Post by goat1981 on Oct 5, 2013 12:10:05 GMT -5
Hello, everybody,
I have recently acquired a Harman Kardon Citation 16 series A from the well-known auction site. The amp was said to be "tested and have no issues with the sound." Wanting the amp really badly (out of curiosity and nostalgia), I overspent a bit to get it.
Upon arrival, I did some readings of DC offset and all was not well... The right channel was fine -- about 25mv before any adjustments. However, the left channel had nearly 1.0 volt of DC at the outputs. Also, the idling current was very high on this channel (the tech manual says there should be 50mv DC across one of the emitter resistors, and in this channel it was around 140mv).
It looked like the amp had some work done on it before I acquired it as well (different input coupling capacitors, and one different power capacitors -- both on the faulty channel).
Not wanting to deal with returning it and knowing I would have to rebuild/recap it anyway, I proceeded to carefully perform a recap of all the electrolytic capacitors (including the power supply caps), and replaced components that are notorious for going out of spec (the 2W 3900 ohm power dropping resistors were about 20% off on the bad channel (in both directions!); and I replaced all the zener diodes with 2% tolerance units).
The variable resistors that are used to adjust bias and DC offset are also notoriously bad, so I replaced them with 15-turn pots that are more precise.
NOW, things are better, but not right.
I can now get DC offset in *both* channels to 1.0mv, no problem.
However, the originally problematic L channel has no bias current now. Tech manual says there should be 50mv across the an emitter resistor on the output transistors. Now there is around 1 mv.... And, adjusting the bias control pot (VR2) does something very strange. For most of the adjustment, bias stays at around 1 - 2mv until one very specific point where it jumps to -1.3v (!!!) ... There is no in-between. Obviously I didn't keep the setting here very long. However, upon a short test with music...this channel sounds like pure crossover distortion (quite awful). Other channel is great.
Attached are the schematics. There are two darlington transistors in the bias circuit -- Q11 and Q18. Could one of those be bad? (I have tested the adjustment potentiometer, VR2 and know it's fine) I have very little knowledge of circuit design, so any help you guys could provide would be great. Thanks in advance!! I really want to get this thing working after the investment I put into it.
I have double checked my work several times and don't see anything stupid I have done (though I have missed dumb mistakes in the past...)
I have recently acquired a Harman Kardon Citation 16 series A from the well-known auction site. The amp was said to be "tested and have no issues with the sound." Wanting the amp really badly (out of curiosity and nostalgia), I overspent a bit to get it.
Upon arrival, I did some readings of DC offset and all was not well... The right channel was fine -- about 25mv before any adjustments. However, the left channel had nearly 1.0 volt of DC at the outputs. Also, the idling current was very high on this channel (the tech manual says there should be 50mv DC across one of the emitter resistors, and in this channel it was around 140mv).
It looked like the amp had some work done on it before I acquired it as well (different input coupling capacitors, and one different power capacitors -- both on the faulty channel).
Not wanting to deal with returning it and knowing I would have to rebuild/recap it anyway, I proceeded to carefully perform a recap of all the electrolytic capacitors (including the power supply caps), and replaced components that are notorious for going out of spec (the 2W 3900 ohm power dropping resistors were about 20% off on the bad channel (in both directions!); and I replaced all the zener diodes with 2% tolerance units).
The variable resistors that are used to adjust bias and DC offset are also notoriously bad, so I replaced them with 15-turn pots that are more precise.
NOW, things are better, but not right.
I can now get DC offset in *both* channels to 1.0mv, no problem.
However, the originally problematic L channel has no bias current now. Tech manual says there should be 50mv across the an emitter resistor on the output transistors. Now there is around 1 mv.... And, adjusting the bias control pot (VR2) does something very strange. For most of the adjustment, bias stays at around 1 - 2mv until one very specific point where it jumps to -1.3v (!!!) ... There is no in-between. Obviously I didn't keep the setting here very long. However, upon a short test with music...this channel sounds like pure crossover distortion (quite awful). Other channel is great.
Attached are the schematics. There are two darlington transistors in the bias circuit -- Q11 and Q18. Could one of those be bad? (I have tested the adjustment potentiometer, VR2 and know it's fine) I have very little knowledge of circuit design, so any help you guys could provide would be great. Thanks in advance!! I really want to get this thing working after the investment I put into it.
I have double checked my work several times and don't see anything stupid I have done (though I have missed dumb mistakes in the past...)
