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Help; need FET sub

preacherJohn

preacherJohn

Active Member
I need to find a substitute for a 2SK68A in a Kenwood KA 3500 I've been working on. I'll try to post the schematic also. There's one resistor (R58) that gets so hot it begins to smoke. I'm not seeing any shorts on my current limiter, but there's a reason it's turning brown on me and I'm suspecting one or more of the FETs Qd1-Qd4 is bad. The other components in the loop with R58 check out alright. I replaced this resistor once with a 1 watt 100 ohm metal film. I can't keep the unit on long enough to check a voltage as it will burn up pretty quick. I've searched and searched for a replacement for those 2SK68As and all subs that are listed in other threads are discontinued. But I did run across one thread that mentioned an LSK189. I found these at Mouser but they're not cheap. If I have to pay 6.74 ea for 4 of them, so be it; I can't find anything else. Will the LSK189 be a decent substitute for the 2SK68A? Or if there is something else that someone found that fits the bill and is stock where I can order them, I'm all ears. Thanks. page 8.jpg
 
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I got the schematic @ HiFi Engine. I'm surprised it doesn't blow up better, I dropped and dragged a PDF out of my file.
 
if it uploads as a jpg it resizes pretty small. If you can upload the PDF directly it should come out OK.
 
Don't know if this will help but, what I do that works very well for me is to enlarge the image (PDF, JPEG whatever) on my screen to where its as legible as its going to get and then screenshot it. I'm on a Mac but it should work the same way on a PC. Then I will either attach that image as is to my AK post or crop the image if it needs it. I've even been known to open it in basic photo editing software and tweak the exposure, contrast and sharpness until I can figure out what that damn value or detail is.


There's a KA-3500 in the AK database BTW.

Like this:

Screenshot 2024-04-09 at 7.03.30 PM.pngScreenshot 2024-04-09 at 7.03.53 PM.png
 
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not to say that fets don't fail but i'd be more suspect of transistors or a cap being shorted.

i'd think with a 40 or 60 watt dim bulb you would be able to get some voltages before having to shut down to get a better idea of where to look.
 

The KA-3500 restoration

Arrival A good many of us AKers have been there. You hop on the auction site and spot something that's just too enticing to pass up. An impulsive click of the buy it now button and now it's yours. You send a message to the seller to please pack well and anxiously cross your fingers. A week or so...
audiokarma.org

Schematic to KA 3500 here in this link.
 
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painttoad said:
not to say that fets don't fail but i'd be more suspect of transistors or a cap being shorted.

i'd think with a 40 or 60 watt dim bulb you would be able to get some voltages before having to shut down to get a better idea of where to look.
Click to expand...
All the caps at least the electrolytic ones have been replaced with Nichicons. That' not to say one could have been fried. I did make a bad mistake from poor eyesight or just being lazy. This thing did play before. A lot of these 50yr old wires break very easily moving them around. I've soldered them back numerous times working on this amp. I was putting it back together after some transistor replacement on the preamp board, where I noticed two wires were broken off. Where one wire looked to be formed to fitting that board, I lazily soldered it to that place. And when I turned the amp on, my dim bulb brightened up and I shut it back down. Upon inspection I noted my mistake on the schematic. Since that time, I've had problems with R58 smoking on me. That's why I suspected the FET's in that area. J4 was the jumper that was wired wrong. It was probably grounded. I thought I checked those caps on that board, but it's been awhile, as I was frustrated with it, and had other items I had to get to.
 
itguy1010 said:
Don't know if this will help but, what I do that works very well for me is to enlarge the image (PDF, JPEG whatever) on my screen to where its as legible as its going to get and then screenshot it. I'm on a Mac but it should work the same way on a PC. Then I will either attach that image as is to my AK post or crop the image if it needs it. I've even been known to open it in basic photo editing software and tweak the exposure, contrast and sharpness until I can figure out what that damn value or detail is.


There's a KA-3500 in the AK database BTW.

Like this:

View attachment 3172789View attachment 3172790
Click to expand...
Yep, this is it.
 
