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Old 8th May 2011, 01:28 AM   #1
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Default Let Smoke Out Of My F5 - Help!

Well I was just setting up my newly minted F5 and a wayward DMM probe let the smoke out of one channel. I have built the stripped down version with no protection so it's probably a resistor or Q1, Q2, Q3, or Q4. I haven't pulled it apart yet but I'm speculating that it's one of the semiconductors. Resistor tend to be pretty hardy. The only clue I have so far is that I'm getting 17VDC at the speakers terminals.

Thoughts anyone?

Regards,
Dan
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Old 8th May 2011, 05:21 AM   #2
cviller is offline cviller  Denmark
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Sounds like you have not put the pots down to zero or you have adjusted one side too much before opening up for the other.
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Old 8th May 2011, 09:38 AM   #3
Itsmee is offline Itsmee  England
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If you let the smoke out, you have probably blown one of the O/P devices.

With the PSU discharged, check Q3 for a short circuit across drain source.

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Sounds like you have not put the pots down to zero or you have adjusted one side too much before opening up for the other.
If Q3 is OK, start again with the bias procedure, P1 & P2 at minimum resistance, begin biasing with P2, slowly adjust counting the number of turns until it starts to conduct, if it doesn't, Q4 is open circuit - no voltage across R12.
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Old 8th May 2011, 09:41 AM   #4
cviller is offline cviller  Denmark
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In my experience, there is also a lot smoke trapped inside the source resistors. And if your amp is oscillating, the feedback resistors may start smoking.

Look for burned components and smoke residual on pcb and tell us about it.
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Old 8th May 2011, 10:28 AM   #5
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Yeah, the source resistors get cooked, rise in value or go open and it puts the bias right off. Check those before the fets.
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Old 8th May 2011, 03:06 PM   #6
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Originally Posted by woodturner-fran View Post
Yeah, the source resistors get cooked, rise in value or go open and it puts the bias right off. Check those before the fets.
Mother's Day here so not much time. Will pull apart and post more info in the next couple days. Thanks for the suggestions.

Regards,
Dan
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Old 9th May 2011, 01:27 PM   #7
cjkpkg is offline cjkpkg  United States
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My bet is the either of the JFETS...I blew a 2sk170 and it immediately put rail voltage to the speakers.
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Old 12th May 2011, 11:41 PM   #8
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This is my circuit. So far all I've found is R11 open. Does that explain the 17VDC at the output?

Regards,
Dan


Click the image to open in full size.
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Old 13th May 2011, 01:18 AM   #9
cjkpkg is offline cjkpkg  United States
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Do R5-8 get really really hot?

When my 2sk170 blew they did to the point of the solder got soft.

A blown source resistor is obviously not good...but I'm not sure that would put DC on the outputs.
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Old 13th May 2011, 01:54 AM   #10
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I have spare 2S's but no spare resistors or MOSFETs. Alittle more continuity checks on the weekend.

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Dan
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