I just got my hands on a Harman Kardon HK6950R. Everything works except the left channel preamp. If I bypass the preamp, both channels work equally the same. When I don't bypass the preamp, the right channel gets a lot louder, but the left stays the same.
How can I fix this? What should I look at?
Thanks
How can I fix this? What should I look at?
Thanks
I have tried cleaning the boards, resoldering joints. Don't know what to do next.
I know the driver and output stage work fine, because when I put the right output into the left channel driver and output stage, it works fine.
The problem seems to have something to do with the preamp.
This particular model has a switch which lets you bypass the amp. I checked the switch as well, and it works fine.
Can anyone give me some pointers?
Thanks
I know the driver and output stage work fine, because when I put the right output into the left channel driver and output stage, it works fine.
The problem seems to have something to do with the preamp.
This particular model has a switch which lets you bypass the amp. I checked the switch as well, and it works fine.
Can anyone give me some pointers?
Thanks
Hi difflvl,
A schematic would help. But from here it looks like a simple case of signal chasing.
Do you have a 'scope and a DVM??
-Chris
A schematic would help. But from here it looks like a simple case of signal chasing.
Do you have a 'scope and a DVM??
-Chris
Nevermind found a faulty transistor on the tone board. Removed it and it works fine. Wierd.
Now I need to know where the test points for the bias is at. I know where to adjust it from, but I dont know where to measure from.
Now I need to know where the test points for the bias is at. I know where to adjust it from, but I dont know where to measure from.
You can take measurement between the emiter resistor on one of each side but don't think you need to do that since the only change you made was to replace a resistor on the preamp board. But if you want to adjust bias, look for about 10 to 12mv on the emitor resistor,
Hi difflvl,
Glad you found it. One down then.
Bias is normally measured across the emitter resistors with no load connected to the amplifier. Measure cold and then allow it to settle down after it's warmed up and measure again. Aloow time for the amp to settle in after making an adjustment.
You really should have the manual for this type of adjustment.
-Chris
Edit: Ostie01, HK normally runs hot compared to the average amp.
Glad you found it. One down then.
Bias is normally measured across the emitter resistors with no load connected to the amplifier. Measure cold and then allow it to settle down after it's warmed up and measure again. Aloow time for the amp to settle in after making an adjustment.
You really should have the manual for this type of adjustment.
-Chris
Edit: Ostie01, HK normally runs hot compared to the average amp.
Here is the service manual. http://manuals.harman.com/hk/Service Manual/HK6950R sm.pdf
Which ones are the emitter resistors?
The part I took out was actually a N channel Mosfet. Q517 on PCB2 Tone Control. Part 2SK364. There is actually another one right by it, which I left in.
Wierd thing is that I took it out and it works. What is it for?
Thanks for all the help!
Which ones are the emitter resistors?
The part I took out was actually a N channel Mosfet. Q517 on PCB2 Tone Control. Part 2SK364. There is actually another one right by it, which I left in.
Wierd thing is that I took it out and it works. What is it for?
Thanks for all the help!
If someone can give me the part designators of the dc bias, and idling current adjustment measurement points, I would really appreciate. I have tried searching, and browsing the internet, but I still cant find it.
Hi difflvl,
R487/489 would be the emitter resistors, and the others like that. I also notice some R15 resistors. They may be measuring across there. If you look at page 4 (of 56) in the manual, it directs you to pages 8, 10, 11 and 25. You need to look at the picture, not the PDF page number. 😡
You may be further ahead to print the darn thing and tape it together so it makes more sense. That's what I would do if I were you.
-Chris
R487/489 would be the emitter resistors, and the others like that. I also notice some R15 resistors. They may be measuring across there. If you look at page 4 (of 56) in the manual, it directs you to pages 8, 10, 11 and 25. You need to look at the picture, not the PDF page number. 😡
You may be further ahead to print the darn thing and tape it together so it makes more sense. That's what I would do if I were you.
-Chris
Hi Chris,
Thanks for your help. I will try to find those resistors, can't seem to locate them on the schematic yet.
