F5 problems - Page 2

Flemming J P · 10th April 2012, 10:39 PM

I was one of the mosfet that had died

Daniel(Del) · 12th April 2012, 02:36 AM

Do you have an idea why?

Flemming J P · 12th April 2012, 02:56 PM

No clue. When I turned the amp off, I head a pop in one speaker. The next time I tried to turn on the amp, it smoked (very hot resistors).

I ran the mosfets at 750 ma each at 32 v rails (350mv drop over 0.5 ohm source resistors).

I have chosen to run with lower bias (600 ma) after measuring how much or rather little power I actually use even when I play loud.

KatieandDad · 12th April 2012, 03:09 PM

This thread started with you suspecting the input JFets. Have you now discovered that one of the power MOS-FETs has failed ?

The MOS-FETs can fail for a number of reasons.

However, the most likely is a short in the speaker leads, OR, overheating, OR, a foreign object shoprting MOS-FET to heatsink.

Flemming J P · 12th April 2012, 06:52 PM

I thought it was the input fets, due to the fact that it was the feedback resistors that got hot/smoked (that was after I had turned the bias down to 0).

After some measurements I found that one of the mosfets was conduction at full power all the time no mater what.

Nelson Pass · 14th April 2012, 01:37 AM

It happens occasionally.

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10th April 2012, 10:39 PM	#11
Flemming J P diyAudio Member Join Date: Oct 2001 Location: Sweden	I was one of the mosfet that had died

12th April 2012, 02:36 AM	#12
Daniel(Del) diyAudio Member Join Date: Dec 2005 Location: São Paulo	Do you have an idea why?

12th April 2012, 02:56 PM	#13
Flemming J P diyAudio Member Join Date: Oct 2001 Location: Sweden	No clue. When I turned the amp off, I head a pop in one speaker. The next time I tried to turn on the amp, it smoked (very hot resistors). I ran the mosfets at 750 ma each at 32 v rails (350mv drop over 0.5 ohm source resistors). I have chosen to run with lower bias (600 ma) after measuring how much or rather little power I actually use even when I play loud.

12th April 2012, 03:09 PM	#14
KatieandDad diyAudio Member Join Date: Nov 2011 Location: UK	This thread started with you suspecting the input JFets. Have you now discovered that one of the power MOS-FETs has failed ? The MOS-FETs can fail for a number of reasons. However, the most likely is a short in the speaker leads, OR, overheating, OR, a foreign object shoprting MOS-FET to heatsink.

12th April 2012, 06:52 PM	#15
Flemming J P diyAudio Member Join Date: Oct 2001 Location: Sweden	I thought it was the input fets, due to the fact that it was the feedback resistors that got hot/smoked (that was after I had turned the bias down to 0). After some measurements I found that one of the mosfets was conduction at full power all the time no mater what.

14th April 2012, 01:37 AM	#16
Nelson Pass The one and only Join Date: Mar 2001	It happens occasionally.