A little help regarding for repair of my Intercom

Thread Starter


Joined Jun 30, 2007
Continued from LAST MSG

And i am using a center tapped transformer....9-0-9V (rms)...on secondary and primary 220V. The actual voltage values i checked from DVM were....13V for the LM386 amplifier and 26V for the buzzer circuit. I kknow that this 26V value is above the value of capacitor 2200uF which has a value of 25V. but i am here concerned about the LM386 Amplifier circuit. One thing i can do is change the transformer ...use something between 8-0-8V..that will solve this problem....but why there is just noise coming from the chip....
can it be so that the DC voltage i am applying on the chip is above 12V(the rated value of LM386) but that chip should burn out but that chip works when it assemble it in another circuit which i have made other than this PCB. can it be so that..the LM386 produces noise for supply voltage above 12V?

Anyone help me now?


Joined Oct 11, 2004
Your measured voltages are obviously with the power supply unloaded. What is the voltage on the 12V rail when the LM386 is connected? How do you know the circuit is operating correctly? Have you checked it using a battery? If so, what current did it draw?

The noise will not increase for a 12V supply - the device is rated at 18V max.

As has been said before, D3 is not necessary. It serves no purpose other than reduce the two rail voltages by 0.7V each.

As the buzzer is only used intermittently, the rail voltage will rise to a peak of 24V when unused. So there's not much headroom with your 25V rating for C1. Go for a minimum of 35V.

Your choice of 1N4148 for rectifiers is not a good one. They are signal diodes not intended for power rectifiers. If you don't want them to pop long term, then go for 1N4001 types.


Joined Apr 27, 2007
I think there is preety much said there. Your intercom, commercial or not, was misconceived. I'm not infering nothing, since actually most comercial circuits are of poor electronic quality, despite the cost and aspect they might have.


Joined Nov 29, 2006
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.?
With any buzzing sound in an audio circuit, one should first look at half waving/unfiltered power supply.
Second thing to do, is to look again.

The circuit that you have provided is two half wave rectification circuits via D1/C1 and D2/C2,R2,C3, god knows what D3 does. I still believe that it is drawn incorrectly, I may be wrong, I'm not there with you, but in thirty + years I have never seen a power supply like that.

Not trying to be nasty, I'm just trying to help. At this point you need to start over and verify all you have and report back accordingly. Assume nothing you have done up to now.

The buzzer can be half waving, who cares about ripple on a buzzer voltage, but the op amp has to be pure filter DC or you will amplify any ripple.

1N4148 are only like .25Amp tops and the current surge to charge the caps would probably be pushing their limit for sure.

Your first message said you replaced the tranny. How did you know what votage replacement to use? Was the original labeled? If the voltage is exceeding the capacitor rating then the tranny is over suppling or the cap was underrated from the get go..

Where the 1N4148 the original type?
Could you have possibly put a diode in backwards????
The 5.6 ohm resistor is shown as 5 watts P=I squared x R which is .96 amps. Are you sure that it is 5 watts? If not then the diodes are underrated.
You still have not answered the question as to: "what the AC component voltage on the filter caps" are. All three of them. And I only see Dc voltage values for two caps.