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Pioneer M90 Amplifier Protection Circuit

Discussion in 'Pioneer Audio' started by IndyAudio, Feb 1, 2015.

  1. IndyAudio

    IndyAudio Active Member

    Messages:
    145
    Location:
    Indianapolis, Indiana
    Last year I picked up a beautiful M90 amplifier that is fully functional, excellent cosmetics and is the heart of my second system. I've used it on a number of different speakers with various loads from 4 - 8 and various efficiency ratings, but nothing like Klipsch, etc that in the mid/high 90's or low 100's db. At low and moderate levels the amp works and sounds wonderful.

    At higher levels, the amp will go briefly into protection mode when pushed reasonably hard. It seems like this occurs way too soon as the spl is not that loud and for a 200 wpc amp, it should not be kicking in like that so soon. The power protection turns from yellow to red briefly cutting the power and then it returns. The dual red power LED meters are usually near the top of the scale by then or hitting it on peaks. I'm assuming this protection is exactly how it should function, but seems to be coming on too early.

    I've read in a few other threads here about the protection relays taking a longer time to kick on/off when the amp is turned on. Mine come on quickly, consistently when I turn on the power amp, so no issue there. I've read here about the need to touch up solder joints when the amp won't come out of protection mode. Is this something that's affecting mine tripping so soon?

    Any thoughts on what I'm experiencing? Thoughts on what might need to be remedied so I can fully experience the glory of this wonderful amp at much higher volumes? I love the looks and it's definitely a keeper. Thanks in advance guys!
     

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  2. hennessy

    hennessy Active Member

    Messages:
    335
    Location:
    Lemont, IL
    Nice amp, and the door is not broken. There is a probability that due to the age some components have come to the end of their life, therefore some overhaul is needed.
    Please, measure the DC offset at output terminals after the amp has heated up. It is controlled by a chip, so no adjustments are possible. Should be as close to 0mV as possible.
    Would be cool if you could get voltage values off the PA0016 chip. Especially between pins 11 - 12 and 10 – 11. See the service manual. For proper idle current at outputs the values are -24mV and +24mV. The difference would reflect the DC offset.
    A common problem is the pre-drivers running too hot. Please see the threads on this forum concerning the replacements.
    Member rlovison found out about the bulletin:
    http://www.audiokarma.org/forums/showthread.php?t=465443
    Here is my thread about a rebuild:
    http://www.audiokarma.org/forums/showthread.php?t=477685

    In my case I suspect one of aforementioned pre-drivers failed, taking out the PA0016, drivers, outputs, and surrounding resistors.
     
  3. rcs16

    rcs16 Super Member

    Messages:
    4,087
    Location:
    Baldwin, Ontario, Canada
    Nice amp!!
    It is always wise to first check/adj per the service manual.
    Without doing some measurements, it is hard to say if it is normal operation or a ckt malfunction.
    Testing out the power-amp/protection circuits using speaker loads is not a good idea. I suggest to use a very large R load for these tests.
    The protection circuit is a std Pioneer ckt, it monitors the v drop across the 4 o/p bjts (npn) and sums them into the wein bridge over current detectors q127,128
    q138-141 act as the over voltage or DCV offset detectors.
    What is that 4/8 ohm tap selector switch (S303) set for?

    Good advise from "hennessy". I do not have a complete schematic!! it cuts off at the OPS.
     
    Last edited: Feb 1, 2015
  4. IndyAudio

    IndyAudio Active Member

    Messages:
    145
    Location:
    Indianapolis, Indiana
    Hennessy, thanks very much for the detailed reply! I had read your posts and you certainly know these amps well. Thanks for the link to your complete overhaul too. I must admit a good deal of what you note is well beyond my pay grade:) But great info for my trusted tech.

    Previously I had the amp working on the CD Direct inputs earlier to use the front panel volume control. I just changed it to Line In. With a single pair of 8 ohm Snell Type E's and the impedance switch in the up position, it's getting the job done. I may have been overdriving the amp with an EQ in line and heavy low bass boost. Running the Snells with tone straight up, it's sounding very nice, good tight bass and gets plenty loud with meters not getting close to clipping.

    I have a multimeter and if you'd provide a couple basic steps to check the DC offset at the output terminals, I am sure I'm up to that! Thanks again!
     
  5. hennessy

    hennessy Active Member

    Messages:
    335
    Location:
    Lemont, IL
    You can use M-91 schematic. It is almost the same. Myself have the paper one.
    Anyway, there is nothing to adjust there :mad:
    PD0016 sets the DC offset in an amp in good repair otherwise. Those chips are not the best solution IMHO. I used to change the DC at the outputs by flipping the chips. The DC varied +/- 8mV. Although it is not that bad, I prefer a Bourns multiturn and a screwdriver.
     
