Kenwood KR9600 dc offset... adjustment?

Anonky

Anonky

Kenwood KR-9600 / Pioneer SX-950 / Sony STR-GX10ES
Have a kenwood kr9600 in pretty good condition after a solid cleaning, working seemingly fine. Checked dc offset because why not and measurements are as follows: L channel: 5mV, R channel: 115mV. Both are stable, but the R channel is mildly concerning.

I'm seeing from an old EchoWars post that there are no adjustments on the unit but if you replace two of the output transistors specified in post #9 of this thread https://audiokarma.org/forums/index.php?threads/kr-9600-dc-offset.346998/#post-4341037 it should rectify this offset issue. Is this correct? Any advice from someone who has done this would be greatly appreciated.

Thanks!
 
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If it gets warm(er) during play or shoots the bed, time to find matching pairs. Might take a small bulk purchase of matching pairs to find the right sets.
 
Hajidub said:
If it gets warm(er) during play or shoots the bed, time to find matching pairs. Might take a small bulk purchase of matching pairs to find the right sets.
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is this simply trial and error?
 
rjsalvi said:
Caveat: Just because I would try things off the beaten path doesn't mean anyone else should. It has zero to do with expertise and more to do with the SicMan axiom: "It's electronics. It breaks, you fix it." I ain't skeered anymore. ;)

Thinking out loud, what if...

Qe3 is a constant current source for the differential pair and Re6 regulates the current. What if one dropped in a 25-turn/500 ohm trimmer into Re6 with the wiper leg soldered to one of the fixed legs, set the initial resistance to 270 and start tweaking?

Kenwood has used similar differential circuit topologies which use a dropping resistor on the rail, then a series resistor into the differential pair. The only difference on the 9600 is that it's utilizing a constant current source as opposed to series resistors.

What am I missing?
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I don’t think I have the expertise to monkey around with things this much unfortunately. My plan is to order several of these transistors and match them for gain and hfe using a meter. Problem is finding these Zetex ZTX694B. They’re not readily available on mouser or digikey at the moment..
 
rjsalvi said:
What am I missing
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You are not missing anything. But why modify the circuit? The original configuration lasted well past the life expectancy of the 9600. $10 to get a batch of transistors to match, then 30 mins to install for another 40 years of service seems like a small effort.

One member modified the circuit to do this for a KA-6004 with good results, but a lot of effort. I just replaced the LTP in my 6004 and got the same result. Could have done it 10 times for the effort of modifying the circuit.

Match 'em up, solder 'em in. Good to go.
 
Anonky said:
That website actually has the ztx694Bs so I’m gonna order like 10 of them. Surely enough to find a match right?
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Most likely. Make sure to measure them all during the same sit down. Room temp and climate has a tendency to alter their readings.
 
Hajidub said:
Most likely. Make sure to measure them all during the same sit down. Room temp and climate has a tendency to alter their readings.
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Noted. I’ve also taken note of whenever I’m installing to amp board to triple check how the pins go in because of the different order of the pins on the transistor vs how they go into the board. Please correct me if this sounds incorrect. Relatively new at this and learning still.
 
Anonky said:
Noted. I’ve also taken note of whenever I’m installing to amp board to triple check how the pins go in because of the different order of the pins on the transistor vs how they go into the board.
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You are correct.
 
Be sure and study the datasheet on the Zetexes. The originals as well as the (twice) recommended KSC1845FTA are center collector, while the Zetex are center base.
 
gort69 said:
Be sure and study the datasheet on the Zetexes. The originals as well as the (twice) recommended KSC1845FTA are center collector, while the Zetex are center base.
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If y’all would recommend the KSC1845FTA over the ztx I’d be happy to go that route. Just following echo wars posting closely as he appeared to be the expert on the 9600. Like I said, I’m new to this and learning.

I’m debating on whether I want to just get this thing functional with a healthy dc offset and enjoy it as is versus a more in depth and comprehensive restore. There is a guy on YouTube who documents him redoing the whole unit so I’m confident I could do it with such guidance. Just a matter of now versus later when I’m forced to with all components at 20+ years past their life expectancy.
 
Matched 1845s will get offset below 10mv and are an easier install for a newbie due to the pinout. Not to mention you could get 100 for $13.20.

The Zetex will work great - no idea if 10 is enough to get a good matched pair.
 
If I am doing a recap of then entire unit, would the whole recap kit on ebay be a terrible option for a first time? It appears they provide a bit of instruction with it so would ~$180 total be too much? I think the addtl guidance and convenience of not having to look up all the parts myself would be worth it. Am I crazy?
 
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