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Old 11th May 2009, 12:16 PM   #1761
flg is offline flg  United States
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Default Re: shorted MOSFETs

Quote:
Originally posted by lgreen
... I think I made a mistake by using arctic silver thermal compond with mica insulators under the MOSFETs, I think the devices were under sufficient pressure that the grease become conductive...
I don't see why this would be a problem? I do it all the time. It's hard to put them under extreme pressure with a #6 screw? And I'm not aware of this thermal compound becoming electrically conductive under these circumstances... You may be refering to the e-poxy like compound they have. That would only be a problem if your thinking of removing them. But, I guess you are
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Old 11th May 2009, 12:52 PM   #1762
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Default Re: shorted MOSFETs

Quote:
Originally posted by lgreen
I think I made a mistake by using arctic silver thermal compond with mica insulators under the MOSFETs, I think the devices were under sufficient pressure that the grease become conductive.

I wouldn't think this would be a problem either. Personally, I use Arctic Silver for CPUs because of it's thicker consistency, but not for semiconductors though. But I don't think it makes a huge difference with either one so why chance it.

Oh and regular white Thermalcote is rated for higher temperature if it makes any difference.

There's no substitute for the right heatsinks. I don't think you're going to make up for undersized heatsinks with some esoteric grease.
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Old 11th May 2009, 06:04 PM   #1763
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Default Re: Re: shorted MOSFETs

Quote:
Originally posted by flg

**It's hard to put them under extreme pressure with a #6 screw? **
There is a screw plus a copper bar with 4 screws that pushes each set of 3 transistors down. So we are gettning a lot of force. But if that's not the problem I will figure it out next go around.

Anyway, what say you on
Question #1
Question #2
??
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Old 11th May 2009, 06:26 PM   #1764
flg is offline flg  United States
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Default Re: Re: Re: shorted MOSFETs

Quote:
Originally posted by lgreen
Question 1: How many transistors do I need to buy and match? and what is supposed to match what? Should I start over with a complete new set of 12 matched transistors for the channel or can I get 3 matched to each other? Putting in 12 matched devices is going to be pain! Maybe 24, have not looked at the other channel yet.
I'm not familiar with the design totally off the top of my head but 4 banks of 3?... Sounds like at a minimum you need to mach a bank of 3. Possibly 2 banks on the same side (top or bottom), 6 total

Quote:
Originally posted by lgreen
Question 2: I recovered some sil-pads from a 1998 receiver. Have these worn out or can I use them? I didn't think that it was possible for these to wear out but I could be wrong.
I would say: do not re-use silpads...

Quote:
Originally posted by lgreen

There is a screw plus a copper bar with 4 screws that pushes each set of 3 transistors down. So we are gettning a lot of force...
Sounds like a better way to apply force evenly but, torqueing to spec is always a good idea. Nelson often points out the torque Pass Labs uses(on 1 screw with a washer) on their product but, I don't remeber off the top of my head. Maybe you want to buy a torque screwdriver???

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Old 11th May 2009, 06:37 PM   #1765
lgreen is offline lgreen  United States
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Default Re: Re: Re: Re: shorted MOSFETs

Quote:
Originally posted by flg

**
Maybe you want to buy a torque screwdriver???

Exactly! Live and learn. I will be more careful.
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Old 11th May 2009, 10:01 PM   #1766
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Igreen

Important to match Q2, Q11 so these should go on the same heatsink.
Q1 and q10 not so important

Quote:
One bank of 3 MOSFETs measures short-circuit between the
pins. I know that the MOSFETs are supposed to be matched but I forget what you are supposed to match to what?
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Old 11th May 2009, 10:25 PM   #1767
lgreen is offline lgreen  United States
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Quote:
Originally posted by audiodan
Igreen

Important to match Q2, Q11 so these should go on the same heatsink.
Q1 and q10 not so important



OK, so the plan will be to buy 3 new unmatched fets and use them for Q10. the other 3 sets are already matched, and I will put the Q2, Q11 banks on the same heatsink.

If this seems wrong let me know.
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Old 11th May 2009, 11:49 PM   #1768
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I also matched Q1 and Q10 fets attached to R8 and R37
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Old 12th May 2009, 04:31 AM   #1769
lgreen is offline lgreen  United States
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Quote:
Originally posted by audiodan
I also matched Q1 and Q10 fets attached to R8 and R37
Well that's the question right? Do I need to match Q1/Q10 too. You say no then yes. What is your position?
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Old 12th May 2009, 09:50 AM   #1770
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Are you guys serious or just fooling around to kill time ?

Why would anyone want to match the current sources and output stages on opposite sides of a balanced amp that employs feedback to set the gain, and has trimpots to adjust the output DC level.

That's similar to matching four gasturbine engines for exact identical fuel efficiency to use on a 747, and then modulating the fuel supply of each individual engine to obtain identical thrusts.
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