My Kenwood KA-9100 Restoration

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by Andrew Page, Nov 10, 2017.

  1. Andrew Page

    Andrew Page Member

    Hi everyone,

    I've been busily undertaking the restoration of my Kenwood KA-9100.

    Many thanks goes to all of the amazing minds on here who've posted about their rebuilds. I have ordered the replacement of all trouble transistors/diodes thanks to AK.

    This mainly just a me-too post, but I'd like you gentlemen to monitor/offer suggestions and give me a hand if I get stuck.

    My approach is more let's get down to business, I tend to leave stuff alone unless it's problematic/recommended to be replaced.

    I will be doing a Triac mod later.

    I'm only left with the control boards to recap as I've already undertaken the protection/diode board rebuild, power supply, power amps and replaced the main power supply caps with an amazing set of Nichicon KG Gold Tune caps. 15,000uf 63 volts. Whoever was in here before replaced one of the originals and while it the same value, was slightly smaller in physical size. Second from the right in the photo below.

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    The rest of the caps are been replaced with Nichicon Audio grade caps. Mostly FW series throughout the amp (or whatever audio grade is available). I've kept all voltages and capacitances the same. The control board is getting totally re-done with FG series.

    The only general purpose caps i've ordered are for the meter driver board :)

    Previous owner/owners did a horrible soldering job on the diode boards, you guys should have seen how this looked before I started cleaning her up.

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    The best I could do, the right hand side in the photo below came up really nice and despite damage to the trace on the left, I was still able to lock it down tight without having to improvise a repair job. The solder damage to the wires was not done by me :)

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    On the diode boards transistors replaced as follows:

    Q1 2SC945 replaced with KSC1845

    Q2 2SC1212A(C) replaced with KSC2690A

    On the Power amp modules, all caps replaced and as per recommendations on this board, the following transistors:

    Q 5 through to 8 is 2SA912(R) supposedly underrated, replaced with KSA1220AY

    Q 11 to 12 is 2SC1885(R) or (S) supposedly underrated, replaced with KSC2690AY

    And Potentiometer 2x 3299Y-501LF-ND

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    And just this afternoon I completed the power supply board.

    Of particular note was Diode 5 next to the relay. This diode went bad some time ago and would not allow the Kenwood to come out of protection. I pulled it out, it fell apart and the amp started coming out of protection again without the diode in there! I made another post about it here: http://audiokarma.org/forums/index....ge-despite-protection-circuit-rebuild.773654/

    It was replaced with a 1N4004.

    Zener diode YZ-140 was replaced with 1N5244B.

    I left the transistors including the TO-220's alone as these don't appear to give trouble unlike some Pioneers and other models. I think Kenwood always got it right with their PS boards, but if I have a freak incident one day, the following are much higher rating Balls to the wall types:

    Q1 2SC1983 replace with TIP122FS-ND

    Q2 2SA755 replace with BD244C

    But I've decided to save those for a rainy day.

    And relay S51 4030-05 was replaced with good old Omron MY4-02-DC24. Dead pins cut out so it could fit. They went flying all over the place but the Kenwood is more important. The vacuum cleaner will get those later.

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    I also touched up the solder on the main PS connecting pins as some of them looked like they didn't have enough solder on them. Don't want something to crack and I've heard of that happening a few times. The ones on the left already done, the ones on the right in this photo show how they looked. Some were even worse than this before I thought I better take a photo.

    [​IMG]

    Besides the Power Supply board I did today (the amp working like a dream) the other work was done about 5 months ago and I got busy with work and other things.

    Now comes the disassembly of the front to gain access to the control boards. I start Saturday afternoon/evening Australian time.

    As others have said, a Royal PITA, but I live for this stuff and want this beauty to shine.

    I welcome your suggestions. If I'm doing something wrong or could do it better, tell me. Be blunt. I'm here to learn and want to get it right.

    I'm 31 years old, so help me keep the dream alive hey?

    [​IMG]
     
    dlucy likes this.
  2. rjsalvi

    rjsalvi Active Member

    Messages:
    312
    Location:
    San Diego, CA
    We like 9100s. :D

    About the only thing I'll add is that it's generally a good idea to use low ESR (equivalent series resistance) caps in power supplies. Your main filter caps are fine, but you chose Nichicon FG for the PS board. Why use low ESR? Series resistance at the input creates high frequency noise across the cap, and at the output, it can actually cause more ripple. Series resistance can also affect transients. Let's say you're watching a movie, you've got the amp cranked up and suddenly, there's an explosion!...an instantaneous large demand is placed on your power supply, to send the necessary current to your amplifier stage so it can reproduce that explosion with authority and as distortion-free as possible. While that may be a gross example, it illustrates why increased ESR to rapidly changing current demands can affect performance.

    Here's an informative video showing various type caps in a circuit, demonstrating the differences in signal integrity due to ESR differences:

    Having said all that, the amp will work fine the way it is. :) Good job, Andrew.
     
  3. Andrew Page

    Andrew Page Member

    I agree with you and thought what I'd already picked would be a good fit for the power supply but in hind site, I probably should have gone for the Nichicon PW series which appear to be rated at 105 degrees. Either that or the KG series. Meantime with the main filter caps, I think those big Gold Tune caps will help things out a whole lot with the amp anyway. But I'll be ordering something different on the next rebuild that I do for the main PS board on what ever amp I do :)
     
    Last edited: Nov 11, 2017
  4. rjsalvi

    rjsalvi Active Member

    Messages:
    312
    Location:
    San Diego, CA
    Andrew Page likes this.
  5. Andrew Page

    Andrew Page Member

    And the front is totally disassembled and control boards pulled out. No turning back for me now.

