Help with Leach amp

[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

 

[BobEllis]

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.

 

[meat]

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.

 

[sss]

u can measure the voltage on c14 and see whats wrong

 

[BrianGT]

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.

--
Brian

 

[BrianGT]

Re: off topic (sorry)

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.

--
Brian

 

[meat]

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.

 

[meat]

Here are the pictures of my boards

1 channel

the other channel

 

[Maxhawk]

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?

 

[larryg]

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

 

[TonyTecson]

did the neg rail fuse blew?

i once encountered this problem, you can check q3,q4,q6&q9 also zenersd15/d16 most likely shorted junctions for these devices...

 

[meat]

Thanks for all of your responses. I think I may have a problem with signal ground and central ground. These are 2 different grounds, correct? His documentation is a little unclear on this.

 

[TonyTecson]

yes, but they all return to the power supply ground, if i were you i'd put the speaker ground wire directly on the psu capacitor junction where the neg side of the positive rail cap, and the positive side of the negative rail cap and the psu transformer center-tap wire meet or are tied together.

 

[dmfraser]

Light Bulb

I suggest next time you fore this amp up that you have a 100W lamp in series with the AC line so you can troubleshoot it without blowing stuff up.

 

[TonyTecson]

hi,
another technique i use is to solder a 10ohm 1/4 watt resistor in series with the output trannies, this way if biasing is way off, you will burn the resistor instead, this is more economical...