op-amp: njm4560

spdf

New Member
Hello guys I tried to measure dc voltages on op-amps in onkyo a-8650 and I found these results:

Q105
1. -0.36V; 2. 0V; 3. 0V; 4. -18.1V; 5. 6.4V; 6. 6.4V; 7. 0V; 8. 18V

Q106
1. 0V; 2. 6.4V; 3. 6.7V; 4. -18V; 5. 0V; 6. 0V; 7. 1.9V; 8. 18V

Does dc voltage on Q105 1 and Q106 7 outputs suggests misbehaviour of these components?
 
Last edited:
You have to tell what opamp they are. From the numbers, they are dual opamps. Pin 8 is the +V and you said it's -18V, you must have reverse the meter probe. Q105 looks ok if I am right that it's a dual opamp and pin 8 is +18V. Q106 seems to have a problem.
 
Q106 readings look weird but it could be another fail not always the opamp. You have voltage at pin 7, but zero at 5-6 what are the inputs. I'd expect the output to be the difference between + and - inputs. It could be a bad opamp or a bad capacitor letting the voltage reach pin 7 from other point. Check schematic to see connections from pin 7.
 
Last edited:
I confirm the njm4560 is just a typical dual opamp pin out. My first post stands. For normal opamp operation, voltage on -ve and +ve input has to be equal. This shows the feedback loop is close ( this is a standard check for opamp function). If the two input equals, then look at the output, if it is not railed, the opamp is at least sort of working.

Q105 looks normal, but Q106 is not. You can see pin 2 and 3 is not equal. This means the loop is not closed. The normal situation in this case, output should rail to about +17V because pin 3 is 6.7V and pin 2 is 6.4V. Pin 3( +ve) is higher than pin 2 ( -ve), so the output should go to the +ve rail.

Also. Your measurement on pin 8 is wrong. It should be +18V, not -18V as said in the earlier post. I don't believe you lose the +18V supply as the opamp Q105 is normal, it got to be you that measured wrong.


I don't know why you ask about the opamp, what is the problem? tell us your problem first.
 
@Alan0354 I believe you may be misinterpreting the OP's method of notation as regards pin voltages. For pin 8 (I believe) he does indeed report +18 and the "-" sign in this case is meant to be a hyphen or a dash, not a negative sign. See pin 4 where there are two - symbols in sequence to (I believe) indicate negative. It is a confusing method of notation.

You have to tell what opamp they are. From the numbers, they are dual opamps. Pin 8 is the +V and you said it's -18V, you must have reverse the meter probe. Q105 looks ok if I am right that it's a dual opamp and pin 8 is +18V. Q106 seems to have a problem.
 
@Alan0354 I believe you may be misinterpreting the OP's method of notation as regards pin voltages. For pin 8 (I believe) he does indeed report +18 and the "-" sign in this case is meant to be a hyphen or a dash, not a negative sign. See pin 4 where there are two - symbols in sequence to (I believe) indicate negative. It is a confusing method of notation.
Ah!!!!
 
Indeed my notation method is confusing I'll try to improve next time ;]

The problem is that on phono input I hear constat hum and the only part in question I can find is this oamp.
 
4560 is a cheap IC, if you doubt of it, just get a new one.

But voltages are way off. Schematic calls for 6 V at pins 2 - 3, and you have zero at Q105. Then , pin 7 should be -1.3V, and you have +2 on Q106, and zero on Q105.

You have a differential pair (Q102, Q101) at the input of the phono preamp. Check voltages there too. All the voltages are detailed in the schematic.
https://www.dropbox.com/s/ntrf084grebf26x/hfe_onkyo_a-8650_schematic.pdf?dl=0

S101 (cartridge selector) is under suspect. I'll deep clean it, even do some passive continuity tests between different points to see if it's working OK.

Is the HUM present in both channels? If so, suspect some ground connection, the mentioned switch, even the power supply to that preamp (C147-148, Q109-110, C151 and C152)

But the voltages that far from the schematic show something is definitely not OK, not just PS ripple.
 
Last edited:
I looked at the schematic, I don't agree with their voltage, I did calculation and wrote on the copy, double check my work.

Onkyo phono.JPG

1) It said Q109 and Q110 are +18V and -18V regulator resp. Why say 17.7 and -18.4, should use +/-18V.
2) The two opamps used in Q106 got swapped, they don't match Q105 if you look at the schematic. pin 2 and 3 should read 0V.
3) I calculated the current through R117 to get the tail current of the differential pair Q101. Assume the Vgs is -1V for jFET, so it's 19V across R117. current I=V/R= 19V/5.6K to give 3.4mA.
4) from 3.4mA, assume the two jFET share equal current, so it's 1.7mA through R119 and 1.7mA through R121. So voltage drop across the two resistors are 1.7mAX5.6K=9.52V, so the voltage at the sources of the two jFET is 18-9.5=8.5V.. So pin 5 and 6 of Q105 is 8.5V, not 5.97V. Even 6.4V is way off.
5) Output of pin 1 of Q105 is not predictable because it is used to adjust the imbalace at pin 6 of Q105 to get 0V at pin 7. You cannot call the exact voltage at pin 1, it can vary up to +/-1V around 8.5V depends on the matching of the Q101. I don't agree the reading of -1.3V.

