My Aleph J dc offset is now at 24V!
I have checked the Q5, Q6, Q7, Q8, Q9 and Q10 and it seems they are working fine. The attached image shows the measurement when it was working fine (my measurement in red). I have now removed all the IRFP 240.
I have 3 pairs of mosfets installed.
Which component is most likely be the fault?
Thanks.
I have checked the Q5, Q6, Q7, Q8, Q9 and Q10 and it seems they are working fine. The attached image shows the measurement when it was working fine (my measurement in red). I have now removed all the IRFP 240.
I have 3 pairs of mosfets installed.
Which component is most likely be the fault?
Thanks.
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I don't think you can make any reasoned measurements with the MOS-FETs removed. Q4 for one might well be turned ON as a result of their being removed.
Hi and thanks for helping. The dc offset was 21v with the mosfets still intact. So I removed them and check if they are faulty but it seems that they are working fine.
Should I install them back? Will it get burn?
Should I install them back? Will it get burn?
Use a light bulb tester to minimize damage after reinstalling the output mosfets. Install one in the upper and one in the lower position to get meaningful measurements. (keep track of them if your current source and output devices have different Vgs) If it works with one, add another, and repeat. You may find one or more of the output devices check OK with a DMM but cannot handle the stress of operation.
As PKI suggests, check the voltage across R7 and between the negative rail and the gates of the lower mosfets. These should be the same, unlike the voltages shown on your schematic. If the 4.32V was measured on the gate pin and you really did have a drop across the gate stopper resistors, something is amiss. It could be an oscillation or a leaky gate. Leaky gate means replace the mosfet(s). Oscillation means you need to check your layout and install C5 if you haven't already.
As PKI suggests, check the voltage across R7 and between the negative rail and the gates of the lower mosfets. These should be the same, unlike the voltages shown on your schematic. If the 4.32V was measured on the gate pin and you really did have a drop across the gate stopper resistors, something is amiss. It could be an oscillation or a leaky gate. Leaky gate means replace the mosfet(s). Oscillation means you need to check your layout and install C5 if you haven't already.
DMM Diode Test Function
Like I have always pointed out, the diode test function is an easy enough test in or out of the circuit for these power mosfets. The IRFP240 N-type has a reversed biased diode across its D-S junctions meaning D has cathode (black probe).
Test this D-S diode with the power OFF. This diode is usually shorted out on bad mosfets and will return a very low mV reading. A good one will show a normal diode junction at 0.4V to 0.6V. Reverse probes for P-type.
Like I have always pointed out, the diode test function is an easy enough test in or out of the circuit for these power mosfets. The IRFP240 N-type has a reversed biased diode across its D-S junctions meaning D has cathode (black probe).
Test this D-S diode with the power OFF. This diode is usually shorted out on bad mosfets and will return a very low mV reading. A good one will show a normal diode junction at 0.4V to 0.6V. Reverse probes for P-type.
That much offset tells me that there might be a problem with one of the J74s. Maybe there could be a simple dry joint problem there or one of the J74s is faulty? But I am just a novice that is thinking out loud.🙄🙂
If the output fets are still good, I would pull them and group R16 with R18 and R17 with R19. It should improve current sharing.
If offset is -24V likely Q7/Q8 or if +24V then Q5/Q6...their smoke spirits are now wandering in the great beyond or are in mosfet heaven.
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Thank you everyone for the help.....and many thanks to Mr. Pass, too.
I re-check all the Mosfets again and discovered that one of the Mosfets is not soldered nicely. So I re-soldered it and installed it back.
Now, the output offset is a funny thing. When the amp was first switch on, with a test speaker attached, it jumps to about 50mV and drop quickly drop down to 20mV and then it climbs back up again. In about ten minutes, it goes up to 25mV and looks like it keeps going on.
I also notice that Q8's heatsink is hotter than the rest. It is about 55 degrees C, compared to the rest which is about 45-47 degrees C.
I will turn it on for half an hour and see what's going to happen.
Many thanks.
I re-check all the Mosfets again and discovered that one of the Mosfets is not soldered nicely. So I re-soldered it and installed it back.
Now, the output offset is a funny thing. When the amp was first switch on, with a test speaker attached, it jumps to about 50mV and drop quickly drop down to 20mV and then it climbs back up again. In about ten minutes, it goes up to 25mV and looks like it keeps going on.
I also notice that Q8's heatsink is hotter than the rest. It is about 55 degrees C, compared to the rest which is about 45-47 degrees C.
I will turn it on for half an hour and see what's going to happen.
Many thanks.
if your fets are ok, switch them as suggested in post 10 and you will get better current sharing. If you have fet hogging current, it will cause multiple problems and is ultimately dangerous to your speakers and amps life. Also make sure that they are mounted correctly and evenly and that your sink is clean, ensuring good thermal contact. THe AJ can have a turn on and turn off thump. This is not unusual, but bias should be pretty steady after about an hour. Ultimately you will haev to let it heat about for about an hour and check offset, as it may change as the sinks warm. It is possible that you have a fet oscillating, but i would try swicthng them first to see if the current sharing improves.
It's half an hour with the amp switch on and it seems like the offset have drop back down to 20mV.
But Q8's heat sink is higher than the rest. (Each Mosfets are mounted on individual heatsink). Q8's heat sink is at 57 degrees C while the rest is at 50-51 degrees C.
I do not know what you mean by grouping the R16-R19. Yo mean to match them?
But Q8's heat sink is higher than the rest. (Each Mosfets are mounted on individual heatsink). Q8's heat sink is at 57 degrees C while the rest is at 50-51 degrees C.
I do not know what you mean by grouping the R16-R19. Yo mean to match them?
continue trying
however - placing heatspreader on several heatsinks is smart thing to do , regarding common temp. behavior of parts on them
when you have several individual heatsinks , you must have tightly matched everything - both output mosfets and source resistors
however - placing heatspreader on several heatsinks is smart thing to do , regarding common temp. behavior of parts on them
when you have several individual heatsinks , you must have tightly matched everything - both output mosfets and source resistors
I check the offset again and it is now at 18mV.....Hmmm. Heatsinks temperature remains unchanged....Q8's at 57 degrees C while the rest at between 50-52 degrees C. After an hour and 15 minutes.
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