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Old 18th November 2011, 05:30 AM   #1
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Default Help cannot identify Amplifier Power transistor

does any have idea what Power Transistor used in this circuit? because here in my country if assembled circuit they erased the number of the components so that you can only buy on their store....

my idea it is MJ2955 or 2N3055 but im not sure....or both

hope some one can help....

Here's the diagram i based it on the circuit board....

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The Supply voltage is Between +- 18-36Vdc

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By art_vip at 2011-11-17

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By art_vip at 2011-11-17

I hope someone can help me... i really want to revive this amp...thank you
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Old 18th November 2011, 06:01 AM   #2
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Are the unknowns mounted elsewhere? Of the options you listed it would have to be the 2955 as the 3055 is an NPN do if you have the schematic correct I'd go with 2955. But from what I see on the bottom pic it looks like the TIP42C (second in on left) is bad. TIP42C is a valid part number.
Doc

Oh, I see the output device pads on the edge of the board now... Need pics of output devices.
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Last edited by thaumaturge; 18th November 2011 at 06:05 AM.
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Old 18th November 2011, 10:39 AM   #3
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thanks for the reply...

i just made the schematic based on this board i dont know if is NPN or PNP because all of it is shorted... you can see on the left channel that there are traces of burned.. i already replace the defective parts on this circuit except those power transistor....

part no. of power transistor mounted on a heat sink has been erased...

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By art_vip at 2011-11-18

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By art_vip at 2011-11-18
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Old 18th November 2011, 12:54 PM   #4
jaycee is offline jaycee  United Kingdom
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The schematic shown is incorrect, with NPN/PNP transistors interchanged. Judging by the schematic with this in mind, the power transistors are 2N3055. They may have also used MJ15003, MJ15024 or similar.

The circuit is a very basic quasi-complimentary amplifier. The VAS device (top transistor in the middle column, incorrectly drawn as NPN) should be a PNP device, presumably the TIP42C. That's a pretty poor choice of device for VAS duty.

Despite board markings, there must be two TIP42C's (one as VAS, one as driver for the lower output transistor) and one TIP41C (driver for the upper power transistor).

I would not recommend this circuit. Another serious problem appears to be thermal compensation - there is no active VBE multiplier device (instead a single diode appears to be used) and it is not even coupled to the main heatsink.

With much the same parts you could build a much better circuit.

Last edited by jaycee; 18th November 2011 at 01:00 PM.
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Old 18th November 2011, 01:41 PM   #5
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Do any of the output transistors test OK ?

If one does you can measure its gain to get a better idea what it might be.
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Old 18th November 2011, 02:44 PM   #6
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thanks for your replies....

i could not build another circuit because it will cost me much... that's why i just like to revive this... the only problem i found here is the Bias Trimpot... when it gets older it doesn't do what it supposed to do.... that's what happen burning the power output transistor, but i got figured it out what will i do with that.... what just i need is the value (part number) of the power transistor.... thanks
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Old 18th November 2011, 03:01 PM   #7
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Please see POST 5.

We can guide you with how to try and identify the Power Transistors.
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Old 18th November 2011, 03:51 PM   #8
wg_ski is offline wg_ski  United States
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If the output transistors are shorted, measuring them won't tell you anything - other than that they're bad. If the power supply voltage being used is +/-30V or less, 2N3055's are perfectly fine. Not an ideal choice, but it won't be the limiting factor on how it sounds. With higher voltage, you could use ANY of the MJ150XX types (as long as it's NPN). There are probably a hundred old types that could be used, as long as they're used within voltage capabilities, and are designed to be used as amplifiers. In circuits like this where the performance is compromised by so many factors, you might be surprised to find out how non-critical the transistor types are. Chances are, an older TO-3 that's too fast to work reliably will be expensive, and ones that will work with no trouble are cheap.
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Old 18th November 2011, 06:05 PM   #9
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ok thanks i re draw the schematics... do you think it should go like this......

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Old 18th November 2011, 06:45 PM   #10
Mooly is offline Mooly  United Kingdom
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Identifying the outputs should be easy...

Pick one location on the PCB and see where C and E go. NPN always has the collector positive with respect to emitter and for PNP it's vice versa.

If the audio output of the amp is from where E of one goes to E of the other or if by chance C of one goes to C of the other in a pair then they are complementary. One NPN and one PNP.

If E of one goes to C of another for the audio output then they are both either NPN or (unlikely) both PNP.

Deteming which is which in a complementary pair is easy... just look at the supply polarities to the transistors.

If it were in front of me it would take seconds... very hard to put into words what to do


To determine which look at polarities of supply and determine
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