Hello.
I've got a Kenwood KR-910 receiver. In the left channel transistors Q8,Q10,Q12,Q14,Q16,Q18 was dead, also resistors R54,R58 and R76.
After replacing them - Q8/10 - 2SC1845; Q12/14-BC639/640 (original 2SC2003/2SA954) and BD911/912 (original 2SB633/2SD613) voltage values were proper. The receiver worked, but after increasing volume above 1/2 BD911/912 were blown. I thought, that was "from China" transistors so I replaced them with original 2N6107/2N6292. After that I noticed, that the left channel radiator is much warmer than the right one so I immediately cut off power. It appears that the quiescent current raises quickly. The quiescent current value of good right channel is 75mA and about 160 mA (!!!) in left channel with (+,-) 33V (supply from external stabilized power supply with current overload protection). There is no trimmer to regulate the quiescent current, so I replaced one 22k resistor - R36- with 47k trimmer. Now, I can regulate the current to 75mA but it is not so stable. I checked out elements, seems to be good. I'm afraid to test receiver with loudspeakers because it has not got a protector.
What I'm to do? Need help.
I've got a Kenwood KR-910 receiver. In the left channel transistors Q8,Q10,Q12,Q14,Q16,Q18 was dead, also resistors R54,R58 and R76.
After replacing them - Q8/10 - 2SC1845; Q12/14-BC639/640 (original 2SC2003/2SA954) and BD911/912 (original 2SB633/2SD613) voltage values were proper. The receiver worked, but after increasing volume above 1/2 BD911/912 were blown. I thought, that was "from China" transistors so I replaced them with original 2N6107/2N6292. After that I noticed, that the left channel radiator is much warmer than the right one so I immediately cut off power. It appears that the quiescent current raises quickly. The quiescent current value of good right channel is 75mA and about 160 mA (!!!) in left channel with (+,-) 33V (supply from external stabilized power supply with current overload protection). There is no trimmer to regulate the quiescent current, so I replaced one 22k resistor - R36- with 47k trimmer. Now, I can regulate the current to 75mA but it is not so stable. I checked out elements, seems to be good. I'm afraid to test receiver with loudspeakers because it has not got a protector.
What I'm to do? Need help.
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It probably wont be very stable, I would say that was normal. You should tag some wires to the bias test point so that you can monitor the current and then play the amp. Set the bias on the low side when the amp is really hot.
Modern transistors do have slightly different characteristics due to different doping processes being used and that will be why the bias was out of range to begin with.
As far as distortion goes, you wont here any difference once you get over a couple of milliamps flowing so fret over it, just set it a little on the low side when hot and enjoy.
Modern transistors do have slightly different characteristics due to different doping processes being used and that will be why the bias was out of range to begin with.
As far as distortion goes, you wont here any difference once you get over a couple of milliamps flowing so fret over it, just set it a little on the low side when hot and enjoy.
I haven't a scope, must to do a hf probe for multimeter.
Actually the quiescent current in repaired channel is set on 70mA and still is swinging in a range 3-5 mA; both radiators are slightly warm even after 1/2 hour. After that time tested with an old loudspeaker 8OHm with low power lewel - looks (and sounds) ok. Still external power supply.
So, I'll test it for a longer time, than try with normal supply.
Actually the quiescent current in repaired channel is set on 70mA and still is swinging in a range 3-5 mA; both radiators are slightly warm even after 1/2 hour. After that time tested with an old loudspeaker 8OHm with low power lewel - looks (and sounds) ok. Still external power supply.
So, I'll test it for a longer time, than try with normal supply.
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3 to 5 milliamp variation is nothing. Even 40 to 100ma (from stone cold to really hot after playing loud music) would be acceptable for a circuit like that.
Sounds like you are all good with that 🙂
Sounds like you are all good with that 🙂
Thx, Molly.
This Kenwood is the first which makes me problem with repairing so simple amp, so I thought it is a good idea to ask somebody more experience.
This Kenwood is the first which makes me problem with repairing so simple amp, so I thought it is a good idea to ask somebody more experience.
In addition to replacing the broken parts, I recommend adding measures to counteract the oscillation tendencies which this kind of CFP output stage is prone to.
There are different ways of accomplishing this. One way would be add a small-value capacitor (for example 47pF~100pF) between the base and collector of Q11 and of Q13.
An alternative would be to add a similar small value capacitor between the base and collector of Q15 and of Q17.
But pick one way, whichever is more effective - don't use both countermeasures at the same time.
There are different ways of accomplishing this. One way would be add a small-value capacitor (for example 47pF~100pF) between the base and collector of Q11 and of Q13.
An alternative would be to add a similar small value capacitor between the base and collector of Q15 and of Q17.
But pick one way, whichever is more effective - don't use both countermeasures at the same time.
Tested. Kenwood works properly. Rf probe shows nothing so it doesn't oscillate. The quiescent current is set by replacing resistor R36 from 22k to 33k and is ok now.
I think Mooly was right:
"Modern transistors do have slightly different characteristics due to different doping processes being used and that will be why the bias was out of range to begin with."
Thank you for help.
I think Mooly was right:
"Modern transistors do have slightly different characteristics due to different doping processes being used and that will be why the bias was out of range to begin with."
Thank you for help.
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