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Old 29th January 2004, 03:46 PM   #1
meat is offline meat  United States
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Default Help with Leach amp

I am trying to build the Leach amp, and I keep smoking C14 and r33, which would say I have a short in the negative side, but I can't find it.

What is wierd is that it has done it on both channels. I have gotten one channel working just fine and then I try the next channel and poof. I have had to replace some of my output transisitors also. I have checked all of my transistors to make sure they are not shorting to the heat sink. I am confident the boards are populated correctly, because I have tested them with a scope before I installed the output transistors.

Thanks
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Old 30th January 2004, 03:26 PM   #2
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could you have installed C-14 backwards? Been there. I cannot think of a reason for that cap to blow unless it is seeing the wrong polarity.

Also check that you have connected the power rails properly - reversed rails could cause similar problems. I did that too - checked the boards out before final installation then in the fnal hookup got it backwards. took a long time to figure that one out.

try using 100R 1/4 watt in place of the fuses, as Dr. leach suggests for those without a variac. you may go through a few of them before figuring it out, but they are a lot cheaper than output transistors.

hope that this helps.
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Old 30th January 2004, 07:51 PM   #3
meat is offline meat  United States
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Thanks Bob for your response. I have checked the polarity of my caps and they are correct. What is odd, is that this has happened on both channels. I have gotten one channel working and then it smokes. I will try the resistors in the fuse holders though.
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Old 30th January 2004, 10:19 PM   #4
sss is offline sss  Israel
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u can measure the voltage on c14 and see whats wrong
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Old 30th January 2004, 10:29 PM   #5
BrianGT is offline BrianGT  United States
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Default Re: Help with Leach amp

Quote:
Originally posted by meat
I am trying to build the Leach amp, and I keep smoking C14 and r33, which would say I have a short in the negative side, but I can't find it.

What is wierd is that it has done it on both channels. I have gotten one channel working just fine and then I try the next channel and poof. I have had to replace some of my output transisitors also. I have checked all of my transistors to make sure they are not shorting to the heat sink. I am confident the boards are populated correctly, because I have tested them with a scope before I installed the output transistors.

Thanks
Check the voltage across the front 2 metal can transistors (Q12 and Q13). The voltage is the sum of the base-emitter voltages for all the devices, and should be around 8*(0.5v-0.65v). If this number isn't correct, it means that all of your output devices aren't working properly. Measure the base-emitter voltages on all your output devices if this is the case

Also, take a picture of your board and post it.

By the way, when you tested the boards, both the VBE multiplier circuit and the output devices were not tested, so they are probably part of the problem.

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Brian
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Old 30th January 2004, 10:42 PM   #6
BrianGT is offline BrianGT  United States
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Default Re: off topic (sorry)

Quote:
Originally posted by Bricolo



Hi Brian, did you recieve my mail (from yesterday)?

Alex
yes, i will get back to you. I haven't had time to sit down and look at it yet.

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Old 30th January 2004, 11:14 PM   #7
meat is offline meat  United States
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Thanks for all of your responses. Shouldn't the voltage drop between q12 and q13 be 6x, since 2 of them are in parallel?

I will post some pictures when I get a chance.
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Old 31st January 2004, 12:44 AM   #8
meat is offline meat  United States
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Here are the pictures of my boards

1 channel

the other channel
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Old 10th February 2004, 01:30 PM   #9
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Your cap polarities are correct. I just compared them with mine since I happen to have a stuffed board sitting here on my desk.

You say it worked for a short time and then blew up. Have you checked to make sure you haven't biased the output transistors to maximum?
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Old 10th February 2004, 05:44 PM   #10
larryg is offline larryg  United States
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im also building this amp, comparing my boards with yours, yours is missing R16 and R51. (version 4.5) this may not be your problem, thought i would point this out. larryg
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