I already have cut and pasted the schematic together.
Im also wondering where I take measurements for the idle current.
Im planning on replacing all the electro caps. Im also debating if I should change the small signal transistors as well. Will replacing the transistors improve anything? What else should I change to bring the circuit back to spec.
Again thanks for all your help!
Regards,
Ray
Thanks for your help. I will try to find those resistors, can't seem to locate them on the schematic yet.
I already have cut and pasted the schematic together.
Im also wondering where I take measurements for the idle current.
Im planning on replacing all the electro caps. Im also debating if I should change the small signal transistors as well. Will replacing the transistors improve anything? What else should I change to bring the circuit back to spec.
Again thanks for all your help!
Regards,
Ray
Hi Ray,
Matching the differential pair can help sound quality and DC offset stability. Thermally couple them when you are done. In a hot running amp like that one, replacing the electrolytic caps may be needed anyway.
I would suggest measuring across the 0.15 ohm resistors and see if the average voltage measurement is close to the value that is recommended. You can also measure across the 0.22 ohm resistors. Once I figure out what the current they want is, I would typically measure across pairs of emitter resistors. This depends on what parts are easy to get at while the amp is on. Measure where you can.
The schematic was so broken up, I didn't waste too much time trying to figure it out. I would if I was working on that amp.
-Chris
Matching the differential pair can help sound quality and DC offset stability. Thermally couple them when you are done. In a hot running amp like that one, replacing the electrolytic caps may be needed anyway.
I would suggest measuring across the 0.15 ohm resistors and see if the average voltage measurement is close to the value that is recommended. You can also measure across the 0.22 ohm resistors. Once I figure out what the current they want is, I would typically measure across pairs of emitter resistors. This depends on what parts are easy to get at while the amp is on. Measure where you can.
The schematic was so broken up, I didn't waste too much time trying to figure it out. I would if I was working on that amp.
-Chris
Chris,
Those resistors you just mentioned. That is for DC bias, or idle current? I dont know where the idle current measurement point is at.
When you say the differential pair, are you talking about the huge output stage transistors? or the small signal ones?
The output stage transistors are 2SA1302 & 2SC3281 Toshibas, 3 sets per channel. I hear there a lot of counterfeits of that particular transistor. Don't know where I can get reliable replacements. The PCB also has the space and pads for an extra set. If I add another stage, will it boost power output? Or just offer more power for lower ohm loads?
Thanks,
Ray
Those resistors you just mentioned. That is for DC bias, or idle current? I dont know where the idle current measurement point is at.
When you say the differential pair, are you talking about the huge output stage transistors? or the small signal ones?
The output stage transistors are 2SA1302 & 2SC3281 Toshibas, 3 sets per channel. I hear there a lot of counterfeits of that particular transistor. Don't know where I can get reliable replacements. The PCB also has the space and pads for an extra set. If I add another stage, will it boost power output? Or just offer more power for lower ohm loads?
Thanks,
Ray
I know the replacements for the 2SA1302/2SC3281 are 2SA1943/2SC5200.
Can i use 2SA1987/2SC5359 instead? Will this give me any type of gain in power?
Can i use 2SA1987/2SC5359 instead? Will this give me any type of gain in power?
Hi,
There is no need to replace output transistors if they are working.I have found a drawing mistake in the power output section;refer to pg.42 of your pdf doc.It seems that if this is an emitter follower design,the emitters(output transistors) are in the wrong place.They should be at the signal output line to speaker with the emitter resistor in tow.The B+,B- are marked 5.2V and -5.2V respectively when it should be +/-52V.As long as you know the standard circuitry it is not a big issue.
The bias/idling adjustment in the schematic is for the left channel,which is R406 1K ohm potentiometer.If you look at the power transistors Q466,Q470,Q474 (2SA1302- PNP) the collectors (which should be EMITTERs are marked 0.030V or 30mV (with respect to ground?).If you measure that to be true then the idling current should be as per original design.Also measure across both resistors and see what is the voltage in milivolts.R488/490 pair or R496/498pair ( 0.22ohm).This is the aproximate way of setting the bias,it can be from 0mV to 45mV across 2 emitters.The NPN 2SC3281 is the coressponding half.
