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Old 29th April 2013, 02:14 PM   #21
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take your time

just enjoy in ride
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Old 8th June 2013, 03:52 PM   #22
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Hi All
back again

well the saga continues.
Fets have been matched, to within 0.01v

All semi's on suspect channel have now been replaced..
except diodes..
the problem remains , one side is till running cold - the negative side.
No heat from mosfets, no voltage across the source resistors and -32v at the speaker terminal


still essentially back where i started after a lot of work - and some expense...
Any ideas..?

thanks in advance.
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Old 8th June 2013, 03:54 PM   #23
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I should add that even the irf9610's have been replaced as well
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Old 8th June 2013, 04:44 PM   #24
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time for pics
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Old 9th June 2013, 06:21 PM   #25
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Hi All
Back again

new developments.
In short it appears on of the lower mosfets keeps on "shorting"
In my amp i have 3 upper and 3 lower output irfp240's

the one that keeps shorting is that which is fed by a 220ohm resistor -not only on the gate but also the source-fed from one of the mpsa42 transistors (or in some iterations this appears to be a ztx450 or mpsa18).
I have managed to replace the mosfet - powerup and get 20mv offset at the speaker terminals and a sensible Vgs measured across all the output mosfets.
after a few minutes the offset shoots back up to rail voltage and the VGs dissappears...on all the lower mosfets.

Upon removal of the suspect mosfet -testing of it shows VGS back to normal whilst it is out of circuit, ie : the mosfet itself tests ok as do those still in circuit.
Replacing the mosfet back into the position from where it came causes immediate failure....
what gives.?

Zenmod- i'm not sure what photos would show as it's unchanged for the last 4 or so years...

many thanks in advance
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Old 9th June 2013, 06:41 PM   #26
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what can I tell you , except - observe for burned parts , cold solder joints , proper mosfet fastening on heatsinks ,shorts , traces continuity, blahblah

considering that's my day job - those things are simple for me ...... so I'm having just one approach developed - start from start , go methodically , and finish ..... on end

replace small bipolars ( cheap , no matching ) , start with just one pair of mosfets in output (one upper , one lower ) , then add second then third pair
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Old 10th June 2013, 02:45 AM   #27
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Pardon my ignorance , but i am confused by mosfet passing Vgs test but failing in circuit....
Am i progressively destroying one mosfet after another by replacing the dodgy ones?

Electronics is not my background

Thanks in advance
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Old 10th June 2013, 10:43 AM   #28
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funny - I'm sort of visual oriented kind of guy ........

so - we can chat till Judgment Day , and I can't give you more than generic advice , if you don't give more details

pull out all mosfets , check them all at current as in circuit , then mount back just one pair and test the amp ;

if one pair passes , add another pair .......... test ......... add another pair

if pcb isn't defective , I see no reason that amp not behave

do you have a CRO ?

use attached file for detailed info what range of measuring values to expect
Attached Files
File Type: pdf Aleph30by Algar_emi.pdf (29.9 KB, 17 views)
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Old 10th June 2013, 11:58 AM   #29
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Thanks Zen mod

I'll give it another go
I did post earlier voltage settings based on the attached image , But i think they may have changed as i "seem" to zero in on the one dodgy mosfet position.

As i stated earlier the mosfets when removed from circuit and tested as though I was matching them again seemed to be working...VGS was consistent with earlier readings.

Again many thanks for the support.
Cheers

I'm also going to replace the small pcb that is home to the 3 lower mosfets to see if that has any effect.

Yes i do have a CRO, though only a single channel....
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Old 10th June 2013, 12:02 PM   #30
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i am running out of mpsa42's can I use something like a bc546 instead?
Thanks
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