A little help regarding for repair of my Intercom

Discussion in 'General Electronics Chat' started by arsalanarahim, Jul 3, 2007.

  1. arsalanarahim

    Thread Starter New Member

    Jun 30, 2007
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    Hi u All

    i have been working on my intercom which was faulty. I removed....its burnt transformer and the diodes of the rectifier (power supply of the INtercom)....then i had my circuit On but there is a loud BUzz coming out of the speaker(handset)....the LM386 chip is being used. I also changed it because may be it had also blown due to over voltage as it has only 12v absolute rating. I placed a new one...but there is so much noise coming out.....the voice is being amplified it feels..when i remove the 10uF capacitor from pin 1 and 8...... The voice is amplified but there is so much noise that.....it is practically impossible to hear. even when there is no input to the amplifier still it does have the noise. I read all the post on this forums regarding LM386 but could not solve it. I placed ripple capacitor too....for the power supply. i am using a 1uF capacitor at input to block the DC and 220uF at the output of the pin between the pin and the speaker. i even checked at the output pin voltage without input applied..to check whether the chip is blown or not....but it is giving full half voltage of the supply voltage. I checked the current on my Ammeter for the whole this circuit...it came out to be 80mA. this is ridiculous.. according to datasheet of the chip it can only 4mA current....how it is taking 80mA.(this is whole circuit current just including chip and...capacitors and mic only)
    if it is blown then why its also amplifying the voice...but with noise. i am really mad about this circuit now.....can anyone help me in this regard. And the circuit for this is very simple....the same given in the data sheet of LM386. Can anyone tell me....whats wrong....? is the chip not right or what?
     
  2. arsalanarahim

    Thread Starter New Member

    Jun 30, 2007
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    The datasheet link is this:
    http://www.national.com/ds.cgi/LM/LM386.pdf


    I need immediate help anyone please. I have used 5 chips...overall but no one has satisfied..have i blown all my chips or whats the problem
     
  3. Gadget

    Distinguished Member

    Jan 10, 2006
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    Have you "scoped" the DC to see if there is excessive ripple..?
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
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    Can you post your power supply schematic? Datasheet shows only the amp.
     
  5. arsalanarahim

    Thread Starter New Member

    Jun 30, 2007
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    I am attaching the power supply of my Intercom Circuit...
     
  6. Gadget

    Distinguished Member

    Jan 10, 2006
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    Cant work out the purpose of D3. Also is only half wave rectified. I would be trying something more like this to see if it works better...
    [​IMG]
     
  7. thingmaker3

    Retired Moderator

    May 16, 2005
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    With D3 in place, the power supply is "riding" on a 0.7V square wave (with a very high duty cycle). The caps won't filter it out because they are also "riding." Get rid of D3!
     
  8. arsalanarahim

    Thread Starter New Member

    Jun 30, 2007
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    Ya u all are saying right..that this diode has no meaning at all....
    But i copied this circuit from a PCB...and this INtercom was working...purely in working condition....when it burnt due to some supply voltage surge...

    After changing the transformer and the diodes.... the LM386....it is giving noise........

    Can anyone comment on the condition of LM386.....if this whole LM386 chip and the associated capacitors are taking 80mA. dont u think it is faulty chip...infact 5 of them are...then faulty....

    Now i am really sick of this....intercom....anyone please help
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
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    Are you sure the filter caps are still good?
     
  10. pfofit

    Member

    Nov 29, 2006
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    Are you sure you have the schema traced out correctly?

    Check your filter caps, 2200 and 1000. remove them one at a time and fit something close in value to see if your buzz, buzzes off.

    Were the diodes fried, open or shorted? Or did you just change them for the fun of it. What's the voltage rating of the filter caps, they probably got toasted in the surge, or if the diodes shorted then they got AC'ed.

    If you can't scope the filter cap, then put your dvm on Ac and read the ripple.

    What Dc value do you have on the filter caps?
     
  11. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
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    we had been trying to point to the psu as the culprit for the noise. however "arsalanarahim" has pointed out that he has a quiescent of 80ma which we can interpret as that the circuit is drawing too much current. in my opinion its the 386 that's at fault. maybe you may not have heard that there is a huge proliferation of fake LM386 especially here in asia.

    moz
     
  12. arsalanarahim

    Thread Starter New Member

    Jun 30, 2007
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    Ok now i rechecked the PCB which i was tracing..it was so that....there are three points where three wires from the transformer secondary can be attached. May be i got wrong in interpreting the connections of center tapped.....for this i have again....drawn the circuit diagram in PDF format and attached.
    When i changed the diodes....one which is in reverse direction.....was open...i just changed all....for precaution.

    pfofit....u are very much concerned about...power supply....but i think after the correction u should not have much concerns.. About the DC values....of Capacitor .....2200uF..25V...and 1000uF are of 16V....

    mozikluv..what u mean by this .. "we had been trying to point to the psu as the culprit for the noise." i cant get what is psu....ya u are right...80mA...is the total current of the chip and associated capacitors...
    i am from Pakistan.....so really i cant find a good chip from here...all are faulty....
     
