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Old 9th October 2003, 11:25 AM   #1
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Default symmetrical 400 problem...bias setting

Ok, I'm having trouble setting the bias on my aussieamplifiers symmetrical 400. There is NO mV across the source resistors (emmitor resistors) at all, no matter how much I turn the pot. Now, i built monoblocks...and both are behaving the same way. weird huh? I get almost zero mV offset...2 mv or so, and I do get sound out of the outputs...although it is quite distorted (either driver stage or preamp driving the speakers). There is voltage on the rails...and also taht rail voltage is getting to the mosfets...irfp240 and 9240. Any ideas where there is a bad solder joint or a bad connection? I have brought back the pot to zero ohms, per instructions, and measured 4.7k across q10's base and collector...again...per instruction... i have not check all the Anthony's checkpoints, but the ones I did check came out just fine. any ideas at all why no current is flowing? thanks!

-Matthew K. Olson
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Old 9th October 2003, 12:18 PM   #2
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Hello Matthew,
I am at the end of my re-construction of this amp with 2x300 VA toroids and the caps you sent me , and it works just fine....

Unfortunately today I can't access the net from home , but when I get home I'll check if I had to make changes to get it work and tomorrow will post the reply.

Are you using Anthony's pcbs or did you make them yourself?

Cheers

Andrea
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Old 9th October 2003, 12:24 PM   #3
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Thanks Andy. The power supply works great...no problems w/ rail voltage...just don't know where I went wrong. I did "alter" the traces a tiny bit b/c i drilled some holes in the boards to be a little bigger. :-D So I'll go back this afternoon and make sure that I did everything ok. But still, i'm pretty sure I checked that stuff. Anyway,......i just need someone to look at the schematic and see where ....around the pot somewhere...I may have messed up a trace so I know what I'm looking for. Thanks! Later!

-Matthew K. Olson
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Old 9th October 2003, 01:08 PM   #4
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It seems that the BD139 is conducting too much, stealing part of the voltage needed from the gate of the output fets.

I'd first check the resistance seen between C and E of this BJT, if it's fairly low you have found the guilty one!

Cheers

Andrea
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Old 9th October 2003, 01:16 PM   #5
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Ok...i'll check it. ok...suppose the resistance is low between c and e of Q10....it should be around 1k right? what to do if it is low (remember both amps are exibiting the same properties)?

-Matthew K. Olson
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Old 10th October 2003, 01:04 AM   #6
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Ok....its 6kohm resistance between the c and e of q10 (bd139). I'm getting only about 1 volt across the gates....but everything...all of Anthony's checks are fine. The pot works...there are no shorts that I can find, and all of the voltages are correct...except the gate voltage. (too low). Not only that...but its the same voltage for both monoblock amps, and no matter how much I turn the pots....the voltage stays at 1v across the gates ...easier to check...the outside of the ZD3 and ZD4....that run to the input resistors. .99v. I checked to see if the zeners were backwards, they are not. The bd139 is wired correctly...the main drivers get barely warm...bd139 does not. It handles rail voltage no problem....hmm...what else. This is gonna drive me nutso. I can change out the bd139's...just to make sure that they are ok...but would 2 of them be bad?...in both amps? too weird! any any any ideas????

-Matthew K. Olson
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Old 10th October 2003, 01:22 AM   #7
jleaman is offline jleaman  Belgium
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i've got lots of problems with my n-channel now. Now i guess ill have to take all the fets off and check them all now GRRR>>..!
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Old 10th October 2003, 03:09 AM   #8
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All is better!! Ok, for anyone doing the anthony holton symmetrical 400 amplifier...uh...the pinout on q10 is out of order. the collector on the bd139 does match up w/ the board....as does the emmiter and the base...BUT...the order on the board is cbe...rather than the rest of the board which is bce. Its weird...but as long as you are VERY careful to place the bd139's collector, base and emmiter leads (flying leads) in the right placed...everything is ok.

i hope its ok to post this link

http://www.farnell.com/datasheets/43480001.pdf

its to the bd139 pdf file.... this is to help anyone w/ the same troubles. Later!

-Matthew K. Olson
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Old 11th October 2003, 07:11 AM   #9
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Before I could help you you managed to find the solution, very well!

I use my own boards so there was no need to swap any pinout...

Cheers

Andrea
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Old 11th October 2003, 10:21 AM   #10
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Took me 8 hours to figure it out though. What a drag. If i thought Anthony would respond...I'd email him about it and tell him to post it on his site. Its just NOT cool that the base and collector are switched and in a different order than EVERY other transistor on the board. not cool at all. Thanks for the help though! later!

-Matthew K. Olson
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