Sx 850 protection

Discussion in 'Pioneer Audio' started by mifune, Feb 1, 2018.

  1. mifune

    mifune Member

    Messages:
    58
    Hi, I have a friend Sx850 which was going in protection after warm up.
    I read around and I went ahead replacing Q1, Q2, Q3, Q4 in the power amp board with matched pairs.
    The issue did not go away. So before to move to the power supply while I was at the power amp I decided to replace the electrolytic capacitors on that board.
    After power up I think I shorted something, there was smoke on the left side of the board. I think I shorted some components with the metal plate that goes between power amp board and output transistors frame. My bad!

    So now the unit is in protection.
    I checked all the output transistors and are fine, so are the resistors that got a bit toasted.
    Voltages are fine on the board except that between terminals 10 and 9 I have -26v

    Any idea about where to look next?
    Thanks!
     
  2. mifune

    mifune Member

    Messages:
    58
    No one? :)
     
  3. markthefixer

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

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    I don't know where you got your ideas for troubleshooting, or perhaps just reporting - and if just reporting, lots of useful stuff was left unsaid.
    Probably accounts for the crickets....
    Hopefully the things on the forum here haven't deteriorated that badly that basic troubleshooting can't be called up with searches.

    Standard procedure for a receiver:

    First you need to verify all the power supply voltages are OK, even when in protection.
    IF they are ok, or WHEN they are ok, then you need to read and post all voltages at the protection board, so we can decide if the protection board is operating properly, and there is a REASON why it is not activating the protection relay.
    When that is squared away, and we know the problem is in the power amp, then ALL the voltages on ALL the pins of the amplifier board need to be posted.

    Now, as to the idea of slavish attention to established procedure - where NOTHING is assumed and EVERYTHING was checked - My best friend used to be a roving troubleshooter for the Motorola Business Radio stuff - he was sent in when the locals were stumped and just wasting time trying to untangle customer problems. He was paid a LOT of money, and was ALL over the Conus... WHAT did he do? You guessed it: assumed NOTHING, checked EVERYTHING. Usually caused raised eyebrows when he stomped a supposedly intractable problem out... On rare occasions he got to send lightning bolts back at Engineering. But not very often.

    ok - and in your case:
    Pictures of "toasted resistors" would help - showing enough of the board around them to be able to identify which resistors they are.

    pins 9 and 10 of the power amp board are the left channel output (10,11) and earth (9).
    pins of the awh-059 power amp board board are numbered 1 through 30 on the bottom row and 31 through 50 on the top row.
    I can assure you that far more than the pin 10 voltage are "off".

    The amp NEEDS +/- 51.5v dc regulated, +36.5v dc regulated, +/- 45v dc UNregulated.

    I can make some assumptions that the voltage on the right channel output pin 25,26 to earth (24) is close to zero and adjustable - thus if so, the right channel is OK and the left channel is damaged.

    Changing out q1 and q3 with ksa992's is a good first step for restoration - but restoration really shouldn't start until the circuit is operating properly.
    q5 is a constant current source, while q7 is the vas transistor - where most of the voltage amplification happens.
    D5 and Vr3 set the idle current in the outputs by varying the 2.4 volts (+1.2v and -1.2v) across the drivers Q9 and Q11 for the four output (q13, q15, q17,m q19) transistors which will get +0.6v and -0.6v

    IF the 150 ohm emitter resistors R37 or /and R39 look "toasty" but read ok, it means you popped R51 and R55 or R53 and R57 which then possibly stopped R37 or R39 from continuing to burn.

    So Check (WITH ONE LEG LIFTED) ALL THE 0.5 OHM EMITTER RESISTORS for blown resistors. If you find EVEN ONE, all 4 outputs should be replaced - as I am fond of saying - they were racing to destruction and lost the race - and are waiting to self-destruct at the slightest provocation.

    The two halves of the circuit are interconnected, and thus what blows out one side usually has to travel through the OTHER side, damaging that side too.

    Quite frankly - I would probably just check ALL the resistors, and deal out a whole new "hand" of silicon.

    DO NOT piecemeal in transistors one at a time trying to fix this, with power turned on in between - that 's the FASTEST way to destroy more transistors,
    WE NEED to understand the damage, and repair it completely before applying power again.
     
    Last edited: Feb 4, 2018
  4. mifune

    mifune Member

    Messages:
    58
    I am sorry for my poor reporting.
    I did check the power supply board and the values were fine ( forgot to write them down but I will re-check when I can power on again) I did not check the protection, my bad. I assumed the issue was at the power amp.
    As for the power amp voltages, I only checked the ones that had a value in the schematics, but I understand now that I missed some.

    Today I checked and all the resistors near to the toasted ones (R53, R57, R65, R67) are they are fine:
    R65, R53, R47, R67, R57, R49, R61, R51, R45, R59, R55, R43 (if my old eyes can read those small numbers well).
    R37 AND R39 are fine too and do not look toasty.

    I removed the output transistors so I can work better on the board without breaking the fragile connections to the output transistors assembly and I checked them again. They are fine. I will apply new paste s suggested.
    On the right side I checked the emitter resistors and they are good too.

    I am attaching pictures of the area, as requested.

    You suggested to check all the resistors: do you mean all of them in the whole board (left and right)?
    Thanks for your help

    [​IMG]

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    [​IMG]
     
  5. petehall347

    petehall347 the brandy coffee man

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    looks to me like you screwed up the left channel with possible damage to protect circuit .
     
  6. WE6C

    WE6C Active Member

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    486
    Location:
    Colfax, Northern California
    I would check em all. Just pull one leg of each and check. You really can't afford not to at this point. If they all check good then the peace of mind is priceless.

    Bob
     
  7. markthefixer

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

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    I'll bet that one or more 0.5 ohm emitter resistors opened up. And that generally means that the output transistors are damaged, even if not fatally.

    71-CP5J-0.5
     
    Last edited: Feb 9, 2018
  8. mifune

    mifune Member

    Messages:
    58
    The emitter resistor all tested good.
    I can replace them for peace of mind. Same for the outputs. I think I still have some MJ21193G/MJ21194G.
    I will go ahead and test the rest of the board today and report back.
     
  9. mifune

    mifune Member

    Messages:
    58
    I checked all the resistors on the board, Then double checked and found out R65 and R67 testing 34 ohms instead of 100 ohms. They look toasty. R51 and R59 test good at 100 ohms.
    After I replace them and put back all the other resistors legs, what should I do?
     
  10. mifune

    mifune Member

    Messages:
    58
    I don't have enough MJ21194G and they seem to be back ordered where I usually buy. What do you guys suggest to use instead?
     
  11. petehall347

    petehall347 the brandy coffee man

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    i have yet to see a resistor reading much less than spec . something else is wrong if read in circuit .
     
  12. petehall347

    petehall347 the brandy coffee man

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    Location:
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    r65 r67 ought to test at 50 ohms but wont because of other stuff . not important right now anyway .
     
  13. petehall347

    petehall347 the brandy coffee man

    Messages:
    24,495
    Location:
    uk.. the middle bit

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