Hello🙂
Can anyone help me how to adjust the offset voltage.there are 2 potentiometer which I believe is for adjusting the bias. I have a kenwood kr5400 series amp receiver,I notice one of the output voltage of the speaker terminal is around 50mv and the other is 12 something. this is the L&R terminal.I'd adjust the Bias current to it's specs of 28.2mv. Is there anything that I missed.
Thanks,
Ann
Can anyone help me how to adjust the offset voltage.there are 2 potentiometer which I believe is for adjusting the bias. I have a kenwood kr5400 series amp receiver,I notice one of the output voltage of the speaker terminal is around 50mv and the other is 12 something. this is the L&R terminal.I'd adjust the Bias current to it's specs of 28.2mv. Is there anything that I missed.
Thanks,
Ann
Hello,
You are right, the potentiometers are for the output transistors bias current.
There is no simple way to set the output offset voltage.
"speaker terminal is around 50mv and the other is 12 something"
50mV begin to be too hight.
12 something is not an electrical unit !!
You can try to replace Ce5 and Ce6 with 10µF/25V electrolytic bipolar capacitors.
You are right, the potentiometers are for the output transistors bias current.
There is no simple way to set the output offset voltage.
"speaker terminal is around 50mv and the other is 12 something"
50mV begin to be too hight.
12 something is not an electrical unit !!
You can try to replace Ce5 and Ce6 with 10µF/25V electrolytic bipolar capacitors.
KENWOOD KR-5400 KR-4400 KA-4006 SUPPLEMENT Service Manual free download, schematics, eeprom, repair info for electronics
There are no adjustments for DC offset on this model. In fact, very few amplifiers have them because the offset is directly related to transistor Vbe and beta variation between the input pair transistors, Qe1+3 and Qe2+4 in the other channel. That is to say, it doesn't require zero offset, beyond what the designer intended in using (probably) sorted transistors, suggested by the part number (2SA620) suffix WN4 5. You won't find this special, so I'd suggest inexpensive BC560B or C, KSA992 or (if still available) 2SA970, as you'll need reliable, low noise substitutes. Don't mix types and note the pinout of BC560 is different.
In the amplifier's long life, offset could have drifted beyond what I would call just fine at <50 mV. This means swapping or replacing the 2 transistors for a better match such that offset is reduced to insignificance, as you have in the channel measuring 12 mV. There is no need to zero the offset to a gnat's whisker tolerance, which will change with temperature anyway as the amplifier clearly wasn't designed for it. Gilding the lily may make us feel better but you won't hear a blind bit of difference or endanger any speaker worth the title if you do nothing about it.
Having said that, if you'd feel better about reducing it, the old parts won't be easily available nor worth the bother of avoiding fake copies to get them. In any event, you'll need to compare the transistors for the pair you want to fit, by measuring Vbe (voltage drop between base and emitter) with a diode check range on your DMM first, looking for a close match in the region of 0.65V (most budget DMMs have a transistor socket for measuring Hfe or gain, which is also part of the match and within +/- 50 would be nice but not mandatory as long as the Hfe is over 300). Google the pinout by simply typing "ABC 123 pinout" for that.
The difficulty of matching is having enough parts to get a good match and 10 would be an absolute minimum. These parameters are temperature dependent so don't hold the parts whilst you are measuring.
Depending on where you buy, they could be cheap or expensive but don't use Ebay as a source of genuine parts. There is Mouser, Digi-key, Newark and several other major US distributors online + local agencies.
Also, have a look at the fun and games here + more substitute comments by DIYaudio member Echowars: Kenwood KA-4006 "new" not working - Page 3 - AudioKarma.org Home Audio Stereo Discussion Forums
Good luck and take care.
There are no adjustments for DC offset on this model. In fact, very few amplifiers have them because the offset is directly related to transistor Vbe and beta variation between the input pair transistors, Qe1+3 and Qe2+4 in the other channel. That is to say, it doesn't require zero offset, beyond what the designer intended in using (probably) sorted transistors, suggested by the part number (2SA620) suffix WN4 5. You won't find this special, so I'd suggest inexpensive BC560B or C, KSA992 or (if still available) 2SA970, as you'll need reliable, low noise substitutes. Don't mix types and note the pinout of BC560 is different.
