th July 2011, 09:48 PM   #9
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Default Hitachi J162 Mosfet

Hi, Have found that Aura Note Premier uses these devices. I cannot find any Hitachi devices on the www I can only find Renesas, would these be drop-in replacements for the faulty one or would I be better to replace all to keep everything ballanced? I am a bit wary of buying on the www from China source due to the stories of fake items being produced there.
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Old 11th July 2011, 02:56 AM   #10
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Just a suggestion - if the dc currents seem ok, use or find someone with a scope and check for oscillation. High frequency oscillation will cause an amp to run hot, also. Good luck.
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