Another Kenwood KR-9600 Power Supply / Protection Mode Repair
- Thread starter bhuskins
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ecluser
Super Member
The current sources Qe3 (2.22mA) and Qe8 (6.6mA) seem to work as they should.
With the large offset that you have at the output(+32V), it is normal that only Qe2 is conducting in the first differential pair. The voltages that you have at the collector of Qe2 (+22.8V) and the cathode of De3 (+23.6V) indicates that you have 2.05mA in Re5. If we consider the tolerance of Re5 and Re6, these results are coherent. I don't see anything bad in the first differential stage.
At one point in my reflection I thought that there was probably no collector currents in the first differential pair, and all the current from the current source Qe3 has to pass through the be junction of Qe2 and the feedback resistor Re15. That would give an offset voltage of the order of +32V. This is why I asked for the voltages at the collectors of Qe1, Qe2 and the zener. Now I am quite confident that the first differential pair works. BUT, where is going the current in the feedback resistor?? I think that capacitor Ce3 is shorted actually, and the base voltage of Qe2 is close to +0.8V. Can you confirm this, please?
Now, we will look at the second differential stage. The base of Qe4 is at +23.6V, and the base of Qe5 is at +22.8V. With these base voltages, I presume that the voltage at their emitters is close to +23V and only Qe4 should conduct. But the voltages that you reported at the positive supply +B (+53.3V) seem to indicate that there is some current in Re9, although most of the current is flowing in Re8. It is always preferable to measure the voltage drop directly at the resistors when we want to compute the current flowing in the resistors, instead of measuring the voltages on each side, at different times. I computed a larger collector currents than the emitter currents, but not by a large value. I think that this stage works reasonably well.
I have a problem with the third differential stage. The voltage at the base of Qe6 is so low (+50.2V) by comparison to the base voltage of Qe7 (+52.8V) that, in principle, only Qe6 should conduct. But the current source Qe8 is sinking 6.6mA from the pin 8 of the power module. Normally this current should come out of the collector of Qe7, and pass through the bias voltage source inside the power module (R1 and D1 in the internal schematic of the TA-200W). If Qe7 doesn't conduct, it is possible that the current sinked by Qe8 comes from the output of the current module and R3, Q2, and R9, R14 and R15 (see the internal schematic of the TA-200W). So, I think that Qe7 doesn't conduct actually because the large positive offset drives Qe2, Qe4 and Qe6 into strong conduction (almost saturation).
It is possible that the power module is bad. Also, I am not totally confident about Ce3 and Qe7. If I had this receiver on my bench I would remove the power module, install the resistors like I said previously (between 3-5 and 8-5 of the power module), replace Ce3, and make a new test. If the amp section is good without the power module, but bad with the power module, then we know the power module is in fault.
With the large offset that you have at the output(+32V), it is normal that only Qe2 is conducting in the first differential pair. The voltages that you have at the collector of Qe2 (+22.8V) and the cathode of De3 (+23.6V) indicates that you have 2.05mA in Re5. If we consider the tolerance of Re5 and Re6, these results are coherent. I don't see anything bad in the first differential stage.
At one point in my reflection I thought that there was probably no collector currents in the first differential pair, and all the current from the current source Qe3 has to pass through the be junction of Qe2 and the feedback resistor Re15. That would give an offset voltage of the order of +32V. This is why I asked for the voltages at the collectors of Qe1, Qe2 and the zener. Now I am quite confident that the first differential pair works. BUT, where is going the current in the feedback resistor?? I think that capacitor Ce3 is shorted actually, and the base voltage of Qe2 is close to +0.8V. Can you confirm this, please?
Now, we will look at the second differential stage. The base of Qe4 is at +23.6V, and the base of Qe5 is at +22.8V. With these base voltages, I presume that the voltage at their emitters is close to +23V and only Qe4 should conduct. But the voltages that you reported at the positive supply +B (+53.3V) seem to indicate that there is some current in Re9, although most of the current is flowing in Re8. It is always preferable to measure the voltage drop directly at the resistors when we want to compute the current flowing in the resistors, instead of measuring the voltages on each side, at different times. I computed a larger collector currents than the emitter currents, but not by a large value. I think that this stage works reasonably well.
