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Restoring SX-3800, my first - a little background and a couple questions

Discussion in 'Pioneer Audio' started by John Hale, Feb 28, 2017.

  1. John Hale

    John Hale Member

    Mark - I do not fully know your situation. I only caught glipmses as a read through the forum. I can only say that you are educating me and others in a way that probably only you can do. For that I am grateful.

    I am not on a DBT, however I am on a BK precision 1653 (isolation transformer and Variac in one) but limited to 3 Amps. Will check with straight line voltage from what you said above.

    The voltages I reported are from my Oscilloscope (Rhode and Schwarz 1202) It is set on dc volts but also has a built in DMM that can show the DC portion along with the AC portion. The waveform also shows the AC "pulses". My DMMS are Fluke 89v (2), Fluke 289, Brymen 869, Keithly 2015.

    sorry for our time disconnect but I live in Germany right now.

    Again your and all other inputs are greatly appreciated - I am learning

    Best to you


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  2. markthefixer

    markthefixer On Hiatus, dealing with Dad's estate full time Subscriber

    no sweat, nice to know what the data is coming from. Good stuff.

    When I say "guys" I am addressing ( inviting ) the gallery - the crowd - the spectators (as such) , which has quite a few quite adept people that can follow the map I laid out and walk you through it, doing a lot of typing, link finding and such that I just cannot give justice to at the moment.
    Time offsets across the planet, no problem, I'm used to it. the thing to resist is blasting through this with action and not a lot of thought on the whats and whys as one tries to answer the posts too quickly. MY side as well as yours.
  3. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    USAF John,

    Bitte ein Bit, fur Mich Bitte!

    Und Vielicht, that is maybe some are wondering where these .6V (.7V)
    keep cropping up from?

    We must remember that the PN junctions take that much voltage before they conduct.
    It's the same number our meters display when checking a silicone diode...about .6V.
    The diode is one PN Junction.

    A Transistor, such as a BJT for instance, has two junctions, either NpN or PnP.
    They will also take .6V for each junction to conduct. In a BJT, one junction is forward biased
    and the other junction is reversed bias. each junction still has to over come .6V to conduct.

    Or when it is conducting...we still have to account for the .6V.

    Alles klau?

    Last edited: Mar 18, 2017
  4. John Hale

    John Hale Member

    Thanks Spike! I took a little break to let my thoughts clear-- will be back on this tomorrow morning.

    BTW - I live in Bitburg -- Home of Bitburger

    Veilen Danke
    Ja Alles klau

  5. USMC Spike

    USMC Spike Active Member

    Tejas, North Central
    Mein Lieber Gott!

    I am so jealous!


    Mit Vergnuegen,

  6. John Hale

    John Hale Member

    Gang - I am stuck. I can't even get passed the power supply. So to recap (pun intended) . ALL components have been checked and rechecked. New transistors, new diodes, new electrolytics. I have it attached to inputs from the transformer and ground from pin 19 and33.
    mains voltage 120.9 VAC
    Input voltages
    pin 1 - 52.5, -23 VDC
    pin 2 - 52.5, -24VDC
    pin 3 - -24 VDC, 12.6 VAC
    pin4 17.5 VAC, 2-5 VAC
    pin 5 11.5 VAC, 2-5 VDC

    All above VDC are jumpy

    Pin 26 35.2 VAC
    Pin 27 35 VAC
    pin 28 - 0

    Output voltages (target voltage)
    pin 35 48.2 VDC (45.5)
    pin 29 48.1 VDC (45.5)

    pin18- - -39VDC (-47)
    pin20- 39 (47)
    pi22- 30.29 (32)

    Chassis ground as reference
    D204- -96.9 VDC, (-62.5)
    D201- 45 VDC (62.5)
    d206- 21 VDC (21)

    Q210 E- 1.2 VDC (5.5) not too worried here, I beleive it must have feedback from other curcuits to activate
    B - .6 VDC
    C- 21 VDC (20)
    Q209 E- 14.5 (12.8)

    Q205 E- -39.1 VDC (-47)
    B- -29.5VDC
    C- -100.7 VDC
    Q201 E- 38.9 VDC
    B- 39 VDC
    C- 39.7 VDC
    Q203 E- 14.55 VDC (14)
    B - 14.2 VDC
    C - 38.7 VDC
    Q208 E - -27.4 VDC (-32)
    B - -27.6 VDC
    C- -37.9 VDC
    Q204 E- 27.6 VDC (32)
    B- 27.6
    C- 38.9
    Q206 D - -32.3
    center pin - -100.13
    other outside pin - -100.4
    Q209 E 14.5 VDC (12.8)
    B - 13.98 VDC
    C 16.4 VDC (20)
    D207 upper leg - 14.4 VDC

    The junction between R213 and R212 is at -0.6v.

    I re- pulled components and checked again. Negative rail seems way High (low however you look at it), Maybe it is pulling down positive rail? I don't have any other boards connected will they influence readings?? Very little current drw from mains, as expected.

    Guys and gals -- I am stuck and don't have enough knowledge to know where to focus efforts on next. I suspected a diode proble because of the high (low) negative rail but they all check good (probably the fourth time I have checked those. SOrry I feel frustrated as I am stuck on step one of this project -- but I will not be defeated. But I need a wizard to point me the right way.
    As always thank you!


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  7. USMC Spike

    USMC Spike Active Member

    Tejas, North Central

    My bad, mis-spoke there about transistor. The reversed biased junction
    must overcome a lot more than .6V, because it has a lot higher resistance.
    Actually, it would be the reverse breakdown voltage in BJTs.

    Now that that is out of the way.

    USAF John, get the data sheets out and check each transistor you replaced.
    Also check that the board isn't mis-printed. Did you take pics before the

    Check the polarity of the caps compared to the transistors and diodes.

    I think they need some sort of load. BUT they are not negative enough.
    Post Pics if someone has one of these it will help you out.

    I don't have same amp and haven't look at schematic.
  8. John Hale

    John Hale Member

    Thanks Spike - I do have pics-- maybe that I'm stuck in the mud , I'll figure how to post them.
  9. John Hale

    John Hale Member

    Hello Gang! - well it's been awhile. I was very stuck with this power supply. Oddly enough TWO SK34's were bad (yikes). anyway I bought another board from flea-bay. it was working but ugly. it was very useful to be able to compare working vs non-working.

    I did some part switching but could never get the original board to work. Although the SK34's were bad swapping still did not make the unit work. the closest I can figure is the Q 201wouId not pass current and never turn on Q 203 (or visaversa) . I was able to really get into the schematic and eliminate working areas from non. quite a bit of learning accomplished. BUT in the end I switched new parts from the original to the working (e-bay) board one at a time - testing each and every component swap. SO at this point I have a "functional" power supply (8 months later. OK i admit ,I took the summer off from this)

    I say "functional " in quotes cause output power is slightly low (like 44 V on the 47 V pin). Voltage at D201 is slightly low,too (50 ish V vs spec 62.5 V) . I haven't switched over diodes yet but that is next. Not sure if these differences will be fixed by the swap.

    MTF's long post above was my constant companion as I went through this - I feel like I almost understand it.

    I'm pretty exited about progress (however slow)

    I did find a source for SK34's (i ordered 4, just in case)


    Hopfully more successfull posts , as we go along


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