You can see from the calculation, it does not even agree with the reading measured by OP. Take a look at my calculation. You sure the schematic is the same as the real circuit given so many things wrong on the schematic already? Double check my calculation AND trace through the pcb to make sure the real circuit match the schematic.


BTW, pin 2 and 3 in Q106 still wrong according to the reading by OP in the first post.
 
Last edited:
It seems the OP readings are OK, having Q105 pins 5 and 6 with 6.4V (schematic calls for 5.97, your math gives 8.5, so 6.4 could be just fine if steady).

If Q106 pins are OK in the schematic, the OP readings are OK. 6.4 and 6.7V at pins 2 and 3.

So, time to compare the schematic with the real circuit: I'd identify pin 7 or pin 1 going to R139 and C129. That will confirm the IC position, perhaps the readings are OK, minor differences due to differential pair mismatch of drift.
 
It seems the OP readings are OK, having Q105 pins 5 and 6 with 6.4V (schematic calls for 5.97, your math gives 8.5, so 6.4 could be just fine if steady).

If Q106 pins are OK in the schematic, the OP readings are OK. 6.4 and 6.7V at pins 2 and 3.

So, time to compare the schematic with the real circuit: I'd identify pin 7 or pin 1 going to R139 and C129. That will confirm the IC position, perhaps the readings are OK, minor differences due to differential pair mismatch of drift.
Check my calculation, I calculated the tail current of the differential pair to be 3.4mA based on assumption of Vgs=-1V. That lead to 1.7mA at the drain of each transistor. From that I calculated to have 8.5V +/-0.5V. It cannot be 6.4V. See the circuit is kind of symmetrical, R117, R119, R121 are all 5.6K. R119 and R121 SPLIT the current of R117, so the voltage across R119 and R121 has to be EXACTLY half the voltage drop across R117, which is 17V/2=8.5V according to the schematic. This is quite accurate. Only variables are the Vgs of the jFET and it can only be between -0.5 to -1.5V. You cannot get 6.4V. Something is really missing.

That's why I question the accuracy of the schematic all together. If the schematic does not reflect the circuit, we got nothing to base on for trouble shooting.

Even if I accept pin 2 is 6.4V and pin 3 is 6.7V. This means the opamp cannot close the loop, the output has to rail to 17V as pin 3 is higher than pin 2. That opamp is wrong. I updated my post to give a little more detail on the calculation.
 
Last edited:
Hello guys I tried to measure dc voltages on op-amps in onkyo a-8650 and I found these results:

Q105
1. -0.36V; 2. 0V; 3. 0V; 4. -18.1V; 5. 6.4V; 6. 6.4V; 7. 0V; 8. 18V

Q106
1. 0V; 2. 6.4V; 3. 6.7V; 4. -18V; 5. 0V; 6. 0V; 7. 1.9V; 8. 18V

Does dc voltage on Q105 1 and Q106 7 outputs suggests misbehaviour of these components?

Another thing that does not make sense is pin 1 at -0.36V. If the schematic is correct, it will draw 0.45mA out of R121. This will totally offset the balance of the differential pair. Something really wrong with the schematic. I don't think this cannot be ignore. See the green writing below.

To SPDF

I think you should take your time tracing out this section with the meter to verify the schematic of this section. I don't think I can help without confirmation of the correctness of the schematic.

Onkyo phono.JPG

There are a lot of time you can say the reading is close enough, BUT not this and the last post. It is very exact, the reading is way off from calculation and in this case, it's not acceptable. Vgs of the two jFET have to be -3V or over to give you 6.4V reading. That is not possible.
 
I think it's time to check the PCB, identify the IC pinout, and check if the channels produce sound.

Perhaps bad OPamps or Fets? Or some leaky capacitors drawing current ?
 
A quick way to determine if the op amp in a feedback loop is working is to look at the voltage on the + and - inputs. If it is the same, then the op amp is working fine and is driving the output to whatever is required to meet the condition of +input = -input. That is its job, and if they are the same then the op amp is not at fault. The problem is elsewhere in the circuit.

Terry
 
To SPDF

I think you should take your time tracing out this section with the meter to verify the schematic of this section. I don't think I can help without confirmation of the correctness of the schematic


Here is what I found. "There isn't R117 resistor (empty holes, but solder joints looks like made in factory. Does that make any sense?), but there is R118" not true!
2.png
 
Last edited:
No that absolutely does not make sense. If you don't have R117, you loss the tail current of the jFET differential pair and the amp won't work. No way you can miss R117. Where is R118? I can't find it.
 
R118 is the same as R117 connected with Q102 instead of Q101. This scheme shows only one side of the symmetrical components found on real pcb. I made this conclusion from silkscreen, I'll double check when I get home.

My bad! Found it and updated my previous post with measured voltages.
 
Last edited:
Back
Top Bottom