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Old 27th February 2014, 05:55 PM   #1
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Default 5 pcs 2sk170 burn, speaker burn, help!

after 5 pcs of 2sk170 burn and my speaker dead ( DC eat them) i need to write a post.

1) i have one good channel to compare, so after change to new 2sk170 the bad pcb is same like the good channel in resistance term. i add picture for the value.

now even with no source connected and no load.

in the start everything work fine, i start to rise the bios to 0.6v and still ok.

i don't know how suddenly i see 30vDC on the output ! and of course the 2sk170 gone again.

i don't know what to check more!. now i disassemble the irfp240 and i look ok:
0.5v between the gain source, all the rest is INF.
maybe its the 2sj74 burn the 2sk170?

i don't know what to do, everything seems ok to me.
thanks
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Old 27th February 2014, 06:34 PM   #2
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Did you check that the output devices are properly insulated from the HS? Sometimes those rubber or mica insulators fail...
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Old 27th February 2014, 10:37 PM   #3
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output devices? i don't have chance to connect some CDP.
the problem is that after replace the 2sk170 it work ok, after rise to 45 tmp and 0.7V bios still everything ok. just get the RCA and before connected it to CDP i see the 30v in the output.
just don't know what cause this problem.
I see that the F5 do not make a lot of trouble in this forum.
Hope to solve this and enjoy from this amp.
Thanks for the help
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Old 27th February 2014, 10:53 PM   #4
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There could be a fault further back in the driver or bias stages.

What I do is leave out output transistors and connect driver stage back into ltp.
In the state the amp shouldn't blow fuses or output transistors.
Check the bias is going up and down ok on the pot.
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Old 28th February 2014, 12:15 AM   #5
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Output devices = output transistors

Are you very slowly turning the pots while keeping an eye on the voltage across the source R's and the offset at the speaker outputs? It's a delicate balancing act...
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Old 28th February 2014, 12:18 PM   #6
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What voltage are you rails? Are you getting 30Vdc on +/-25V?

Did you plug in an RCA cable into the amp while it is still on and
then things went awry?
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Old 28th February 2014, 01:52 PM   #7
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hi
now i remember that before i get 30vDC output i see o.2vdc in the output and try to reduce it with the pot but with no success, this not normal, it was easy with the good channel.
i use 30v dc in the output stage.
the first transistor blow after connect RCA, the lest transistor burn with no RCA connected.
i do not fast with the bios, but not slow too.
can i turn on the amp with no output transistor? and with no 2sk170? i can show you the results!
maybe its the 2sj74 burn the 2sk170?
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Old 2nd March 2014, 02:22 AM   #8
eyoung is offline eyoung  United States
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Another question should be what music source are we using and is a pre amp in the chain or is it alone (a single ended pre plugged into a direct coupled amp can produce very high voltage DC at the input of the amp if no DC blocking caps are present)

Regards, E
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Old 2nd March 2014, 07:44 AM   #9
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i use cdp, so its dac with op amp in the output.
can i turn in the amp with no IRF transistor?
thanks
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Old 2nd March 2014, 02:30 PM   #10
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some measurement for the IRF fet:
simple test i connect to G about 3.5vdc and the D-S resistance give 0 ohm, when i unplug its back to INF slowly. (IRFP9>+3.5v, IRFP>-3.5v)
this test is ok? i check the same thing with power on D-S and the results is the same.

friend of my told me that the 2sk170 very sensitive, and i need to connect RCA to GND when i torn on the F5. and than turn on the volume.

"turn in"="turn on"
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