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Old 19th April 2011, 01:12 PM   #1
xcicc is offline xcicc  Italy
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Default 2n3055 + mj2955 ampli

Hi, has anyone tried to build this amp?

Electronics Circuits: Power Amp OCL by 741+2N3055+MJ2955

I bought the printed circuit board and mount it all.
I put in series to power a 100W lamp to limit the current.
But if I keep it on for a few seconds I get out of the smoke from the speaker.
(fortunately cheap speakers for testing)
I can not adjust the bias in the sense that rotating the trimmer in one hand and the other does not change anything.
any ideas?
Following are some pictures of my amp under construction / testing
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File Type: jpg CIMG0803.JPG (431.5 KB, 458 views)
File Type: jpg CIMG0804.JPG (417.9 KB, 413 views)
File Type: jpg CIMG0805.JPG (375.8 KB, 389 views)

Last edited by xcicc; 19th April 2011 at 01:20 PM.
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Old 19th April 2011, 01:58 PM   #2
infinia is offline infinia  United States
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Hi
Maybe you have a small mistake somewhere, check the pcb for backward transistor, diode or wrong value, etc.
disconnect the collectors of both TO-3 transistors (the 2 big ones) and measure and record all the DC values. The bias adj should be very minimal in range. BTW, These circuits from that web site are not very good, more like text book experiments.
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Old 19th April 2011, 02:03 PM   #3
xcicc is offline xcicc  Italy
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thanks for the reply, I checked everything, and everything is alright.
then you say; disconnect the wires of the two collectors? sure? will not burn anything?
and the speaker? must be connected right?
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Old 19th April 2011, 02:10 PM   #4
infinia is offline infinia  United States
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yes keep the light bulb for safety. no speaker connection until everything checks out.
markup and post schematic with all your measurements.
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Old 19th April 2011, 02:16 PM   #5
xcicc is offline xcicc  Italy
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Quote:
Originally Posted by infinia View Post
yes keep the light bulb for safety. no speaker connection until everything checks out.
markup and post schematic with all your measurements.
damn, I just disconnected the two collectors, but with the speaker connected, the result, the lamp is turned on for just a moment and then goes off so it's a good sign, unfortunately I saw a beautiful cloud of white smoke from the transistor BD139

I do not have an electric schematic, the website has the schematic
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Old 19th April 2011, 02:20 PM   #6
xcicc is offline xcicc  Italy
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to be precise, the PCB is slightly different from the drawing, there is no capacitor between pin 1 and 8 of the two transistors 741 I have changed the bd139/140 respectively.
but this should not affect the failure
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Old 19th April 2011, 02:25 PM   #7
infinia is offline infinia  United States
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Just looking at your very good images, I can NOT see that you used two (2) insulating nylon shoulder washers for the TO-3 mounting screws. The case (collectors) of both transistors should have very high ohm resistance to the heatsink. BTW your heat sink as shown is probably way too small!
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Old 19th April 2011, 02:30 PM   #8
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isolations of the two power transistors are ok!
the insulation of the transistors are mounted at the top to the contrary in order to connect the collectors.
if the heatsink is too small at most there would be a substantial heating not you agree?
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Last edited by xcicc; 19th April 2011 at 02:37 PM.
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Old 19th April 2011, 02:31 PM   #9
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Because this is a cfp topology (very primitive) and oscilates badly try to use compensation caps (local feedback) betweent the base and collector of the driver transistor bd139 and bd140. Start from 680pf and lower the value to 22 pf. if you succed to stay between to 100pf ~ 33pf you will be OK. Otherwise you will probably have a loss of bandwidth. If it still oscilates with 680pf try also to fit caps between the output transistors between the base and the emitter. Th e same method as the driver. As lower value the better.
Good luck
Ps keep the bias current low and increase gradully
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Old 19th April 2011, 02:33 PM   #10
infinia is offline infinia  United States
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Quote:
Originally Posted by xcicc View Post
to be precise, the PCB is slightly different from the drawing, there is no capacitor between pin 1 and 8 of the two transistors 741 I have changed the bd139/140 respectively.
but this should not affect the failure
agreed,
mark any part changes on the schematic along with yer DC values. Im looking mainly for the max and min current flowing thru the driver transistors with both the output collectors floating disconnected.
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