N-channel experts advice needed.
A long time repair issue:
This N-channel amp has a bad habit of occasionally taking out the lower FET (Q102) on power up.
I am reasonally experienced with both complementary and sziklai output stages, but not so much with FET quasi, and this one annoys me a bit
It can run beatifully for weeks, and then at turn on - boom, lower FET shorts.
What could cause this seen from a schematic point of view?
Could it be a result of uneven supply start-up voltage timing?
I am especially thinking of the positive 15V driver pull-up supply (Q14)...if this starts up quicker than the main supply, could this then keep the gates of the FETs fully open when the main supply kicks in?
Are there any potential oscillation issues not covered in the schematic that could cause it?
Can you think of any special reason that it is always ( three times) the lower FET that goes?
Background info:
The amplifier is a Sirius DMX100...a predecesser of the old Gamut series...probably not comparable with the newer Gamut stuff since ownership and constructor are different.
The schematic is reverse engineered, originally not by me but I can verify that the gainstage and output stage is acurate, and I have added the component values.
The original FETs are unknown since they were debadged by the constructor, so I am using IRFP240 with 100 ohm gatestoppers (was 27ohms with the unknown original FETs) and 15V zeners between gate and source.
Power supply is +-60 volt with 10.000uF per half.
Some other repairs have been done by me, including a new bias-pot.
As before said: It runs fine for weeks, maybe months, so I am kind of sure that the cause is not down to a faulty repair.
Any hints will be much appreciated.
Tell me if you need specific info.
Regards Henrik
A long time repair issue:
This N-channel amp has a bad habit of occasionally taking out the lower FET (Q102) on power up.
I am reasonally experienced with both complementary and sziklai output stages, but not so much with FET quasi, and this one annoys me a bit
It can run beatifully for weeks, and then at turn on - boom, lower FET shorts.
What could cause this seen from a schematic point of view?
Could it be a result of uneven supply start-up voltage timing?
I am especially thinking of the positive 15V driver pull-up supply (Q14)...if this starts up quicker than the main supply, could this then keep the gates of the FETs fully open when the main supply kicks in?
Are there any potential oscillation issues not covered in the schematic that could cause it?
Can you think of any special reason that it is always ( three times) the lower FET that goes?
Background info:
The amplifier is a Sirius DMX100...a predecesser of the old Gamut series...probably not comparable with the newer Gamut stuff since ownership and constructor are different.
The schematic is reverse engineered, originally not by me but I can verify that the gainstage and output stage is acurate, and I have added the component values.
The original FETs are unknown since they were debadged by the constructor, so I am using IRFP240 with 100 ohm gatestoppers (was 27ohms with the unknown original FETs) and 15V zeners between gate and source.
Power supply is +-60 volt with 10.000uF per half.
Some other repairs have been done by me, including a new bias-pot.
As before said: It runs fine for weeks, maybe months, so I am kind of sure that the cause is not down to a faulty repair.
Any hints will be much appreciated.
Tell me if you need specific info.
Regards Henrik