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Problrms with Pioneer SX-780 source.

Discussion in 'Pioneer Audio' started by ReynaldoGonz, Oct 1, 2018.

  1. ReynaldoGonz

    ReynaldoGonz New Member

    Messages:
    20
    What do you think about source flutuations? Any clue?

    [​IMG]
     

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    The TAPE MON and FUNCTION switches need a thorough cleaning.
     
    merlynski and ReynaldoGonz like this.
  3. ReynaldoGonz

    ReynaldoGonz New Member

    Messages:
    20
    In general purpose:
    what is the goal of use an electrolytic capacitor conected to chasis?
    what is the goal of a resistor conected to chasis?
    Why system have ceramic capacitor conected between main power switch terminals?? and another one ceramic capacitor in parallel before AC feeds power transformer?

    For example: I know that plug a capacitor in a diodes bridge is to improve DC signal that suplies the DC circuit.

    I would like to learn in practical way, what function does those arrays.


    Thank you.

    upload_2018-10-6_0-3-11.png
     
  4. Watthour

    Watthour Electron Rancher - JS3600

    The chassis of the unit is also the electrical ground (common) point. Many of the signal cable shields, components, and even circuit boards can connect there as a common. You will probably also discover several wire-wrap posts connected to the chassis as grounding points. The main filter caps connect there as a common.

    The capacitor installed across the power switch terminals is intended to absorb any EMF/surge which can occur when the switch contacts are opened. This can greatly prolong the life of the mechanical contacts in the switch (or on a relay) by suppressing any arcing which can result from this surge.

    The capacitor connected at the power transformer primary also helps to absorb the CEMF generated when the transformer field collapses as power is turned off, further protecting the switch and fuses.

    You may also notice a high-value resistor connected from the power input to the chassis. This is intended to provide a safety coupling between the chassis and the power supply to prevent voltage on the chassis from floating up to line potential. Since most of these units were not provided with a polarized power cord nor a safety grounding plug, this is the only method of limiting the chassis potential. Always be certain this safety resistor is in place and intact.
     
  5. ReynaldoGonz

    ReynaldoGonz New Member

    Messages:
    20
    i think it is something more, I cleaned up again those switches and checked continuty and everything is ok.

    I noticed a kind of innestability on a 13.5 VDC node, because it is on 10.4 VCD. This node looks important, it feeds Led sourse indicator that sometimes looks innestable and Q27 that is PA3004 (I don't know what is for this chip).

    In the output of a rectifier diodes bridge:
    I have 56.9 VCD in a node that should be 51.7 VCD, this node feeds coil protection relay then collector of Q26 (2SC1384), I have -58.8 VCD on a node that should be -53.5 VCD.

    The other bridge should be +40 VCD and -40 VCD but it is on 44.6 VCD and -44.6 VCD

    Something is not ok, maybe on rectifier diodes bridge.

    upload_2018-10-6_8-21-25.png
     
  6. Watthour

    Watthour Electron Rancher - JS3600

    The supply voltage to the unit may be a bit high. That would explain the higher bus voltages.

    Is this a multi-voltage unit with an input voltage selector?
     

     

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  7. ReynaldoGonz

    ReynaldoGonz New Member

    Messages:
    20
    It is a normal wall plug, even on regulated sourge for CPU had my problem with Pioneer SX-780's flutuation green led sourge. Why others Pioneer SX receivers works nice and smooth?
    But I'll keeping looking for the problem and update soon.

    Thank you
     
  8. Watthour

    Watthour Electron Rancher - JS3600

    What is the measured AC supply voltage?
     
  9. ReynaldoGonz

    ReynaldoGonz New Member

    Messages:
    20
    125 VAC
    What do you think of this glue on ceramic capacitor in parallel before main power transformer? I don't know if ceramic caps have glue, maybe that glue is for 2 big caps (8000uf that are upside in this part of the board, I think this glue could be from previous caps, when someona recap this unit maybe didn't cleaned up the glue. I must check it.

