Hello,
I have two circuits above. I am trying to figure out why pin 40 (A1) is reading 2.7V vs pin 39, 2, 1.

I suspect that its probably the Transistor is bad but I don't remember how to check for the transistor if its bad or not?

Do I do this with the power on or off?

I have check the resistors and they all seems okay (right value). I also checked the CAP for short but there's no short.

The only other thing I can think of is the power, but I am sure the powers are there since I am receiving 2.7V out of pin 40 (J1-5).

I have checked all the diodes and they seems okay too.
Can someone tell me where to check next?
thanks

 

What are the voltages at pins 3 and 35?

 

14.1V

 

No clue there then.
Can you measure the base, emitter and collector voltages of Q6, Q7, Q13 and Q14. Also the voltage at the cathodes of VR1 and VR2.
I do not understand the circuit on the collectors of those transistors - as drawn it doesn't seem possible!

 

thanks for your advice!
I measured the base/emitter/collector and found that Q14 is lower than the rest.
one reads 0.6V and one reads 0.2v so I believe its a bad transitor.
I will remove it and find out if it works!
thanks!

 

According to the circuis then the voltages of 8 pins should be comparing as:
1. pin-12(3.7V) comparing with pin-23(3.7V).
2. pin-1(2.4V) comparing with pin-39(2.3V) ... 0.1V
3. pin-2(2.4V) comparing with pin-40(2.7V) ... 0.3V
4. pin-6(3.8V) comparing with pin-28(4.1V).... 0.3V
5. pin-3(14.1V) comparing with pin-35(14.1V)

So there are 3 pins are different, the pin-2 and pin-39 in some application maybe is ok, but the pin-40(2.7V) is higher than pin-2(2.4V) 0.3V, and the pin-28(4.1V) is also higher than pin-6(3.8V) 0.3V.

You said that you already checked the resistors, did you desolder them from the PCB?

Please measure the voltages where I used arrows to shown in the circuits.

 

Scott, can you see where that -3.9V on the collectors of Q6, Q7, Q13 and Q14 comes from? Note the polarity of the two zeners and the electrolytics in parallel with them.

 

Scott, can you see where that -3.9V on the collectors of Q6, Q7, Q13 and Q14 comes from? Note the polarity of the two zeners and the electrolytics in parallel with them.

1N748A is a 3.9V zener, so it clamp the voltage from ground to -3.9V, but the circuit seems lost the resistor Rz for zener, please see the circuit that I redraw.

1N748A 3.9V zener.

 

1N748A is a 3.9V zener, so it clamp the voltage from ground to -3.9V, but the circuit seems lost the resistor Rz for zener, please see the circuit that I redraw.

Yes, that makes sense, but odd that they missed out both resistors.

 

Yes, that makes sense, but odd that they missed out both resistors.

I'm not sure the circuits belongs to which product, but when the manufacturer released the circuits, they could be modified the contents.

 

I used a DMM to measure the voltage across the points you mentioned.



the red font is the value using an osscillascope.
the green font is the value using a dmm (Vdc).

I only see that at Q5 and Q12 is where the major different in voltage drop.
I have tried replacing Q12 but it doesn't seems to fix the problem.
Maybe I am re-using Q12 from old component?

 

The -15V supply is defunct, only -4.7V.
Do you have the schematic of that bit of the circuit.

 

The -15V supply is defunct, only -4.7V.
Do you have the schematic of that bit of the circuit.

If the voltage is correct then it could be some kind of voltage regulator as 7915 was damaged.