  6. hennessy

    hennessy Active Member

    Messages:
    335
    Location:
    Lemont, IL
    Switch the amp on, let it simmer for a while, or keep playing for up to 1 hour. Disconnect all interconnects, speaker wires. Put it into Control amp mode. Put the speaker switch into A. Take a multimeter. Put it into DC voltage mode. Red probe to Speaker +. Black probe to Speaker -. Note the reading. Repeat the other channel. It should be in mV range as close to 0. Usually it is 2-5 mV in PA0016 bias chip controlled amp.
     

     

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  7. IndyAudio

    IndyAudio Active Member

    Messages:
    145
    Location:
    Indianapolis, Indiana
    Excellent, will do after the game is over and guests are gone. Let you know what the readings look like, thanks again!
     
  8. luridbeatle

    luridbeatle New Member

    Messages:
    25
    Hello everyone, first of all forgive my English ...
    I reopen this old conversation hoping to find help!
    I have a M-90 for a long time and yesterday decided to go in "protection"! I have no components visually broken, I tried the relays and they work... the MOSFETs heat, the source selection works...
    When I turn on the M-90 the source relay closes and then close the two Osram condenser relays but then ... nothing!
    I measured the voltages on both PA0016 and are indicatively correct (24mV + - 2.5mV)
    Thanks for the suggestions, I do not know what to do, I only have a tester, a soldering iron and a lot of good will !!
     
  9. hennessy

    hennessy Active Member

    Messages:
    335
    Location:
    Lemont, IL
    What are voltages on PA0016? Pins 8, 9, 11, 13, 14 to ground DC. What are rail voltages? Where did you find the MOSFETs there?
     
  10. luridbeatle

    luridbeatle New Member

    Messages:
    25
    Sorry ... Power Transistors... I'm a little confused ... I
    I'm a little confused ... vague looking for information and are tilting !!
    Now I measure the tensions!
     
  11. luridbeatle

    luridbeatle New Member

    Messages:
    25
    So, I answer you but something strange happened ...

    each ground pin (pin121 on awz1025):
    8 --> 3 V
    9 --> 1,68 V
    11 --> 7mV
    13 --> 1,7 V
    14 --> 3,06 V

    each pin on the ground (pin147 on awz1026):
    8 --> 22,4 V
    9 --> 23,6 V
    11 --> 25,4 V
    13 --> 27,4 V
    14 --> 29,3 V

    After a few moments, everything went with a value of 0, the amplifier came out of the protection!
    Now I turned it off, I wait half an hour to repeat the operation ... It's all soo strange!
    That there is to change some condenser?
     

     

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  12. luridbeatle

    luridbeatle New Member

    Messages:
    25
  13. luridbeatle

    luridbeatle New Member

    Messages:
    25
    good morning, this morning I turned on the amplifier again ... it went into operation!
    4-5 seconds and the Red light change in yellow.
     
  14. luridbeatle

    luridbeatle New Member

    Messages:
    25
    and now what do I do?
     
  15. redk9258

    redk9258 AK Subscriber Subscriber

    Messages:
    1,598
    Location:
    Illinois, close to St. Louis.
    Don't know if it is your problem, but someone has been in there doing half-assed work. Look at the wire wraps. How could that be expected to work correctly?
    [​IMG]
     
  16. luridbeatle

    luridbeatle New Member

    Messages:
    25
    Hi, do you think of a false contact? can I solder? but the defect presented itself suddenly, without touching anything!
     

     

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  17. hennessy

    hennessy Active Member

    Messages:
    335
    Location:
    Lemont, IL
    Yes, can be a failed solder joint, among any other things. The board runs hot over all, therefore full overhaul is recommended.
     
  18. luridbeatle

    luridbeatle New Member

    Messages:
    25
    Good evening (in Italy), small update:
    - the amplifier is no longer in protection;
    - I replaced the pairs of a1145 and c2705
    - all relays click
    - I replaced 7 capacitors (c106,132,134,136,138,149,150) on board awz1026
    - I redid all the welds, but ... I have new problems !!

    No amplification, the right channel heatsink is cold and the vu-meter does not work, it's completely off (without backlighting).

    I found that on the 137-139 connector (red and blue cable) I have 0.0V while on 101-103 (red and blue cable on awz1025) I have 136.5V ...

    If I use the headphones, I hear music on the left and right channel

    And now??
     
  19. hennessy

    hennessy Active Member

    Messages:
    335
    Location:
    Lemont, IL
    Assuming you did the work properly, check RY 401.
     
  20. luridbeatle

    luridbeatle New Member

    Messages:
    25
    Verified RY401, there was oxide on a contact, now it's perfect ...
    The amplifier returns to protection mode with the backlight... then I replace Q124 and 126 ... the FU2 fuse blows! I replace the fuse, I turn on the amplifier and in a second, the resistors R192 and 216 burn ... they are those on Power Amp D. I found two shorted C3907 transistors.
    Tomorrow I replace the components but the question is: "are they the problem or are they victims?"
     

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