Which (of these three) resistors to measure for bias adjustment?

[dlucy]

I've just finished recapping a fine little Kenwood KA-4002 integrated amp and I'd like to adjust the bias. There are two trimpots on the main amp portion of the one an only board in this unit and the generic Kenwood bias adjustment guide link indeed listed the KA-4002 and a bias to shoot for even though the KA-4002 service manual doesn't cover it.

Normally, I'd just follow the schematic and measure voltage across the final output transistor's emitter resistor... but there are more than one in this unit (to my wholly untrained eye). So, which one (or more) of these resistors should I be measuring across as I adjust trimpot VRe1? And please help explain why you pointed out the ones you did so your advice might be applied to others (i.e. me when I'm working on the next little gem) as they do something similar in a different unit.

If I was taking a wild guess, I'd say I should measure from the "top" side (as per diagram) of Re91 (since that looks like the emitter on Q1) all the way through to the "top" side of Re 93 (since that looks like the emitter on Q2), but there is cap Ce59 in the path between those and that makes me think that can't possibly be right. Help!

(Image below is just one half of the schematic, just one channel)

Thanks!

 

[catrafter]

Power OFF
Volume all the way down
Meter set to read DC millivolts
Red meter lead to Point 41
Black meter lead to Point 45 (This is across Re91)
Power ON
Set VRe1 for a reading of 9.4millivolts (20ma times 0 .47 Ω = 9.4mV)
Power OFF
The reading from Point 47 to Point 49 (across Re93) should be about the same.
Re99 and Ce59 form a Zobel network to limit high frequency response.

Tom

 

[dlucy]

Power OFF
Volume all the way down
Meter set to read DC millivolts
Red meter lead to Point 41
Black meter lead to Point 45 (This is across Re91)
Power ON
Set VRe1 for a reading of 9.4millivolts (20ma times 0 .47 Ω = 9.4mV)
Power OFF
The reading from Point 47 to Point 49 (across Re93) should be about the same.
Re99 and Ce59 form a Zobel network to limit high frequency response.

Tom

Awesome. Thank you, Tom!

 

[burmashave]

I've just finished recapping a fine little Kenwood KA-4002 integrated amp and I'd like to adjust the bias. There are two trimpots on the main amp portion of the one an only board in this unit and the generic Kenwood bias adjustment guide link indeed listed the KA-4002 and a bias to shoot for even though the KA-4002 service manual doesn't cover it.

Normally, I'd just follow the schematic and measure voltage across the final output transistor's emitter resistor... but there are more than one in this unit (to my wholly untrained eye). So, which one (or more) of these resistors should I be measuring across as I adjust trimpot VRe1? And please help explain why you pointed out the ones you did so your advice might be applied to others (i.e. me when I'm working on the next little gem) as they do something similar in a different unit.

If I was taking a wild guess, I'd say I should measure from the "top" side (as per diagram) of Re91 (since that looks like the emitter on Q1) all the way through to the "top" side of Re 93 (since that looks like the emitter on Q2), but there is cap Ce59 in the path between those and that makes me think that can't possibly be right. Help!

(Image below is just one half of the schematic, just one channel)

Thanks!

View attachment 914332

Awesome. Thank you, Tom!

I cant find pin 41 no matter how hard I look. Any hints?

 

[Goldie99]

Q1 Emitter

 

[Hyperion]

Yep, across Re91 or Re93 (but not Re99).

 

[dlucy]

I cant find pin 41 no matter how hard I look. Any hints?

The underside of the PCB has numbered pads. I'm pretty sure that's what I used to follow @catrafter advice.

 

[burmashave]

awesome. never occurred to me. thank you very much!

 

[dlucy]

awesome. never occurred to me. thank you very much!

No problem! I onestly couldn't remember either, so I had to go back to my collection of before and after photos.

The KA-4002 sounds great when restored, so don't hesitate to ask further questions.

 

[burmashave]

Sounds awesome now. Thank you very much. I’ll post some nudies and I’m open to suggestions after that

 

[tortelloso77]

Hi guys, I have a ka-4002A and I cannot find the bias point 41, not even on the bottom side of the board, could you please help? Thank you

 

[tortelloso77]

I manage to find tge point, it was n.43. I assume this was for 1 channel (vr1), what points for vr2 please? Thank you

 

[pumptronics]

I cant find pin 41 no matter how hard I look. Any hints?

Old thread I know but I was also trying to find this point recently. On my KA-4002A this has been mislabelled on the PCB. There are two pin 44’s on mine and one (nearer the front) is actually pin 41. It has a yellow lead and goes directly to R91 on the board.

Grateful to you all on this site for the process of bias adjustment. Thank you!