Recheck after say 30 minutes or longer if the setting is stable.
There is no need to replace output transistors if they are working.I have found a drawing mistake in the power output section;refer to pg.42 of your pdf doc.It seems that if this is an emitter follower design,the emitters(output transistors) are in the wrong place.They should be at the signal output line to speaker with the emitter resistor in tow.The B+,B- are marked 5.2V and -5.2V respectively when it should be +/-52V.As long as you know the standard circuitry it is not a big issue.
The bias/idling adjustment in the schematic is for the left channel,which is R406 1K ohm potentiometer.If you look at the power transistors Q466,Q470,Q474 (2SA1302- PNP) the collectors (which should be EMITTERs are marked 0.030V or 30mV (with respect to ground?).If you measure that to be true then the idling current should be as per original design.Also measure across both resistors and see what is the voltage in milivolts.R488/490 pair or R496/498pair ( 0.22ohm).This is the aproximate way of setting the bias,it can be from 0mV to 45mV across 2 emitters.The NPN 2SC3281 is the coressponding half.
Recheck after say 30 minutes or longer if the setting is stable.
Hi Ray,
I haven't checked singa's post against the schematic, but it sounds logical. I would tend to measure across a pair of emitter resistors to double the voltage. This cuts your reading error down. The is the bias, or idle current. The same thing. DC offset is another measurement again.
The diff. pair is the pair of transistors at the input of the amplifier. An example would be Q755,757,759,761. However these are all Jfets and it would be difficult to source and match these for the average person. Don't do it, just forget I mentioned it altogether.
-Chris
I haven't checked singa's post against the schematic, but it sounds logical. I would tend to measure across a pair of emitter resistors to double the voltage. This cuts your reading error down. The is the bias, or idle current. The same thing. DC offset is another measurement again.
The diff. pair is the pair of transistors at the input of the amplifier. An example would be Q755,757,759,761. However these are all Jfets and it would be difficult to source and match these for the average person. Don't do it, just forget I mentioned it altogether.
-Chris
Okay got it, thanks.
What parts do you think I should change to bring the circuit back to spec? Im not new to electronics, just new with this amp stuff.
Thanks.
What parts do you think I should change to bring the circuit back to spec? Im not new to electronics, just new with this amp stuff.
Thanks.
I don't think you should change anything.If it aint broke don't fix it.😀
Just recheck the bias voltage and you are fine.
edit-- Of course you may want to change the electrolytic caps but that's optional.
Just recheck the bias voltage and you are fine.
edit-- Of course you may want to change the electrolytic caps but that's optional.
Hi Ray,
As singa said, maybe replace some electrolytic caps.
I don't believe that you shouldn't touch something just because it appears to be working properly. But then again, I am a technician. I do know when to look and when to touch!
-Chris 😉
As singa said, maybe replace some electrolytic caps.
I don't believe that you shouldn't touch something just because it appears to be working properly. But then again, I am a technician. I do know when to look and when to touch!
-Chris 😉
Hi,
Okay Ill just change all the caps. Don't know where I can find the main big filter caps though. 10000uf 85volt.
I have a question about the output stage. The pcb has pads and traces for a 4th pair. Can I put in the 4th pair of output transistors or the power supply or rail voltage wont be good enough.
I also want to know if I can swap out the 2SA1302/2SC3281 for 2SA1987/2SC5359? Or does the power supply need to be upgraded?
Thanks again.
Okay Ill just change all the caps. Don't know where I can find the main big filter caps though. 10000uf 85volt.
I have a question about the output stage. The pcb has pads and traces for a 4th pair. Can I put in the 4th pair of output transistors or the power supply or rail voltage wont be good enough.
I also want to know if I can swap out the 2SA1302/2SC3281 for 2SA1987/2SC5359? Or does the power supply need to be upgraded?
Thanks again.
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