Restoring SX-3800 - From Power supply , moving to Amp section

Discussion in 'Pioneer Audio' started by John Hale, Nov 14, 2017.

  1. John Hale

    John Hale New Member

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    Hello all - I am declaring victory on the pwer supply (see thread " Restoring SX 3800... I have a few questions"). I thought I should start another thread so readers could differentiate the lines of effort. I have re-transistored (if that is a thing), re-capped and tested ALL resistors and diodes with one leg lifted. I have connected the Amp board to power supply (amp pins 1,2,6,7 and the plug ins at 29,30,31 and 35,36,37). I did NOT attach final output transistors.

    Status: brought up on variac. no explosions or Hot components (yippee). All the higher voltage test points (47, 45.7, -45.7, etc... test pretty well. However all the low voltage point are not correct :

    Q104 = - 8ish , should be 0
    Q104= -16ish, should be .7
    Also mirrored on the other side

    And all other subsequent Lower voltage points all reading in the negative teens rather than the required .7 or 1.2 volts.

    Can you give me a starting point? turning VR1 and VR2 potts had influence on Q104 but in no way could overcome that much variance.

    All input voltages from PS are within spec.

    I appreciate ya'll
    John
     
  2. John Hale

    John Hale New Member

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    Any takers?
     
  3. John Hale

    John Hale New Member

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    45
    well folks - I am at my wits end. I was real optimistic when I got the power supply running, but the amp is driving ( ha get it) me crazy. I continue to chase some negative voltage coming from somewhere ( hate to be so vague) but i have traced the schematics , tested seemingly hundreds of components.... And I cannot find a "smoking gun". seems like I truck down a path, just seeing if I can make an impact on this -11V to -16V reading on all the places where ther should be + .7 v or +1.2V. The relay never kicks in (obviously).

    Please help ( and thank you in advance) . I have tried to narrow things but , nothing seems to effect the readings. I have worked for two weeks on this, I'm not the wicked smartest but I am no dummy either. I am going to give this one more day, then I'm going to pack it up and move on to a Marantz re-build.

    I feel terrible about my progress (or lack thereof) but I might have to get allot smarter before I can concur this NSA amp.

    I don't think I keyed any interest in a reply thus far, I would be happy to elaborate on details if that would generate some interest.

    Thank You
    John
     
  4. Dingman

    Dingman Do you know where your towel is? Subscriber

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    Dang, usually there's help for Pioneers. I can't help, sorry. Hitting a wall like that can be quite discouraging.
     
  5. John Hale

    John Hale New Member

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    45
    Woo HOO! I FINALLY figured it. But wait ya'll gtta at least feel a little of my pain. I took the week of Thanksgiving off-- hoping to finish up this SX-3800 and make a good splash into my Marantz 2265b... but that was not to happen. I was a wisker from giving up on the 3800 but it kept sucking me in by just minute progress. I checked , rechecked, EVERYTHING except one -- I'll get to that in the end. To reveiw a little the problem was High voltage (+40ish and -40ish)where I should have been getting low (.7, -.7, 1.2) at the base of certain resistors. Obviously this turned them all on to Blast and was getting 42 V , -42 V out the speaker terminals with no input (bad stuff). SO the long journey of checking all transistors, all ECB, BCE against the board, the drawing (there is a mistake in the drawing for Q123, the emiter should be toward the signal,pwer supply terminals -top of board) and the schematic. SO here it is (drum roll) the two electrolytic capacitors (which I had replace early in the process) by the book are speced C133 (470/6 ?) and C134 (100/25). I had upped the voltage rating to 10 and 35 respectively. But they had been introduced to 45 volts. I'm not going to take allot of credit for finding this I was reading in the Marantz forum and found a similar problem and they tracked it to the electrolytics that were involved with the protection circuit. With similar symtoms to my problem. I didn't think it could be that easy of course and came home and retraced checking transistors and such (yeah insanity but...) But It was on my list to check so... checked out the electorlytics--- basic tests -- they passed (Capacitance, ESR) -- but that was not what fault I was searching for so hooked the up to a power supply and dialed the voltage up-- they still held up pretty well but they are in a tough environment. I switched to higher rated caps 470 at 25V and 100 at 63V.

