Do not understand how this circuit work in car audio amplifier

Thread Starter

Linas Lapeikis

Joined Mar 19, 2017
21
I want to know how the marked transistor is controlled which is responsible for the relay control. I do not understand under what conditions the marked transistor is on. Thanks!

Here is the picture of the schematics:
https://ibb.co/f21nfw
 

AlbertHall

Joined Jun 4, 2014
8,551
That circuit connects to wires that disappear off the top of that diagram. Can you give us a complete and legible schematic please. The original pdf, not a screen grab.
 

AlbertHall

Joined Jun 4, 2014
8,551
That transistor is on under normal conditions - no fault. It is turned off and disconnects the speakers if there is residual DC on the outputs of the amplifier which could damage the speakers.

C651 is connected by four 4.7k resistors to the amplifier outputs. This capacitor removes the audio and stores the DC level of the amplifier outputs. Then R655 feeds that voltage to the base of Q654 and via D651 to the base of Q655.

If the voltage is zero (as it should be) then both Q654 and Q655 will be off, and R656 will bias Q653 (the marked transistor) on enabling the speakers.

If that voltage is positive then Q654 will be on, if it is negative then Q655 will be on, and in either case the bias for Q653 will be removed and it will switch off disconnecting the speakers.
 

Thread Starter

Linas Lapeikis

Joined Mar 19, 2017
21
That transistor is on under normal conditions - no fault. It is turned off and disconnects the speakers if there is residual DC on the outputs of the amplifier which could damage the speakers.

C651 is connected by four 4.7k resistors to the amplifier outputs. This capacitor removes the audio and stores the DC level of the amplifier outputs. Then R655 feeds that voltage to the base of Q654 and via D651 to the base of Q655.

If the voltage is zero (as it should be) then both Q654 and Q655 will be off, and R656 will bias Q653 (the marked transistor) on enabling the speakers.

If that voltage is positive then Q654 will be on, if it is negative then Q655 will be on, and in either case the bias for Q653 will be removed and it will switch off disconnecting the speakers.
Thank you very much. So I need to check is there is some DC on every channel of amplifier output section? And what does the R653? Is this DC protection circuit is somehow controlled by the Power/Muting IC?
 

AlbertHall

Joined Jun 4, 2014
8,551
Yes. DC on the amplifier outputs is bad.
When pin 3 of IC601 is positive it turns on Q654and disconnects the speakers as above. It also feeds Q501 and Q502, muting the PREOUT outputs.
 

Thread Starter

Linas Lapeikis

Joined Mar 19, 2017
21
Yes. DC on the amplifier outputs is bad.
When pin 3 of IC601 is positive it turns on Q654and disconnects the speakers as above. It also feeds Q501 and Q502, muting the PREOUT outputs.
I always worried that IC601 is one that turns the relay off in my situation. It gives positive voltage on pin 3. The main problem is that I read its datasheet and still cant breafly understand under what condition it is muting amplifier. Can you explain me? Sorry for being so noob :/
 

AlbertHall

Joined Jun 4, 2014
8,551
Pin 3 will be high if pin 9 is >0.8V. This will be the case if the current in one of the amplifier outputs is too high (D6*3) or it is too hot (D9*3)(TH901).

* = unreadable digit.

[EDIT] Also if the voltage on pins 10 or 12 is high enough (I don't know how high that is!).
 

Thread Starter

Linas Lapeikis

Joined Mar 19, 2017
21
Pin 3 will be high if pin 9 is >0.8V. This will be the case if the current in one of the amplifier outputs is too high (D6*3) or it is too hot (D9*3)(TH901).

* = unreadable digit.

[EDIT] Also if the voltage on pins 10 or 12 is high enough (I don't know how high that is!).
 

AlbertHall

Joined Jun 4, 2014
8,551
Q656, 657, 658, and 659 monitor the voltage across the current sense resistors and, if too great, turn on Q651 which sends voltage through R6*7 and D6*4.
 

Thread Starter

Linas Lapeikis

Joined Mar 19, 2017
21
Q656, 657, 658, and 659 monitor the voltage across the current sense resistors and, if too great, turn on Q651 which sends voltage through R6*7 and D6*4.
Thank you very much. I am getting to realise and understad how it works. The last question would be: For the whole power supply to be on and working, the muting IC pin 4 must be at 0 volts? Or I am wrong? How the pin 4 of the IC interacts with Q603?
 

Thread Starter

Linas Lapeikis

Joined Mar 19, 2017
21
Pin 3 will be high if pin 9 is >0.8V. This will be the case if the current in one of the amplifier outputs is too high (D6*3) or it is too hot (D9*3)(TH901).

* = unreadable digit.

[EDIT] Also if the voltage on pins 10 or 12 is high enough (I don't know how high that is!).
So for normal working condition pin 10 and 12 must be low?
 

AlbertHall

Joined Jun 4, 2014
8,551
I think I got that the wrong way round for pin 12 as this is connected to the power input (which should be 12-15V).
I'm really not sure about pin 10.
 

Thread Starter

Linas Lapeikis

Joined Mar 19, 2017
21
If I solder out one suspicious transistor and every leg pair shows infinite resistance in both measuring way. Is it a bad transistor?
 

AlbertHall

Joined Jun 4, 2014
8,551
Yes, providing your meter is in the diode test position. Ordinary ohms ranges may not have enough voltage to turn on a silicon diode transistor junction.

Which transistor?
 

Thread Starter

Linas Lapeikis

Joined Mar 19, 2017
21
Yes, providing your meter is in the diode test position. Ordinary ohms ranges may not have enough voltage to turn on a silicon diode transistor junction.

Which transistor?
I checked Muting IC. The one reason for muting signal was the current sensing circuit transistor Q651. Desoldered it and it was dead shorted. Whitout that shorted transistor Muting IC mute out signal was off (low) but the relays was still off. Then I removed Q654 and Q655. And still Q653 was off. Checked the Q653 base voltage and it indicates 12V. I think that is the right voltage to turn this transistor on as collector voltage is 12V too. Am I right? So I removed the Q653. Measured it with diode check, and every situation measure infinite resistance. The same is with Q654 and Q655.
 

AlbertHall

Joined Jun 4, 2014
8,551
I can see no possible cause of the death of Q651 other than suicide. It has resistors on each lead that would limit the current.
As the emitter of Q653 is at 0V its base voltage should never be higher than about 0.7V. That would be enough to turn it on. If the base was at 12V then that indicates that the base/emitter was open circuit as your unsoldered test confirms. I cannot see any component that could blow up these four transistors. It is just about possible that a short across the relay coil (in the relay - unlikely - or the diode across the coil (D656, more likely) could damage Q653 and might also get Q654 and Q655.

Has it been struck by lightning, I ask almost seriously!
 
Top