Set to restore a Kenwood Model 500

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by hopjohn, Apr 6, 2016.

  1. hopjohn

    hopjohn Silver Face Subscriber

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    I've been on the hunt for a Kenwood Model 500 or 600 for more than 4 years, but the prices have just been too far out of reach. Over that span I've collected and restored many different models of Kenwood integrateds, even a few receivers, but never a Supreme. I'm happy to report that last week, after stumbling across a newly listed auction, that has all changed. I can now call myself the proud owner of a Kenwood Model 500.


    I welcome advice, opinion, and fun so please join in as I document my progress with this legendary amp.
     
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  2. Alobar

    Alobar Pulling out of the Last Chance Texaco.. Subscriber

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    That's a long time to wait. Congrats! :thumbsup:
     
  3. DrumminDaddy

    DrumminDaddy Hit it, Baby !!

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    Lost in musing circumstances
  4. gort69

    gort69 AK Subscriber Subscriber

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    Looking forward to it!
     
  5. SPL db

    SPL db It's all about the music!

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    And well worth the wait coming from an owner of three Model 500s and one Model 600 :thumbsup:
     
  6. turboyam

    turboyam The Kenwood fan!

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    Congrats!. Isn't it fun to be able to buy now, what we lusted for in our younger years!. Plus we all know that with a lil love and a few caps,
    it will be good for decades more!. I'm not sure people in 30 years will be collecting/ refurbing black plastic crap of today!?...
     

     

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  7. hopjohn

    hopjohn Silver Face Subscriber

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    Yes, it is. The four figure prices in some cases made it a bit easier though.

    Haha. Get your popcorn ready.

    Thanks gort!

    You're a lucky man.

    There will be pieces in every decade worth lusting over, but true certainly the 70s, particularly the late 70's, has more than it's fair share.
     
  8. hopjohn

    hopjohn Silver Face Subscriber

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    The seller described this unit with the following:

    "For the last 6 years, this amp has been my daily driver. It worked great, and has a ton of power. Then a few months ago I decided to add a set of Magnaplaner speakers to my set up.
    Not being too smart, I didn't realize the difference between 4 ohm and 8 ohm and tried to blast all 3 sets of speakers at the same time. "
    As a result, one of the channels is almost completely lost....some sound still comes thru, but not much."

    Best case scenario I was thinking after reading this was that the relay must still be engaging if he was still getting sound through it, so the outputs may have survived. The first thing I did was hit the rear Normal/Seperated switch with some Deoxit just to eliminate it as a source of problems. After bringing it up on full power it does indeed come out of protection.

    I measured the offset to give some indication of how unhealthy it is. There is a pretty steady 290mV of DC on the right channel with the left channel starting out around 200mv and quickly dropping to around 80mV and likely beyond, but I dared to leave it powered up only for a minute. Listening through headphones at low volume there is a constant crackle regardless of balance position in the right channel with some very distorted signal getting through. The left channel is clean, but has an AM sound quality to it, where the frequency output is very narrowly positioned somewhere in the high mid range.

    After checking the above I removed the bottom plate, and faceplate and checked the right-side outputs with my transistor tester. All 4 outputs there check good. I'm very happy about that as these Sanken devices in the Model 500/600 were the best of their day. If I'm right about what I've researched, using MJ21193/MJ21194 will work, but certainly the original output devices are the preference.

    [​IMG]
     
    Last edited: Apr 7, 2016
  9. hopjohn

    hopjohn Silver Face Subscriber

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    A couple of things I'd like some feedback on you guys:

    1. The measurement after the rectifiers at pins 14, 16 of protection board seems low. 65V nominal, with a measurement of 57V. -8V, is this is a concern?
    Seems like the power supply board is pretty close though so maybe the nominal is a bit more than to be expected.

    2. I've got some poor voltage measurements on the right power amp board at pins 2, 6, and 8 which lead me to believe I've got some real issues there.

    Voltages

    Power Supply
    Pin # (Nominal)
    1 (-28V) -28V
    4 (40V) 38V
    8 (0.3V?) 175mV
    11 (2V) 2.3V
    14 (40V) 38V
    15 (28V) 28.9V
    18 (-28V) -28V
    [​IMG]
    Protection Board
    1 (45VAC) 43.7VAC
    2 (-65V) -58.4
    3 (45VAC) 43.7VAC
    4 (65V) 58.4
    14 (65V) 57V
    15 (45VAC) 43.5VAC
    16 (-65V) -57.2V
    17 (45VAC) 43.6VAC
    [​IMG]
    Power Amp Right, Left
    2 (1.35V?) -0.79mV, 1.23V
    6 (12mV) -344.7mV, 18.9mv
    8 (0.6V) -349mV, 0.567V
    10 (-65) -56.9V, 56.2V
    15 (65V) 56.9V, -56.4V
    [​IMG]

    Tone Amp
    14 (28V) 28V

    Pilot Lamp
    (7.5VAC) 7.43VAC
     
    Last edited: Apr 9, 2016
  10. hopjohn

    hopjohn Silver Face Subscriber

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    My first task will be to gain access to the right channel amp and take a close look at the components there. Here is the bottom view of the 500.

