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Old 10th December 2004, 09:07 PM   #1
Mlaen is offline Mlaen  Croatia
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Default Problems with building an amp...please help!

I'm trying to build amp from http://www.electronics-lab.com/proje...001/index.html
Problem is that whem i turn on the amp the quiescent current is 0 no matter what value R11 is
I don't know where the mistake could be (everywhere!), all transistors are OK, and elements are where they sould be (I checked that MANY times)
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Old 10th December 2004, 09:51 PM   #2
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Do you have any voltage across R10 and R12? If no you haven't got any current through the R11.

Make sure that you have all expected currents in the input stage and the VAS stage. Just approximate Vbe to 0.7V and current gain to 100. With this assumption you can calculate pretty accurate.

The voltage between the gates of the mosfet's should be 5-7 volts for normal mosfets and 0-2 volts for 2SK135/2SJ50 and similar "old" Hitachi types.
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Old 10th December 2004, 09:56 PM   #3
MikeB is offline MikeB  Germany
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Hi !

The circuit looks okay, have you measured voltages across the
r10/r12 ? These should be somewhere at 0.6-1.0 volts.
Maybe you got some pinout incorrect ?
The current in vas should be ~20ma, with 500ohm this gives
far enough voltage to open the mosfets.
With a voltage of ~7v across r11, the mosfets should open.

Ooops, peranders was faster than me...

Mike
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Old 10th December 2004, 09:58 PM   #4
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Safe to assume you have checked you power supply is delivering properly?
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Old 11th December 2004, 12:16 PM   #5
Mlaen is offline Mlaen  Croatia
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Default More problems...

Yesterday when I turn on the amp nothing was going on and there was no currnet enywhere so I posted the question here and went to sleep... today I turn on the amp and R13 and R16 start to BURN?! WTF!? I did nothing diffenet than yesterday and yesterday was on for about an hour...
I don uunderstand...I checked the board now and couldn't find any mistake with my eyes...
I dont have any spare parts so I'll have to wait untill monday to buy them
Thanx all for posting anyway...

PS:
Papageoff:
Yes, PS is giving -+35V
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Old 11th December 2004, 12:30 PM   #6
MikeB is offline MikeB  Germany
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Maybe simply one of the transistors was defect ?
You should measure the transistors before building, this has 2
advantages:
1, You can sort out defective ones
2, You see if pinout is different. Typically pinout is CBE, but i have
seen TO92-transistors using BCE...

And don't forget, the bd139/140 has a reverted pinout (compared
to bigger powertransistors), it does not have BCE, it uses ECB.
(order when looking to the printed side)

Mike
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Old 11th December 2004, 02:55 PM   #7
Mlaen is offline Mlaen  Croatia
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something like this (picture)?
that's the way I'm using it...
It's when you look at him at front (side to attached to heatsink is back)

Edit:
I forgot the pic
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File Type: jpg untitled.jpg (3.0 KB, 67 views)
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Old 11th December 2004, 02:56 PM   #8
MikeB is offline MikeB  Germany
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Yes, that's the correct pinout !
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