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    Control board front:

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    Control board back

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    May be a few hours or a day or so before my next update. This is the part where I REALLY don't want to screw something up!
     
  6. Andrew Page

    Andrew Page Member

    The front control board is done!

    [​IMG]

    Now for a question guys, when I was pulling the beast of a front panel apart, this bracket fell out without me being able to catch where it goes and so far, I can't find it in the service manual.

    Does anyone knows where it goes to save me the wild goose chase when it comes time to reassemble :)

    [​IMG]
     
  7. rjsalvi

    rjsalvi Active Member

    Messages:
    312
    Location:
    San Diego, CA
    It goes there. :) It's, or it was, glued to the side of the meter board. Also, take a look at the back side of, and towards the top corner of your volume control and you'll notice a "tang"...a small cutout one could wrap a wire around. Mine had a ground wire from there to the chassis. Some units had that ground wire, some didn't.
     

    Attached Files:

    Last edited: Nov 11, 2017
  8. Andrew Page

    Andrew Page Member

    Excellent, thanks for that :) There appears to be no ground wire on my volume control.
     
  9. Andrew Page

    Andrew Page Member

    The rear tone control board is re-capped and as I write this post, re-assembly is underway.

    Sure enough, my ESR meter has found some bad caps (very high ESR) on the tone control boards. Virtually all of the tiny 25v capacitors less than 10uf test bad! The bigger ones appear to mostly be ok.

    I will post some results here later.

    [​IMG]
     
  10. Andrew Page

    Andrew Page Member

    My Kenwood KA-9100 restoration has been a total success! Oddly enough, the part that I found most difficult about this part of the rebuild was reassembling the front of the amplifier. Taking apart the front, removing the boards and old caps from them to solder the new ones in was pretty straightforward for me.

    The improvement in sound is amazing. I listen to mostly 50s and 60s jazz in my spare time. The music has so much more detail. I've been listening to the Art Farmer Jazztet recordings from the 1960s. The bass player plucking the strings, hammers hitting the strings on the piano and Art's trumpet has so much more presence. And it's not a small difference. It's a huge difference! Old caps really seem to make vintage amplifiers sound muddy. Some people might mistake that for a warm sound, but to me, vintage is all about power, punchiness, articulation, detail and smoothness. The vibraphone almost leaps out of the speakers on George Shearing with the Montgomery brothers as well. I can now clearly hear the hammers hitting the vibes.

    Now for some ESR meter results on some of those old ELNA low leakage caps from the control boards.

    10uf 25 volt low leak cap: ESR 7.6 ohms about 5.3 appears to be more acceptable

    1uf 50 volt low leak cap: ESR 18 ohms! This rating doesn't come up on my meters chart but appears to be way too high.

    10uf 25 volt low leak cap: ESR 6.5 ohms about 5.3 appears to be more acceptable

    1uf 50 volt low leak cap: ESR 15 ohms again appears to be another very high reading but not on my chart for this value.

    Bigger values all appear to have "mostly" tested really well:

    220uf 35 volt standard cap: ESR .31 ohms about .17ohms appears to be acceptable

    100uf 35 volt standard cap: ESR .35 ohms about .32 appears to be acceptable

    470uf 50 volt standard cap: ESR .10 ohms about .09 appears to be acceptable

    47uf 35 volt standard cap: ESR 1.8 OHMS about .68 appears to be acceptable (yes this one a little off)

    100uf 35 volt standard cap: ESR .32 ohms about .32 appears to be acceptable (on the money)

    220uf 10 volt standard cap: ESR .29 ohms about .6 appears to be acceptable.

    This is only a few capacitors and obviously, it's a real mixed bag of results. Some test really well so one would wonder why would you want to change any, some are questionable but then others just tested horribly! Either way, the results are amazing and I'm so glad I finally got around to doing this.

    Here are some more pictures of the work.

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    And by the way, here is the thyristor modification. My first ever attempt at doing one of these.

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    I think this is the only part of the restoration where I had a brain fart!

    I followed the diagrams carefully and made sure I got the connections right on the power switch, when I power up, the off position on the power switch was really on and the on position was off! It really freaked me out for a second and was thinking how could I have screwed that up! Rather than go through the painstaking exercise of trying to desolder and do it all over again, I just dealt with it in a lazy fashion by flipping the whole assembly upside down. I'll re-do it one of these days soon so the triac goes in the place I intended it to. For now, works like a champ and I hear no more arching on the power switch.

    This is the triac I used: http://www.littelfuse.com/data/en/data_sheets/littelfuse_thyristor_qxx40xx.pdf
     
    rjsalvi likes this.
  11. SicMan

    SicMan Fire up those speakers Subscriber

    Messages:
    3,030
    Location:
    Wolcott - CT. USA
    Looking good, nice wood case.
    :music: should sound sweet when done. :beerchug:

    John
     
  12. Andrew Page

    Andrew Page Member

    Thanks John :) She's sounding amazing! :bigok:
     

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