Getting a recapped/serviced KA-8100. A question...

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by ethangsmith, Sep 4, 2018.

  1. Watthour

    Watthour Electron Rancher - JS3600

    I will let you decide that for yourself after reviewing some experiences of others:

    http://audiokarma.org/forums/index....owing-fuses-after-triac.834626/#post-11893645

    "I put one in a KA-7100 a while ago and it made the transformer hum. Not related but seems they are not 100% all the time. No idea what my problem was, hope you find the solution to your problem."


    http://audiokarma.org/forums/index.php?threads/kr-4070-switch-fix-with-triac.794467/#post-11005282

    "Did you experience an increased hum of the power transformer after installing the Triac switch? I've done this mod to several of my units, but the last one I did (Yamaha CA-810) suffers a hum of the power transformer. Very odd."


    http://audiokarma.org/forums/index.php?threads/nad-3020-buzzing-noise.645230/#post-8579268

    "Do you hear it through headphones? If not, your speaker wires could be acting as antennae and picking up some radiation from somewhere. Is the unit closed up, or are the top and/or bottom covers off? I had this problem with a triac power switch mod. Closing the unit up eliminated it."


    http://audiokarma.org/forums/index....d-update-a-ka-7100.713661/page-6#post-9727960

    "Turns out my transformer hum was coming from the inclusion of the triac. Its been removed for the moment until I can work out whats going on with it, zero hum again."


    http://audiokarma.org/forums/index.php?threads/au-777a-fix-up.628727/page-2#post-8382410

    "EDIT: I'm getting some non-volume-dependent hash through the speakers with the triac installed. Moving it 18 inches away from the unit with some jumpers makes no difference. Taking the triac out of the circuit eliminates the hash. I haven't noticed any hash on the two amps and the receiver I've installed triacs on. Gotta give this some thought." .
     
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  2. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    Interesting. I've been talking with another member about alternatives to the Triac as well. Seems like a relay or doing the jumper and safety cap would be my best options. I don't want transformer noises or any other issues!
     
  3. Watthour

    Watthour Electron Rancher - JS3600

    A jumper doesn't necessarily save the switch from the load. A safety cap is a good effort regardless of how the power is switched. I know it's not SS, it's not Bluetooth/WiFi compatible, and is not "high-tech", but a power relay is fairly robust and straightforward.
     
  4. leesonic

    leesonic Hold on, here comes the bass. Subscriber

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  5. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    Al was kind enough to walk me through a relay install. It's much easier than I suspected it may be. I'm compiling parts and I plan on adding it in as soon as I have time.
     
  6. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    It's been a month. Still waiting on parts. The 8100 continues to play beautifully (Haven't tried to crank it up yet) but puts out what seems to be a lot of heat. Mostly coming from the diodes it seems. I have 5A diodes on hand waiting for installation when the other parts arrive. ETA is the end of the month.
     

     

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  7. ManyMoonsAudio

    ManyMoonsAudio AKA paul79 Sponsor Subscriber

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    I had a KA-8100 here that someone installed a Triac into. Worse sounding Kenwood I ever heard. Took that out, and it sounded fantastic. Don't use them is my recommendation.
     
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  8. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    I'm not doing the Triac. Doing a traditional 120V relay.
     
  9. Porkbuns

    Porkbuns New Member

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    Don't bother with a triac.

    Honestly the ugly but also ideal solution is to have a separate power strip and leave the KA-8100 on. I have the same thing with my GFA545 and a few other bits of kit where the current inrush eventually arcs and causes a noticeable popping sound. Cleaning the contacts every so often also helps a lot (as clean contacts reduces carbonization) but honestly for a lot of the old gear I just turn it off manually then turn off the power strip and when powering on, i flip the switch on first then turn on the strip. No arcing and honestly these things last forever.

    Alternate solution; you can always find a replacement on/off switch that's sufficiently high amperage. Might not be original shafts, but it'll do the trick. Works on the KR-9600 I have and the litany of other gear too.
     
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  10. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    Here it is 2 months later and my relay is still on backorder via Mouser. I have all the other parts ready and I already got the new diodes soldered in.

    An issue I'm beginning to experience with the unit is the Subsonic/Bass/Treble filter switches don't seem to work well. I've noticed the unit seemed very lifeless over the past few weeks and yesterday fiddled with those three switches. Suddenly I have nice clean treble and wonderful deep bass again. How the heck do you clean those switches!?!?
     
  11. gort69

    gort69 AK Subscriber Subscriber

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    Dribble some Deoxit D5 or CRC Contact Cleaner in the slots shown by the arrows and work the buttons a bunch of times. Repeat as necessary

    P1011588.JPG
     
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  12. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    Sounds good. I'll see what I can do! Thanks!
     
  13. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    Huge thanks again to Gort69. Switches are functional again and I've got nice treble and bass response!

    Yesterday I picked up another 8100 for a good price. Haven't put any sound through it but it seems functional. Previous owner said the tone controls didn't work and noted that the transistors on the tone board get super hot. And they do! Board is darkened and within seconds of turning the unit on you can feel the warmth. Traces on the board leading to the purple/blue caps next to the transistors look like they're getting super hot. I know these transistors DO run hot naturally, but this seems excessive. What is generally the cause of this? Fried caps?
     
  14. gort69

    gort69 AK Subscriber Subscriber

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    I assume you're talking about the two heat-sinked transistors, Qi13,14? Those are the +/-25VDC regulators for the pre amp and control amp, and they both share a single heatsink. Check for +25VDC at pin 5 and -25VDC at pin 4 on that board. (red meter lead on the pin, black lead to a clean chassis ground) Qi13 is a 2sc1419 and is failure prone in my experience. If the voltage is drastically wrong at pin 5, I'd suspect it has failed.

    Those do get quite hot, that is why they are heatsinked. I'd expect the board to be darkened.

    Also - the p/o wouldn't be the 1st person to be fooled by the "turnover" switches - the center position is tone defeat.
     
    Last edited: Dec 21, 2018
  15. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    Yea. I need to actually put speakers on it and feed it some music. I checked DC output at the speaker terminals and it holds rocks steady at 2mv on each channel. Amp bias hovers at 12mv on each channel (14mv is spec). Seems to hold everything very steady in that aspect of the unit.

    Are pins 4 and 5 accessible without dropping the front of the unit down? I'll have to check those voltages as soon as I have a few minutes free...
     
  16. gort69

    gort69 AK Subscriber Subscriber

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  17. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    Amazingly, pin 4 shows -24.8V and pin 5 shows 24.7V.
     
  18. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    Here's a photo showing the rear of the board and the heat damage. Note too that some traces seem to have attracted dust to them. I noticed this on the power amp board as well. Certain traces attracted a fine dust over them. Note the traces going from the transistors, under C55, towards the pins. Something is making those transistors HOT HOT HOT!!

    [​IMG]
     
  19. gort69

    gort69 AK Subscriber Subscriber

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    I think that's just indicative of the unit seeing a lot of use in the 40 years it's been around. Very common - the regulators get hot.

    For comparison, here is a regulator on a KR-7600 that has obviously seen a lot of use as well.

    P4020250.JPG

    P4020249.JPG
     
  20. ethangsmith

    ethangsmith AK Subscriber Subscriber

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    Wow. So this leaves me with three questions:

    1. What attracts fine dust to some of the board traces? Electron flow?
    2. Why do the transistors seem to get hotter faster than my other 8100 (That's been rebuilt, I should add.).
    3. The diodes that sit directly above the transistor are very baked looking. The leads are black and orange looking. Should I replace those? And what is a suitable replacement if I do?
     

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