Kenwood KR-4070: DC offset

[kasbah512]

There is no DC offset adjustment on this device and Im trying to figure out how to correct the issue. Im getting -17 mv on the left channel and +35 mv on the right.

Here is the circuit:

 

[spark1]

I would not worry about offsets that small. You could try replacing differential amp transistors with matched and bonded pairs, but doing so is as likely to give no improvement as some improvement.

Q1-4. 1 and 3, 2 and 4 are the diff pairs.

EDIT: depending upon the design of the applicable circuit - which I am not good at analyzing - replacing the diff amp transistors with devices of lower hFE than the originals might actually yield poorer results. Keep this in mind if you decide to pursue modern replacements.

 

[painttoad]

I would not worry about offsets that small. You could try replacing differential amp transistors with matched and bonded pairs, but doing so is as likely to give no improvement as some improvement.

Q1-4. 1 and 3, 2 and 4 are the diff pairs.

agreed. those numbers aren't bad at all for a unit with no adjustment

 

[kasbah512]

I would not worry about offsets that small. You could try replacing differential amp transistors with matched and bonded pairs, but doing so is as likely to give no improvement as some improvement.

Q1-4. 1 and 3, 2 and 4 are the diff pairs.

EDIT: depending upon the design of the applicable circuit - which I am not good at analyzing - replacing the diff amp transistors with devices of lower hFE than the originals might actually yield poorer results. Keep this in mind if you decide to pursue modern replacements.

Ahh, i see. I changed out (Q12, Q14) and (Q11, Q13) and the results were exactly the same, but I suppose that is because they are drivers, and not further up the line. Thanks for the advice. Ill give it a try tomorrow and report back. I personally cant stand to see a signal fully below/above 0v DC, even if its a low signal. I was freaking out over 6.5 mv on another unit recently.

 

[spark1]

What transistors are in there now?

 

[kasbah512]

The part numbers from the ones i put in as driver replacements are mouser part #'s 757-TTA004BQ, and 757-TTC004BQ

 

[spark1]

I meant the differential amp pairs.

EDIT - it appears that they are KSA640. If so, KSA992 should be a good replacement, if this is something you feel the need to pursue.

 

[dlucy]

@kasbah512 your thread would be best in the Kenwood forum. Shall I move it there for you?

 

[kasbah512]

@kasbah512 your thread would be best in the Kenwood forum. Shall I move it there for you?

 

[kasbah512]

@kasbah512 your thread would be best in the Kenwood forum. Shall I move it there for you?

Yes, that would be appreciated. My appoligies.

 

[kasbah512]

I meant the differential amp pairs.

EDIT - it appears that they are KSA640. If so, KSA992 should be a good replacement, if this is something you feel the need to pursue.

Im probably gonna call it a day with this unit when the two 100V caps show up.(4.7 uF, 47 uF) Maybe it will solve the problem if they are leaking. Its a low end unit, but my school of thought was that over 25 mV was unacceptable. For what its worth, i didn't notice until i hooked it up to a meter.

 

[painttoad]

but my school of thought was that over 25 mV was unacceptable. For what its worth, i didn't notice until i hooked it up to a meter.

Amplifier Distortion, DC-Offset, and You!

Time for one of those long, boring semi-technical posts that no one here reads... As a few of you know, I bought a rare Kenwood 700M amplifier a few weeks ago on eBay. It arrived with a weak channel which was taken care of by replacing a bypass cap. Since then, I have gone through the entire...

audiokarma.org

 

[spark1]

Im probably gonna call it a day with this unit when the two 100V caps show up.(4.7 uF, 47 uF) Maybe it will solve the problem if they are leaking. Its a low end unit, but my school of thought was that over 25 mV was unacceptable. For what its worth, i didn't notice until i hooked it up to a meter.

Not likely to have any impact on offset.

But if it really bugs you, KSA992s are cheap, and assuming good soldering skills, there's really no risk in giving them a shot. If it gets better, great. If it gets worse, put the old ones back in.

If you do try it, think about bonding them together (each pair).

 

[kasbah512]

Not likely to have any impact on offset.

But if it really bugs you, KSA992s are cheap, and assuming good soldering skills, there's really no risk in giving them a shot. If it gets better, great. If it gets worse, put the old ones back in.

If you do try it, think about bonding them together (each pair).

I have alot of ksa992 in stock. I may try to see if it has any affect.

Edit: sorry if that came off wrong. Its a cheap unit and after reading the other thread idk if its even worth the time to find a matching pair, but I suppose ill already be in there for the two caps i still need.