    The other glue, I don't know emerges of this electrolytic caps or previous caps and no cleaned to install these.

    (I waiting my trans and caps tester, just bought on ebay, that's why I not unsoldied yet any element).

    I have new caps, same uf and Voltage, but I saw that their size are so smaller compared to those ones that I though that is not ok to change them.
     

    Attached Files:

    Last edited: Oct 6, 2018
  10. Watthour

    Watthour Electron Rancher - JS3600

    Your photos are showing the assembly glue used to hold components in place before soldering. It is not leakage.
     
  11. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,103
    Location:
    uk.. the middle bit
    might be the pics but diode looks blackened above the 3 heat sinks
     

     

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  12. ReynaldoGonz

    ReynaldoGonz New Member

    Messages:
    20
    Yeah, I saw at 1st time and tought same thing (why black painted?, maybe some burned component beside of them on other times). Gonna check every component on energy supply section unsoldering and soldering one by one, and I don't know if could be a good idea to change them whole kit, I think are one of the most work demanded compontents of whole unit.
     
  13. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    28,103
    Location:
    uk.. the middle bit
    regular diodes can be checked in circuit .main failure rate is short . or open . 600mv or so on diode test it is good .
    no need to lift one side if testing good in circuit in most cases .
     
  14. ReynaldoGonz

    ReynaldoGonz New Member

    Messages:
    20
    It could be Q25 that is a regulate voltage transistor. It supose to be a 13.5 VCD source but I have 10.4 VCD.

    People says that this one is one of the main who fails so often on these stereo receiver units.

    Thank you

    By the way, diodes works as it shoud do.
     
  15. Watthour

    Watthour Electron Rancher - JS3600

    Q25 is controlled by D4, which is a 14V zener diode.

    Test the voltages at R1 to verify adequate voltage at the collector of Q25. R1 is the ceramic/cement resistor clipped to the main heat sink.

    Measure the voltages at all three legs of Q25, as well as Q19 an Q17, then post them here.
     
  16. rcs16

    rcs16 Super Member

    Messages:
    4,067
    Location:
    Baldwin, Ontario, Canada
    Suggest to read the schematics and measure all the voltages that they print on them (esp the power supplies), identify what does not match within 5-10% tolerances. Tabulate what is out of spec.
    Q25 regulator supplies ~13V to many places.
    A lot of times the transistor gets so hot that it creates a bad solder joint, touch up the solder joints on Q25
     

     

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  17. ReynaldoGonz

    ReynaldoGonz New Member

    Messages:
    20
    I already resolded Q25 some time ago and I'm thinkin to clean and add thermal grease and add another heat sink plate to Q25 but first fix this problem. Thank you
     
  18. ReynaldoGonz

    ReynaldoGonz New Member

    Messages:
    20
    I think I found out that the problem is R1, this happends sometimes not always, my lucky is that right now relay clicks a lot.
    R1= 54.6 VCD, but on other side was 30.9VCD and now is on 38.8 VCD and should be 25.9 VCD, but I dont know if problem is on R1 or C316.

    R1 reads innestable between 250 Ohms and 450 Ohms, but after 1 or 2 minutes at last stays on 221 Ohms.


    upload_2018-10-7_11-20-11.png
    upload_2018-10-7_11-20-42.png
    upload_2018-10-7_11-21-2.png
     
  19. Watthour

    Watthour Electron Rancher - JS3600

    The voltages you report at R1 should be acceptable. One side should be 52V, and the other side 26V. Since your reported line voltage is about 7% above the design voltage, your measured voltages should be reasonable.

    The most important thing is that there is about 26V at the collector of Q25. It should be able to regulate the 13.2V supply as designed.

    As mentioned before, we need to know the voltage at the emitter of Q25.
     
  20. ReynaldoGonz

    ReynaldoGonz New Member

    Messages:
    20
    It is 38.6 VCD, in attached draws are voltages.
     

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