    I hate to say vIola but VIOLA!!!!! She came into focus low voltages where they should be -- DC on outputs adjustable, Idle current adjustable

    Sorry for rambling but I'm pretty excited ( 8 months of wrestling has come to a Milestone--- gotta go run some signal through it now... I guess the journey continues


    BTW thanks Dingman for the reponse!
     
  6. Dingman

    Dingman Do you know where your towel is? Subscriber

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    Good deal man. Crazy how it works out sometimes.
    Thanks for the details, you know that will help people in the future.
     
  7. John Hale

    John Hale New Member

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    Hey all - I think the is pretty simple one to answer. I'm trying to source the large White resistors (three legged ones) . I'm not sure how to read the code 0.47 (ohm symbol) KX2. I know there are two restors but wonder if there is a drop in replacement or needs to be two single ones if these sre out of production. I am assuming 0.47 (ohms) K means -- 470 ohms -- correct or not?

    Hope someone can help

    Thank you

    John
     
  8. Bruno Primas

    Bruno Primas Member

    Messages:
    58
    You can replace those with two .47k ceramic resistors. There should already be an extra hole in the board for that reason. I did that on a 3600 that had shorted outputs. Works fine.
     
  9. John Hale

    John Hale New Member

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    Bruno - Thanks Man- I already ordered up some regular ones. was hoping somebody had a magic place to find the original types.

    I don't see any extra holes on my unit. will do some figuring when the parts come in.

    Thanks Again
    John
     
  10. Bruno Primas

    Bruno Primas Member

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    Hmm..it would be funny if the 3600 had them but the 3800 didn't. The extra hole would be located right next to the center hole of the 3-legged resistor. (If it has one.)
     
  11. redk9258

    redk9258 Super Member

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    How about a picture of the resistor in question.
     
  12. John Hale

    John Hale New Member

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    Photo attached (I hope) -- it is .47 Ohms from each end to the middle. The Letters in the glare are RGC55


    emitter resoster photo.JPG
     
  13. Bruno Primas

    Bruno Primas Member

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    58
    I mis-typed the value of the resistor in my previous post. It's .47 ohms, 5 watts, not .47k ohms. Sorry about that. Glad you saw it correctly.
     
  14. redk9258

    redk9258 Super Member

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    I see some available on a popular auction site.
     
  15. John Hale

    John Hale New Member

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    Thanks for the help ( I looked on a site that rhymes with sheshay, I guess not supposed to say it) Came close but only .22 Ohm. Not sure where you are refering to.

    Kind of a step back on the Amp -- My rookie mistake -- I was about to do some signal testing across dummy loads and had both channels hooked up. I attached ground lead to one side of the dummy load. It cause the resistance to go down to about 1.7 ohms --- Odd it wasn't the same on the other channel. Same oscilliscope. When I change direction of the ground lead (i.e. to the neg dummy load terminal it would read 8 ohms. After I thought about it -- it was probably really stupid to attach the ground lead because the receiver only has a two prong plug all else grounds to chasis.

    Any way - blew the resistors and took out one side of power transistors , along with some 220 ohm resistors. Damn---- the other channel put out a clean 25 Volt signal @ .005% THD + N (measured on Keithly 2015). Two steps forward one step back..

    Tschuss
    John
     
  16. redk9258

    redk9258 Super Member

    Messages:
    1,318
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    Illinois, close to St. Louis.
  17. John Hale

    John Hale New Member

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    Awesome- thank you--- bought... now just have to wait for it to get to Germany
     
  18. redk9258

    redk9258 Super Member

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    1,318
    Location:
    Illinois, close to St. Louis.
    I didn't realize that you were in Germany. I guess that's why you didn't see it in your search.
     
  19. Dingman

    Dingman Do you know where your towel is? Subscriber

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    This guy can be hard to listen to, but it's worth it.
    "how not to blow up your oscilloscope".

     
  20. John Hale

    John Hale New Member

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    Yeah -- I rather like him--- I've seen just abou every video including that one. Thank you though-- I knew I made a mistake ( I knew too late though).. I'll be waiting on some parts til next week sometime, so more to follow. I'll try to get better at posting Pics. below is the current state of affairs (quite cleaned uo from just a few days ago. starting to loosly place things where they were) sorry bout the focus
     

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