You say it is a '100ohm resistor', the schematic calls for a 1K (1000 ohm) resistor.
Finding problem (with Amp circuit)
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3rd April 2012 06:03 PM #1irishape
Finding problem (with Amp circuit)
I have assembled this amplifer from a pcb I obtained on the internet. When I power up the R4 risistor gets very hot within 2 seconds. The instructions tell me that if the amp oscillates to change C2 from 220pf to 470pf to keep output transistors from getting hot. Is it reasonable to think that this may be my problem with the resistor which is a 100ohm resistor. I am doing this project just for learning but hate to remove something that is not the problem. In my inexperianced opinion I would suspect another reason. Any suggestions?
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3rd April 2012 06:30 PM #2House0Fwax
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3rd April 2012 06:44 PM Thread Starter #3irishape
You are right, it is a 1k resistor. I have been looking at too many numbers lately. That is to say it has a 1k resistor on the board.
0Last edited by irishape; 3rd April 2012 at 06:52 PM. Reason: more info
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3rd April 2012 07:09 PM #4KeepItSimpleStupid
Make sure Q13 and Q14 are installed correctly.
For power up purposes short the collector to Emitter on Q12. What DC offset do you measure at the speaker terminals?0
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3rd April 2012 10:20 PM Thread Starter #5irishape
OK, shorted Q12 but R4 resistor gets hot instantly so can not measure speaker output without damage to board.
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4th April 2012 11:49 AM #6alec_t
If R3, Q13 and Q14 are correctly wired and functional then the maximum current through R4 will be ~2.5mA and it won't even get warm. So, I agree with KISS. Double-check the pin-out and soldering of Q13 and Q14, and the value of R3. Check too that the pcb track layout around those components actually is consistent with the schematic (it's not unknown for pcbs to have layout errors).
0My circuit designs should be regarded as experimental. Although they work in simulation, their component values may need altering or additional components may occasionally be necessary when the circuit is built. Due safety precautions should be taken with any circuit involving mains voltage or electrostatic-sensitive components.
Alec's First Law:-
Every problem has a solution (given the right information and resources).
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4th April 2012 10:36 PM Thread Starter #7irishape
OK guys, I have checked R3 Q13 Q14, even removed them to make sure of proper type. Every thing checks. Went ahead and checked all resistors and transistors. All I have now is to check all tracks in board to make sure they all go where they are supposed to. Being new at this it will take awhile. There are no solder strings or any thing like that. Whatever it is it acts like a dead short because R4 is hot instantly yet it never blows a fuse on either rail. Anyhow I appreciate your help very much so thank you until your better paid. IA
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5th April 2012 02:48 AM #8ronv
only a guess but the transistors may be in backwards. Google a datasheet and make sure which view of the pins you used.
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5th April 2012 04:04 AM Thread Starter #9irishape
Thanks ronv but I have already checked to make sure that npn and pnp are in proper place and the board has pattern as to which way they face. IA
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5th April 2012 01:56 PM #10alec_t
Not all transistors face the same way. Unless the transistor code (type) of your transistor exactly matches the transistor code that the board was originally designed for you could have the transistor pins wrongly connected. Does the board have markings 'b', 'c', 'e' next to the pattern?the board has pattern as to which way they face0My circuit designs should be regarded as experimental. Although they work in simulation, their component values may need altering or additional components may occasionally be necessary when the circuit is built. Due safety precautions should be taken with any circuit involving mains voltage or electrostatic-sensitive components.
Alec's First Law:-
Every problem has a solution (given the right information and resources).
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