I have a problem with the third differential stage. The voltage at the base of Qe6 is so low (+50.2V) by comparison to the base voltage of Qe7 (+52.8V) that, in principle, only Qe6 should conduct. But the current source Qe8 is sinking 6.6mA from the pin 8 of the power module. Normally this current should come out of the collector of Qe7, and pass through the bias voltage source inside the power module (R1 and D1 in the internal schematic of the TA-200W). If Qe7 doesn't conduct, it is possible that the current sinked by Qe8 comes from the output of the current module and R3, Q2, and R9, R14 and R15 (see the internal schematic of the TA-200W). So, I think that Qe7 doesn't conduct actually because the large positive offset drives Qe2, Qe4 and Qe6 into strong conduction (almost saturation).
It is possible that the power module is bad. Also, I am not totally confident about Ce3 and Qe7. If I had this receiver on my bench I would remove the power module, install the resistors like I said previously (between 3-5 and 8-5 of the power module), replace Ce3, and make a new test. If the amp section is good without the power module, but bad with the power module, then we know the power module is in fault.
ecluser
Super Member
I think Ce3 is bad. There is +32V on one side of the feedback resistor Re15, and +5V on the other side. There is 1.5mA in this resistor. This current can not go into the base of Qe2 since the collector current of Qe2 is almost equal to the current sinked by Qe3. There must be approximately 1.5mA in Ce3
You can verify this if you measure the voltage on each side of resistor Re7. I think there is a voltage drop of the order of 0.7V in Re7
You can verify this if you measure the voltage on each side of resistor Re7. I think there is a voltage drop of the order of 0.7V in Re7
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ecluser
Super Member
10 min from Mouser! Like a kid at 10 min from Disneyland (except that there is no entrance fees). By the same occasion, could you buy some transistors to replace Qe7 (if needed)?
Do you have another set of transistors for the first differential pair? I can not say for sure that the transistor on the input side is good, because the stage is driven to saturation on the other side by the large offset voltage at the output.
Do you have another set of transistors for the first differential pair? I can not say for sure that the transistor on the input side is good, because the stage is driven to saturation on the other side by the large offset voltage at the output.
bhuskins
Super Member
Glenn rebuilt the amp modules and replaced all of the transistors so I don't have the exact matches for transistor replacement as he used his private stash of no longer available transistors he saves for these types of projects...I have parts that would work, but not optimal like what Glenn used.
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bhuskins
Super Member
I replaced Ce3, removed the power module and installed 1K 1/2 watt resistors between pins 3-5 and 5-8 of the power module location.
Before testing I checked for shorts on the power module between 1-5 and 5-10 being that it was removed now...it does NOT show any shorts between those pins so maybe the module is still good? Any other tests I can do?
Also, with the 200w DBT still in place the Power On test was successful. The unit comes out of protection in about 6-8 seconds.
So far so good.
What measurements would you like me to take with current set up?
If in fact Ce3 was the problem all along, what would have caused it to fail being that it was new and installed properly, etc.?
I'm thinking a bit ahead here, but if I remove the resistors and reinstall the module and it works, what's going to keep it from happening again? Or another way to ask it is what caused Ce3 to fail?
Before testing I checked for shorts on the power module between 1-5 and 5-10 being that it was removed now...it does NOT show any shorts between those pins so maybe the module is still good? Any other tests I can do?
Also, with the 200w DBT still in place the Power On test was successful. The unit comes out of protection in about 6-8 seconds.
So far so good.
What measurements would you like me to take with current set up?
If in fact Ce3 was the problem all along, what would have caused it to fail being that it was new and installed properly, etc.?
I'm thinking a bit ahead here, but if I remove the resistors and reinstall the module and it works, what's going to keep it from happening again? Or another way to ask it is what caused Ce3 to fail?
