Yamaha RX-397 DC protection?

Discussion in 'Solid State' started by patrick_l, Nov 10, 2017.

  1. patrick_l

    patrick_l Active Member

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    142
    Few days ago, i've been trying to repair this receiver I bought in non-working order. The thing is when you try to switch on, power relay clicks, CD/DVD appear on the display, and that lasts for about 3 seconds, and device shuts itself off. Already got service manual, and tried to enter diagnostic mode, and in that condition I discovered there is DC protection activated. But, there was no dangerous voltage on speaker terminals, what would make sense in this case, but voltage is normal. Checked all voltages on output transistors, everything seems to be ok, and even i hear sound if i put headphones. It is possible in diag mode to switch on and off speaker relays. Didn't try to put speakers, but my guess is I would get sound without any problems.
    As I figured out, DC protection is achieved when there is voltage outside normal on pin 91 of microprocessor.
    Value that is written on display is DC 000, which would suggest there is no voltage at all, at pin 91 (signal called PRD, DC protection). I should have 1.7V at that pin, and I have 0V. On schematics, I saw there is one capacitor connected to that PRD signal line, and it is C147 100uF/6.3V. I don't have voltage drop on that capacitor. Even, there is very small resistance across terminals, around 13 or 14ohm, so I thought capacitor is not ok, but turns out, when I wanted to replace it, suddenly I couldn't measure that low resistance across terminals any more, but anyway i replaced it, to know for sure that it is not the problem.
    Then I realized maybe something else making that low resistance across terminals, maybe some other board, so i tried to disconnect one, by one board from the main one, to see if that would make a change, and it didn't.
    And it was second time to disassemble everything, and first time i had -0.6V on positive terminal of capacitor C147, but later there is only 0V. Checked a lot of transistors in that area, didn't notice single one with problem.
    What could possibly go wrong if i can get sound in diag mode?
     

     

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  2. mbz

    mbz Super Member

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    The microprocessor controlled units are rather complicated and the reasons for protection sometimes obscure.
    In the case of DC 000 I think this means the unit was expecting to see a voltage "somewhere" but saw an
    abnormal value, 0V in this case. It normally suggests a problem in the power supply, typically a voltage regulator.
    Will try and find SM and add more.
     
  3. mbz

    mbz Super Member

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    Location:
    Melbourne, Victoria
    Not much extra help. Suggest checking each of the voltages from voltage regulation
    ie, +5BU, +5M, +5S, +5T,,,
    You will need to bypass protection using the diag menu.

    Things to check
    D602 zener for pin 91
    Stressed caps in voltage regulator circuit
     
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  4. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    Theres no voltage because its in protect mode which disconnects the amplifiers output from the speaker binding posts via protect relay. In other words you must measure for your DC voltage (DC offset) at some point before the relay contacts.
     
  5. patrick_l

    patrick_l Active Member

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    142
    Good news! After intervention, receiver works as it should. Before I even saw replies here, I was thinking about exactly the same thing, to check supply voltages, especially +5S, since it feeds that zener diodes, that are connected to pin 90 (PRV) and pin 91 (PRD). Before, when I discovered that signal PRD on one connector, voltage was 0V, and for PRV there was around 1.6V. So I knew I have voltage in supply, but seemed that something was still missing. Checked all voltages, and everything was ok. So my next step was to find diode D602 on board, since it is the only what is left to consider. Problem was, I needed to disassemble everything, to be able to check SMD diode D602, and it is shorted. Resistance is the same I measured across capacitor, on the other board, around 13 ohm. Since I didn't have tools and proper elements for work with SMD components, I managed to desolder diode with regular soldering iron, and I had "usual" zener diode, with same parameters (5.1V) and soldered it. After that receiver finally went out of protection, and I've been testing it for 2 hours without any problems. Everything works as it should.
    Thank you very much, for precise directions to solve this mistery. I appreciate it!
     
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  6. patrick_l

    patrick_l Active Member

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    142
    I know it sound strange, but I managed to enter diagnostic mode, in which protection is disabled, exactly for the reason of troubleshooting, and I wrote in that mode I can actually switch speaker relays on/off, and it works, but microprocessor interpreted it wrong, because of shorted zener diode near pin that is used for sensing DC voltage at speaker terminals, so IC interpreted that as "dangerous DC voltage across speaker terminals", and that was completely untrue. For some older amplifiers, without use of microprocessor, I know speaker relay wouldn't engage if there is actual DC voltage at speaker terminals. This problem would never exist in old-fashioned amp, because of the absence of advanced ICs, that monitor state of the amplifier, and if their "sensors" go wrong, it may appear as a broken final stage of the amp.
     
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  7. mbz

    mbz Super Member

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    Good work, you fixed it by some good observation, measurement and thinking...
     
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  8. avionic

    avionic " Black Knights " Eagle Keeper Subscriber

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    :rolleyes: Never mind my babbling. :biggrin: Bravo on your success .:thumbsup::thumbsup::thumbsup:
     
    Last edited: Nov 10, 2017
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  9. patrick_l

    patrick_l Active Member

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    142
    Thank you very much. I've been testing it since repairing, no problems whatsoever. Amazing sound, and I am really satisfied. Quality phono section!
    Also noticed, that unit doesn't warm up at all :). Tested idling current, and everything is perfect by specification. I totally recommend this receiver!
    Guys, thanks again for participation! :)
     

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