Vintagecasey
Well-Known Member
Looking for replacement for a Kenwood kr9600 pre amp 2sk68A(M) and 2sk68A (K). Any such thing that's still available? I tried Google's AI and I get different answers when I ask... Thanks!
FET replacement
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better off to use ak's search
Vintagecasey
Well-Known Member
Have to ask how do you know you have a jfet that needs replacing? Did you substitute one from the good channel to the one that's not working to verify that your issue is the jfet?jfets are usually very reliable devices.
It's fairly easy to test a jfet out of circuit to verify its static specifications.
You show a amplifier circuit with feedback, it only takes one component to be out of spec to through the bias voltages off.
Standard jfet specs are
1) IDSS, which is the current through the device with Gate and source shorted or Vgs
2) pinch off voltage is the Vgs that turns the device off
3) gm or transconductance, basically gain how much variation in Vgs to create variation in drain current
4) node capacitances which affect the frequency responses.
So if you are certain it's the jfet, then look up the specs on NEC 2sk68A, (M), (K) are the IDSS gradings. Then search what's available at Mouser or Digi-key. Download datasheets and try to find something that has similar specs. Current manufactures are Onsemi, Linear Integrated Systems, etc
Watch for pin out variations. In a lot of cases drain and source are interchangeable.
Could take a chance on eBay, but so many fakes out there, buyer beware, no guarantees unless you buy from a authorized dealer.
Good kuck
It's fairly easy to test a jfet out of circuit to verify its static specifications.
You show a amplifier circuit with feedback, it only takes one component to be out of spec to through the bias voltages off.
Standard jfet specs are
1) IDSS, which is the current through the device with Gate and source shorted or Vgs
2) pinch off voltage is the Vgs that turns the device off
3) gm or transconductance, basically gain how much variation in Vgs to create variation in drain current
4) node capacitances which affect the frequency responses.
So if you are certain it's the jfet, then look up the specs on NEC 2sk68A, (M), (K) are the IDSS gradings. Then search what's available at Mouser or Digi-key. Download datasheets and try to find something that has similar specs. Current manufactures are Onsemi, Linear Integrated Systems, etc
Watch for pin out variations. In a lot of cases drain and source are interchangeable.
Could take a chance on eBay, but so many fakes out there, buyer beware, no guarantees unless you buy from a authorized dealer.
Good kuck
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Vintagecasey
Well-Known Member
Yeah, I was thinking about just swapping those jfet's and see if the low channel remains the same or changes. I was just figuring if I had them out just replace them if there is a good substitute/replacement. No I'm definitely not an expert in electronics so I'm not really sure if that's where the problem is I'm just trying the shotgun approach to work through where I think the problem area is. Thanks
Vintagecasey
Well-Known Member
Do you have a idea where my problem could be before I look at the jfet's? Opinions are welcome...
So what's the problem, gain, channel imbalance, could be anything in the signal paths from input to outputs? I see you marked up in the schematic, mV level signals, how are you testing these circuits?
suggest
1) Verify the DC voltages as printed on the schematics.
2) apply AC sine wave stimulus to inputs, something like 100mV/400-1kHz, measure outputs, compare each circuit, you have four identical circuits using those jfets.
suggest
1) Verify the DC voltages as printed on the schematics.
2) apply AC sine wave stimulus to inputs, something like 100mV/400-1kHz, measure outputs, compare each circuit, you have four identical circuits using those jfets.
Last edited:
Vintagecasey
Well-Known Member
Problem is the left channel is about half the volume of the right. This only happen la with phono 1 everything else is even (phono 2, Aux, tape 1 &2..)
Vintagecasey
Well-Known Member
Didn't have much time today but I did get a chance to flip Q1 and Q2 and the problem remains in the same channel.
it has 2 separate phono preamps. one with fet inputs, one with bjt inputs. it's an oddball for sure.
Now I have to go look.
By golly, it does. It's been awhile since I've worked on one. I'm still betting on a connection issue.
signal tracer would be my tool of choice
Right after a thorough switch cleaning.
Vintagecasey
Well-Known Member
How would I do that, check AC alone the audio path for differences. Unfortunately everything on this particular board is extremely close together.
Vintagecasey
Well-Known Member
Or what I need an oscilloscope?
you can do it with a scope but i prefer a signal tracer. you also need a way to inject a VERY low level 1khz signal into the phono stage.
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