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Have you seen a KA-3500 with no power supply regulator?

Discussion in 'Kenwood-Trio/Kensonic-Accuphase' started by tusntuk2, Jul 25, 2018.

  1. tusntuk2

    tusntuk2 AK Subscriber Subscriber

    Messages:
    188
    Location:
    Arkansas, USA
    Looking at my second KA-3500 with no apparent regulation of the power supply. Missing parts - diodes, resistors and Qe25, and only two jumpers in place of the diodes. Is this legit? Wouldn't it affect many more parts that were designed for a regulated supply? Would it create more noise? Much to learn here, i'm not an engineer. My guess is someone trying to get more output?
     

     

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  2. hopjohn

    hopjohn Silver Face Subscriber

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    I had to do some serious sleuthing to find an answer for you so I hope you appreciate the response. Here goes..if you look at the third photo of post #9 in this thread you will see that Qe25 is missing. A bit too coincidental to be a random thing so I would suggest to you it was a running change from Kenwood. I couldn't tell you why, but it seems legit.
     
    restorer-john likes this.
  3. restorer-john

    restorer-john Addicted Member

    Messages:
    6,430
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    Well done. I hereby bestow upon you, the official Amateur Sleuth badge.

    sleuth.JPG

    :)
     
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  4. hopjohn

    hopjohn Silver Face Subscriber

    Messages:
    1,095
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    Dallas, TX
    Excellent. I'll have to add that to my "ego wall".
     
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  5. restorer-john

    restorer-john Addicted Member

    Messages:
    6,430
    Location:
    Australia
    I must be out of touch, I'd never heard of an 'ego wall'.

    In Australia, sadly, we have the 'tall poppy syndrome' and an 'ego wall' simply wouldn't fly. :)
     
  6. tusntuk2

    tusntuk2 AK Subscriber Subscriber

    Messages:
    188
    Location:
    Arkansas, USA
    Well done hopjohn! In both cases the amp was described as working, but in one of them there was lots of heat damage. It was very curious a second amp turned up the same way. I decided to rebuild the power supply anyway, adding the parts back per original schematic, so I'll post how it goes. Thanks for the effort! ("tall poppy syndrome - indeed! Love it!)
     
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  7. tusntuk2

    tusntuk2 AK Subscriber Subscriber

    Messages:
    188
    Location:
    Arkansas, USA
  8. slimecity

    slimecity Super Member

    Messages:
    3,006
    Location:
    New Zealand
    We have that in NZ too, maaaaaaaaaaaayyyyyyyyyte.
     
  9. tusntuk2

    tusntuk2 AK Subscriber Subscriber

    Messages:
    188
    Location:
    Arkansas, USA
    Successful reinstallation of PS regulation circuit. Powered up with DBT, just fine. With full AC DC offset was surprising low, at 11mv and 6mv, rare for this model. Unfortunately the left bias trimmer had no effect, and L bias was steady ar 2.4mv (yes, 2 point 4) The Rt side worked, hung out around 40mv after a brief warmup so I left it until things get more serious in here. Now I'm wondering why I could not adjust the L bias. I have replacements for the trimmers and for the transistors Qe7,8, a KSA992 I believe, so I'll try that. Any clues? I'll check some more voltages, too.
     
    Last edited: Jul 26, 2018
    eizner23 likes this.
  10. llwhtt

    llwhtt Super Member

    Messages:
    1,479
    Location:
    SoCal
    Qe25 looks more like a turn on or turn off control on the regulated -24VDC supply than it does a regulator. The regulation is by Zeners De5 and 6. With the emitter tied to ground and the collector on the -24VDC supply line and NO voltage reference on the base it can't be a regulator. Now the Y10,000 question is was it deleted or added in the production run, OR was it on certain versions for certain markets?

    Craig
     
  11. sregor

    sregor AK Subscriber Subscriber

    Messages:
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    Location:
    Mass
    IMHO Qe25 is a turn off circuit. It is normally non conductive, and will only conduct if the + supply is significantly less than the negative supply, and it will pull the negative supply down. As an aside, those zeners probably get quite warm, and will be hottest with nothing connected to the supply. could see mounting them with stand off ceramic insulators... but I digress.
     

     

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  12. tusntuk2

    tusntuk2 AK Subscriber Subscriber

    Messages:
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    Location:
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    Gentleman, Thank you for this education! I lack formal electronic theory, my general understanding assumes a lot. I have rewired a 7100, which seems to have a regulation circuit, so I assumed this circuit had a similar function when I tried to describe it. A closer look shows me the grounding and lack of power to Qe25. Had I thought about it more carefully I should have realized it was inactive.

    So how does unregulated power affect other circuits and parts?
     
  13. tusntuk2

    tusntuk2 AK Subscriber Subscriber

    Messages:
    188
    Location:
    Arkansas, USA
    Steve, In my notes there is mention of replacing D5+6 with 3 watt zeners. I believe it was Echowars' suggestion, as the CZ245 was a 2 watt zener. I was replacing everything on this part of the board with stronger parts, although i am keeping the mains at 6800uf, just at 63v. I was boosting 1w resistors to 2w, 1/4w to 1/2 metal film, other diodes from .5w to 1 w. All in response to apparent heat damage. Figured it wouldn't hurt.
     
  14. mikepick

    mikepick New Member

    Messages:
    46
    I recently rebuilt a KA-3500, mine is missing Q25 and the nearby resistors as well, just as in your photo. I left it as was, the amp is sounding pretty good now with new caps and new transistors (including outputs.)
     
  15. tusntuk2

    tusntuk2 AK Subscriber Subscriber

    Messages:
    188
    Location:
    Arkansas, USA
    Saw another one for sale with this section missing, so I'm starting to believe it was a design switch. I did replace Q25 and the jumpers per the schematic, and the amp runs fine with no apparent differences. Wish I had taken more voltage readings before I did that, but this amp was rather a mess in several areas, so it never got powered up before changing a couple of things like the main caps.
     

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