    [​IMG]

    I decided the best way to access the right channel power amp was by completely removing the heat sink. The heat sink is held in place by 4 screws at the front (mounted to the front sub-chassis), 3 at the rear (mounted to rear sub-chassis), 4 at the interior ( which hold the terminal strips for the T-O3 sockets and the thermistor), and 8 at the exterior (that mount the four output transistors). I start with the removal of the front 8 screws holding the output transistors and next the front 4 screws holding it to the front sub-chassis.. The right angled corner of the rear sub-chassis interferes with the heat sink, so this must first be removed next. The amp's rear panel hinges downward rendering access to the 9 screws needed to remove this rear sub-chassis panel. This will then allow the heat sink to swing out with the ability to gain access to the interior screws (be very careful around the thermistor the leads can break off easily). Once the interior screws are removed, the heat sink can be completely taken out (see photo below). With the rear sub-chassis panel removed the amp gets very flimsy, so I reinstalled it, and also the rear panel temporarily. Doing this provides the structural integrity required to move the amp around in position to be worked on.

    [​IMG]
    The top two corners of the power amp board have plastic locking pins that attach to a stabilizing bar. Once removed the board can swing down with some very respectable access. Notice the heat sink (top right in photo) completely removed.

    [​IMG]
    Right Channel Power Amp (X07-1440-00) in stock form.
     
    Last edited: May 9, 2016
  11. turboyam

    turboyam The Kenwood fan!

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    :lurk: I will follow this religiously :thumbsup:
     

     

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  12. 39cross

    39cross AK Subscriber Subscriber

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    Me too. I have a 500 which I'm using right now...got a good BIN deal on it. I've recapped the power supply board but the rest is stock. I started a restore thread at some point but haven't touched it since. It is quite a lovely thing, even in it's relatively tired state.

    Also, a side note to say thank you again on your excellent posting of your KA-7100 restore. I did in fact perform some of your updates post my own recap, replacing the PS diodes and other silicon, this cleaned up the sound even further. So I am very much interested in seeing this restore as you progress with it. Bon chance!
     
  13. hopjohn

    hopjohn Silver Face Subscriber

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    Right Channel Power Amp (X07-1440-00)

    Every transistor on the right channel power amp board, with the exception of the Dual FET, was evaluated. None of them test shorted. All of the resistors were checked for value in circuit and only a couple of them weren't close to the rated value. Those two were pulled and they checked out fine.

    I suspect either one of the the two 2SA810 (Qe3,Qe4) or the 2SC1452 (Qe6) were going bad though, as these are known to fail. They were replaced with KSA1381 and KSC3503 respectively. I also replaced 2SA673A (Qe7, Qe10) with KSA733C and 2SC1213A (Qe8,Qe9) with KSC945C. The heat sink mounted Qe11,Qe12 were given fresh compound.

    Though they tested fine, I figured I'd replace the zener diodes while I was in there. For De1 EQA01-24 24V 500mA, I used a 1N5252B. For De2 YZ-140 14V 500mA, replaced with a 1N5244B.

    R14 looked pretty shabby (see the before photo), so it was replaced with Vishay/Dale 1K metal film. Having looked at photos of other Model500/600 amp boards and also the parts list in the service manual, my board is the only one one I've seen use a 1K resistor in that position. The others all had a 100Ohm part. I verified that the left channel had a 1K installed also. :dunno:


    [​IMG]

    An orange low leak Elna CERB 10uf 25v capacitor is checked. Notice the ESR measures pretty poorly when compared with say a Nichicon PW of the same size and value which likely has an ESR half that. This is not an anomaly, all of the 10uf orange Elna low leak caps measure this way and seems to indicate the leakage current was of more importance to the engineers than the ESR.

    [​IMG]

    One of the special features of the Model 500/600 Supreme amplfier is this Sony 2SK58 dual FET which is enclosed in this small, shielded metal case due its temperature sensitivity.

    [​IMG]
    Here is the finished right power amp. I've squeezed three Wima 10uf 50v polyester caps in place of the 10uf 25v Elna CEBR caps at Ce5-8 and an Elna Silmic II 10uf 50 at Ce5. Each of the Vr trimmers were replaced with 3386H Bourns single turn. Vr1 and Vr2 got 500ohm, and Vr3 a 50K. Vr1 is the coarse adjust and Vr3 the fine adjust for the offset metered at pin 13 and set as close to 0mV as possible. Vr2 is the bias adjust set to 25mV while metered across pins 5 and 6.

    Edit : July 30, 2018 : There is an error in the service manual that identifies VRe1 as the fine trimmer, and VRe3 as the coarse trimmer. VRe1 is the COARSE trimmer (faced down on the amp board), VRe3 (faced up on the amp board) is the FINE trimmer. Read this thread for further information on the adjustments for the model 500/600/650.