It appears that after the R58 resistor you should have 23 volts on capacitor Cd38. If that capacitor is not the issue, would it be possible to add another resistor is series with R58 to drop the Voltage lower to troubleshoot. The power supply before that resistor shows 24.5 Volts so 24.5 minus 23 V at Cd38 gives you normally around 15 mA. through R58. If yours is getting hot would say you have around a minimum of 5 Volts being dropped across it being .25 Watts to 10 Volts that would make it 1 Watt.

If a resistor of a value of 270 Ohms was substituted in place of the 100 Ohm, you most likely would still have enough Voltage to test the 6 semiconductors in the circuit to troubleshoot. I would start then by looking at the Voltages across RD5, 6, 7, 8, 17, 18, 19 and 20 for excessive drop or imbalance.
 
Jose Jimenez said:
It appears that after the R58 resistor you should have 23 volts on capacitor Cd38. If that capacitor is not the issue, would it be possible to add another resistor is series with R58 to drop the Voltage lower to troubleshoot. The power supply before that resistor shows 24.5 Volts so 24.5 minus 23 V at Cd38 gives you normally around 15 mA. through R58. If yours is getting hot would say you have around a minimum of 5 Volts being dropped across it being .25 Watts to 10 Volts that would make it 1 Watt.

If a resistor of a value of 270 Ohms was substituted in place of the 100 Ohm, you most likely would still have enough Voltage to test the 6 semiconductors in the circuit to troubleshoot. I would start then by looking at the Voltages across RD5, 6, 7, 8, 17, 18, 19 and 20 for excessive drop or imbalance.
Click to expand...

I made a mistake on which resistor was getting hot. I guess my eyes are failing worse than I thought. R58 wasn't the problem, it was R56 that was getting too hot. I removed the preamp board from the power board, then R56 was acting normally except for being way out of spec from the heat it was enduring before. I then removed the JFETs (Qd1-Qd4) from the preamp board, and reconnected that board to the pwr board. R56 was working OK, so I replaced it with a new 1 watt/100 ohm resistor and measured the voltage across it and it read -14.4v. So I ordered 4 new JFETs (LSK189) and should have them later today. My OP was asking if these JFETs would work in place of the others as they are no where to be found. The other components around those JFETs tested good, all resistors and caps. So hopefully those JFETs were the problem. I'll know by tomorrow.
 
I would not install the new JFETs until you clarify something. Re56 is a 330 Ohm 1 Watt transistor in the power supply. It is not 100 Ohms like Re58. Re56 in the power supply is a known issue on these receivers getting hot by a defective Zener diode De6, or a leaky or shorted transistor Qe25. Please check your work and part identification for me so that I may assist you. The capacitor Ce32 could be an issue due to age. I would think the semiconductors are the place to start but again, let's get the identification and values correct.
 
Jose Jimenez said:
I would not install the new JFETs until you clarify something. Re56 is a 330 Ohm 1 Watt transistor in the power supply. It is not 100 Ohms like Re58. Re56 in the power supply is a known issue on these receivers getting hot by a defective Zener diode De6, or a leaky or shorted transistor Qe25. Please check your work and part identification for me so that I may assist you. The capacitor Ce32 could be an issue due to age. I would think the semiconductors are the place to start but again, let's get the identification and values correct.
Click to expand...
You are correct; Re56 is 330 ohms, I must have gotten my values mixed up when I posted upstairs instead of in front of the amp in the basement. It does have a 330 1 watt resistor. Zener De6 & De5 have both been replaced when I went thru that board with 24v/3W zeners. I'm having a hard time locating Qe25; give me a few minutes.
 
preacherJohn said:
You are correct; Re56 is 330 ohms, I must have gotten my values mixed up when I posted upstairs instead of in front of the amp in the basement. It does have a 330 1 watt resistor. Zener De6 & De5 have both been replaced when I went thru that board with 24v/3W zeners. I'm having a hard time locating Qe25; give me a few minutes.
Click to expand...
OK, I couldn't locate it, because it doesn't have a Qe25 installed in it, nor does it have Re53-54, or De3-4. I guess they were never installed; or someone else removed them.
 

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All caps have been replaced earlier including Ce32. Now, what about those JFets? When they're out of the preamp board, there's not any problem with Re56. That why I've been looking there so closely. I could be wrong, it wouldn't be the first time, but I did accidentally short that preamp board, and it may have caused them to fail.
 
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