Yearly Aleph 5 clone breakdown - diyAudio
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Old 24th February 2014, 10:21 AM   #1
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Default Yearly Aleph 5 clone breakdown

Hi All

My Aleph 5 clone has broken down again....though this time i'm getting quicker at narrowing down the problem area..

This amp has been running faultless ly for nearly a year (i think )since a rebuild.
When it runs heatsinks are at abt 45deg C -fan cooled

This time the neg rail fuse keeps blowing.
I've ripped out all the output mosfets and replace one by one until the last one which causes the problem...

Its not the mosfet as I have tried about 6 in this location and it always blows the fuse . (seems to be ok without this mosfet left out)
The mosfet in question is the one which is fed from the base of Q4 (ztx450 )

When power is applied the 1ohm source resistor for the mosfet also glows (pretty!) along with the fuse till it blows.

Any ideas?
Replace Q4 or something else?
Hard to measure voltages with blown fuse.


Many thanks in advance, as per usual I run out of talent pretty quick
Cheers
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Old 24th February 2014, 10:27 AM   #2
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pictures could help
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Old 24th February 2014, 10:30 AM   #3
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OOOOOPPSSS


CORRECTION

Sorry I'm getting tired.
Its a pos rail mosfet
I'll re examine and post correct info
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Old 24th February 2014, 10:36 AM   #4
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check that the heatsink is nce and flat. and that it do not short out the transistor.
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Old 24th February 2014, 10:45 AM   #5
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yup

agree with AS

throrough check of heatsink surface , (non)continuity test and - in your case as must - measuring temp of all mosfets (at mid pin) , to be sure they're all in 10% bracket

just in case - sure that all (on same rail ) need to be matched
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Old 24th February 2014, 10:58 AM   #6
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Hi Guys = you are quick off the mark.
Yes the fets are matched.

No short to heatsink

I attach a diagram indicating the suspect mosfet location.
The diagram is that which was used for earlier trouble shooting (from an Aleph 3 ) (partial circuit only )
but circuit layout is the same in this case.


Any ideas ?
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File Type: jpg george aleph 5a.jpg (222.0 KB, 481 views)
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Old 24th February 2014, 11:16 AM   #7
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Just an update
this seems to be a similar problem to that of last time - see aleph 5 and Dolphin sounds. June /july 2013

Zen mod you assisted last time too
Many thanks


That time the problem was solved by a complete rebuild- cant do that this time (yet)
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Old 24th February 2014, 11:16 AM   #8
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no reason (change that small bjt whatever - it can be damaged ) except physical ones - bad thermal contact with surface at that particular mosfet position

temp probe would reveal any possible discrepancy between mosfets
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Old 24th February 2014, 11:21 AM   #9
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Maybe the 0.47 ohm resistor for that MOSFET has a problem. Maybe there is a bad connection to all the other MOSFET gates in the upper side, so that one is doing all the work.
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Old 24th February 2014, 11:26 AM   #10
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"Its not the mosfet as I have tried about 6 in this location and it always blows the fuse . (seems to be ok without this mosfet left out)"

"When power is applied the 1ohm source resistor for the mosfet also glows (pretty!) along with the fuse till it blows."

this tells me that there is a huge amount of current going here. and since there is no current with the mosfet out. it tells me that the conection between the mosfet and ground has a role in your trouble. (i doubt it's the mosfet itself since you tried 6 of them).
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