    After a partial reassembly and the 4 Sanken outputs reinstalled I fired the 500 back up under full power. I set the offset and bias per the service manual and then shut down long enough to hook up a CD source to Aux 1 and have a listen through the headphones. The right channel distortion is GONE, just a nice clean signal now. As I said before, I can only speculate that I had a failing transistor at Qe3,Qe4 or Qe6 as all parts tested okay. A bit anti-climatic in finding the guilty part, but you can't complain with the end result.

    With the repair completed it's time to move on to the restoration.
     
    Last edited: Jul 31, 2018
  14. EchoWars

    EchoWars Hiding in Honduras

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    The WIMA caps are kinda wasted on the V/I limiter, but if that's what you wanna use, mo power. I just think they're kinda spendy for that application.

    It's not necessary to bend the bejeezus out of the TO-126 leads to get them in place. Not slamming you, just offering some constructive criticism.

    [​IMG]
     
    Last edited: Apr 12, 2016
  15. hopjohn

    hopjohn Silver Face Subscriber

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    I welcome the comments. No offense taken. The lead bending thing was a force of habit with straight line pins. I like your method better here.

    My use of the Wimas was simply that I had designated the parts for the spot based on the parts list number being a "BR" cap when I placed my order. When it came time to put them in I deliberated over it for like 5 minutes and finally I just said eff it. I see you used FC.

    I notice your Re14 is 100 ohm also. Beginning to wonder if mine wasn't a factory screw up. What's Re14 doing precisely?
     
    Last edited: Apr 12, 2016
  16. 39cross

    39cross AK Subscriber Subscriber

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    Glad to see your distortion is gone.
    Are modern caps 'low leakage' compared to what they had available (i.e. the orange caps) in the 70's? I've seen that MTF uses KL's for these in Pioneers.
     

     

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  17. Blue Shadow

    Blue Shadow I gotta get me a new title

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    Film caps are very low leakage because it is film cap, way lower than even the KL.

    The KL is low leakage 'lytic by Nichicon's design group and is one correct 'lytic to use for the low leakage caps being replaced. I don't believe that other 'lytics would be considered low leakage by vintage orange lytic cap standards. Probably a question for Conrad though.
     
  18. hopjohn

    hopjohn Silver Face Subscriber

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    Yes thank you, I'm very happy about that.

    Blue Shadow, roger2, and I all agree about the orange Elnas being lower leakage than a general purpose modern cap. The KL's are designed for low leakage, but even still aren't as low leakage as a film cap will be. According to MTFs experiment, as linked in roger2's post, the KL was an improvement on the original noise spec. I guess it could be argued that the KL were slightly less noisy than the orange caps, but it isn't a fair comparison when you consider the age. The fact that the originals were lower noise than the all PW channel however, is telling.

    My philosophy is pretty straight forward. Anything I remove I do my darnedest to replace it with something better. With transistors there are sometimes compromises sure, but with caps you should be able to find improvement nearly every time. Also, I try not to second guess the original design. If they wanted to use orange Elna low leak caps over the gray (or navy blue) general purpose caps, I have to think there was a reason. The point I was making in my previous post about the higher ESR of the orange caps, if not already clear, was that the designers knew that the gray general purpose caps, size for size, were significantly lower in ESR and chose to use the orange caps for their strengths (low-noise) regardless.

    I've seen inconsistencies with stock equipment having an orange cap in one unit and not in another identical unit. Running changes?, it's hard to say because assuming they made units in order of serial # I've seen parts switch back and forth with no pattern at all. Using the Sansui TU-717 as an example ( I've worked on too many to count) this type of thing would occur frequently i.e...one unit has an orange low leak, the next a tantalum, then the next a general purpose. So trying to understand what the designers were thinking in every instance is futile.

    Even though it may not be a perfect method, I usually try to refer to the service manual's parts list and see if they have designated the low leak caps for that position (Kenwood will use the suffix BR at the end of the part #), if so, then it's tough to argue what Kenwood was shooting for. If I consistently see an orange cap across multiple units where the parts list didn't designate one, again I use one since it's convincing that an intentional design decision was made. The great thing about being able to use film caps is that their versatility makes deciding what to use easy. Regardless if it's polar/non-polar, low-leak/low ESR, signal path, coupling, decoupling, bypass you're covered with a film.

    At any rate EW's point of contention is not lost on me, it's definitely not the best use of $2.53 caps and the cost factored greatly into my deliberation before I ultimately went ahead with them. That said, with this build I'm not in the position of having to explain to a customer that I spent 6 times more on some of the caps then was necessary. I usually draw the line WRT to using Wima poly caps at 1uf (maybe 2.2uf to 3.3uf if only a small number of caps are required), but now 4.7uf to 10uf are small enough to be in the realm of possibility should you not mind forking out the cash for them. To roger2's point... with these Wimas now an option, the debate over whether the KL, Silmic II etc. is the best substitute for orange Elnas becomes trivial should they fit.
     
    Last edited: Apr 12, 2016
  19. gort69

    gort69 AK Subscriber Subscriber

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    Nice work John, and thanks for the info on the "BR" - are there any other top secret codes in the K-wood parts lists re caps?
     
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  20. turboyam

    turboyam The Kenwood fan!

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    Plus one for the BR secret too!
     

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