In the amplifier's long life, offset could have drifted beyond what I would call just fine at <50 mV. This means swapping or replacing the 2 transistors for a better match such that offset is reduced to insignificance, as you have in the channel measuring 12 mV. There is no need to zero the offset to a gnat's whisker tolerance, which will change with temperature anyway as the amplifier clearly wasn't designed for it. Gilding the lily may make us feel better but you won't hear a blind bit of difference or endanger any speaker worth the title if you do nothing about it.
Having said that, if you'd feel better about reducing it, the old parts won't be easily available nor worth the bother of avoiding fake copies to get them. In any event, you'll need to compare the transistors for the pair you want to fit, by measuring Vbe (voltage drop between base and emitter) with a diode check range on your DMM first, looking for a close match in the region of 0.65V (most budget DMMs have a transistor socket for measuring Hfe or gain, which is also part of the match and within +/- 50 would be nice but not mandatory as long as the Hfe is over 300). Google the pinout by simply typing "ABC 123 pinout" for that.
The difficulty of matching is having enough parts to get a good match and 10 would be an absolute minimum. These parameters are temperature dependent so don't hold the parts whilst you are measuring.
Depending on where you buy, they could be cheap or expensive but don't use Ebay as a source of genuine parts. There is Mouser, Digi-key, Newark and several other major US distributors online + local agencies.
Also, have a look at the fun and games here + more substitute comments by DIYaudio member Echowars: Kenwood KA-4006 "new" not working - Page 3 - AudioKarma.org Home Audio Stereo Discussion Forums
Good luck and take care.
Note that DC offset also depends on DC resistor balance at inverting and noninverting input (check values) and, as mentioned, non-leaky DC blocking caps.
y371
Hi Ian, I think, the problem (and the solution) is easier. In such schematic, DC offset is set by 100% NFB (for DC). So even if there is a certain mismatch in transistors' parameters, NFB will keep the offset low enough.
I agree with alayn91 (previous post) - as soon as Ce5 (Ce6) capacitor gets older, it starts draining the current more and more, difference in the input LTP's base currents dis-balances the circuit, leading to offset increase.
I had exactly the same situation once, the offset was around 2.5V DC, see what the NFB cap looked like - definitely not healthy, sort of "inflated".
So, in my opinion, the most probable solution is Ce5, Ce6 replacement.
Cheers,
Valery
Hi Ian, I think, the problem (and the solution) is easier. In such schematic, DC offset is set by 100% NFB (for DC). So even if there is a certain mismatch in transistors' parameters, NFB will keep the offset low enough.
I agree with alayn91 (previous post) - as soon as Ce5 (Ce6) capacitor gets older, it starts draining the current more and more, difference in the input LTP's base currents dis-balances the circuit, leading to offset increase.
I had exactly the same situation once, the offset was around 2.5V DC, see what the NFB cap looked like - definitely not healthy, sort of "inflated".
So, in my opinion, the most probable solution is Ce5, Ce6 replacement.
Cheers,
Valery
Attachments
Be sure to set the bias with no speakers attached (because the DC offset causes an increase/decrease in the reading across the emitter resistors)
The actual offset you have is normal and no problem in use or audibly.
The actual offset you have is normal and no problem in use or audibly.
You can remove the DC blocking capacitor and replace with a new version.
But reform the new cap and/or the old cap, if the leadouts are long enough.
Discharge with a resistor.
Charge up again to the maximum working voltage and then measure the leakage current.
Expect a good cap to have a leakage <1% of the rated leakage current specified by the manufacturer.
But reform the new cap and/or the old cap, if the leadouts are long enough.
Discharge with a resistor.
Charge up again to the maximum working voltage and then measure the leakage current.
Expect a good cap to have a leakage <1% of the rated leakage current specified by the manufacturer.
Thanks
I check part# of the Qe1 to Qe4 it's 2SA620WN I believe that this is the differential part transistor which is not available anywhere.regarding the dc blocking caps is that the orange color caps?w/c is 100uf 25v.So I thinks this amps need overhauling of caps and some transistor.
Ann
I check part# of the Qe1 to Qe4 it's 2SA620WN I believe that this is the differential part transistor which is not available anywhere.regarding the dc blocking caps is that the orange color caps?w/c is 100uf 25v.So I thinks this amps need overhauling of caps and some transistor.
Ann
Install a 100 ohm pot between the LTP emitters, wiper to the 15K resistor. That will give you about +/- 100 mV Vos adjust range.
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