  13. pfofit

    Member

    Nov 29, 2006
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    Ok, that's great.
    Now what is the actual DC and AC voltage across each capacitor?
     
  14. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
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    1. "psu" means power supply unit

    2. have you check the signal input capacitor?

    3. with an 80ma current draw could mean that you have a short circuit in your system. check the foil traces, check for overlapping soldered leads

    4. have you check the output of your psu?

    5. have you check the filter caps if it's still good as "pfofit" suggested?

    6. the 386 replacement that you bought, is it LM386-1 or LM386-4. the 386-1 can take only a maximum of 13.8v while the 386-4 can take a maximum of 18v

    7. the LM386 output should be half of your supply voltage.

    moz
     
  15. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    You need to rethink that power supply.
     
  16. pebe

    AAC Fanatic!

    Oct 11, 2004
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    Arsal.....
    Don't mess around with your own design.
    Use the circuit that Gadget gave in post #6
     
  17. cumesoftware

    Senior Member

    Apr 27, 2007
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    As someone said, taking D3 out might solve the problem, but it also might not. Since the cap is filtering the ripple, any ripple won't pass the capacitor. It is true that the ground floats because of D3, but the amp will float as well. What D3 really does is to agravate a potential ground loop. Also, you really should use a smaller filter cap located near the amp, precisely to avoid ground loops. I had the same experience with a LM386. The supply already had a 1000MF filter capacitor, but the amplifier was motorboating precisely due to a ground loop. The problem was solved by using a small capacitor near the amp (keep the leads short - I used a 100nF capacitor with no problems, and tried a 47nF with an almost unperceptable rise in humming).

    Also, your supply design is mis-conceived. If you want to use a centre tap transformer instead of a simple secondary transformer and a diode brigde, you should take out C2, C3 and R2, since they are only destabilizing, and join the cathode (negative side) of D2 to the cathode of D1, where should be the positive rail. The type of diodes is also wrong, since these diodes wont go past 200mA (you should consider the average current and NOT the DC current). Use 1N4001 diodes instead. You will need more than 200mA (power amplifiers can cause your diodes to fry).

    In the projects collection section, you have a circuit that uses LM358 amplifiers. It is a stereo amplifier. Search for "Active speakers".
     
  18. pfofit

    Member

    Nov 29, 2006
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    I was under the impression that this intercom schematic is NOT home made. rather a 'store bought" one that he has traced out in an attempt to get to the root of his problem.

    The schema is indeed a weird setup, D3, half waving, and dinky diodes. I'm guessing, but I suspect the reverse engineering to get the schema may be in error.

    Attn: Arsalanarahim, can you please confirm if this intercom is home made or store bought??

    Also post the voltage values on the capacitors.
     
  19. cumesoftware

    Senior Member

    Apr 27, 2007
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    I would not be impressed to see unecessary or odd circuits in commercial product. Might be a question of manufacturer incopetence, or just a silly way to make the product more difficult to reverse engineer. I would not copy that design for sure.
    Then again, it might be some error of who copied the circuit to paper (happens a lot).
     
  20. arsalanarahim

    Thread Starter New Member

    Jun 30, 2007
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    ok guys, now listen this is an intercom which was working till last month successfully and the circuit i traced out was on a PCB. It is a full built intercom from market working for the last 2 years great then it burnt due to a surge voltage in the mains supply. The circuit of the PCB is again attached. Its a full wave rectifier. It was two voltage....one nearly 12 and the other nearly 25V. The DC values of Electrolytic Capacitors is that 1000uF has a value of 16V and 2200uF has a value of 25V.
    After so much hectic efforts...i decided to first make a LM386 amplifier circuit on the breabboard and i can say with full confidence that its working perfect. I joined and checked the same circuit on the breadboard by tracing out the PCB.
    Then i connected the intercom PCB and checked out...The buzzer circuit of the intercom is working fine and it buzzez when the button is pressed. the LM386 i am using now is working and fine.....
    Now i think the power supply has some errors..the diodes i have changed. now left part are the capacitors and one resistor 5.6R (5W). Now i want your help for this power supply.
    When i connect the LM386 chip on the PCB...there is buzzing sound...otherwise it should be clear. Why is it so ....is it because of the power supply...is it being shorted....due to some capacitors...or what....anyone please....help me for this.?
     
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