Onkyo TX-NR525 clicking on & off

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  • Khron
    Badcaps Legend
    • Sep 2006
    • 1350
    • Finland

    #1

    Onkyo TX-NR525 clicking on & off

    A couple weeks ago, i had one of these AVR's brought in; fault description = totally dead.

    Took the cover off (coveniently missing screws), and after a quick visual inspection, noticed a glaring issue - see attached photo no.1. Yep, great big honkin' crater in the MCU.

    "Obviously" no chance of finding one of those on its own, and even if i did, i couldn't know if/what else was blown on the same board. Somewhat fortunately, i found a same model board from a seller in the US on fleabay:

    http://www.ebay.co.uk/itm/2626112563...%3AMEBIDX%3AIT

    As can be seen, sold as untested. Then again, nothing with craters in it, so at the very least, it's in better physical shape than the original one. I explained that to my client, and that i'd charge him for the cost of the board regardless of whether it works or not, and he agreed.

    As a side-note, i like how the faceplate of the thing had started to melt (see attached photo no.2). I'm not sure i wanna know how cramped & hot this thing ran, when it still did...

    It finally arrived yesterday. I install it, hook up all the connectors, and that's where the click on / click off thing began. After a few of those cycles, a puff of smoke from somewhere inside the depths of the thing, came out. That turned out to be a crapped out 7915 regulator (measured about 68 ohms input-to-output and vice-versa).

    Didn't have any 7915's in my parts bin, so i "MacGyver'd" one with a 7912 with a LED and a diode in series with its ground pin (photo no.3) - this results in a nice -14.5v on the output - that's within 5% of the 14.9v that comes out of thr 7815 next to it, so i'm calling that "close enough" That didn't help much though, but at least the dropper resistor "upstream" of it stopped smoking

    While i had the main board removed from the case, i noticed a "skidmark" on the inside of the case (photo no.4), as well as what had caused it. The 47k resistor between the AUX_L input and ground was toast, and the associated ground trace had been literally blown clean off the board (photo no.5).

    During the testing and the "automatic" on/off cycles, i noticed the HDMI chip on the "new" board was getting unreasonably hot, in a big hurry. Turned out the 3.3v AVDD rail for the HDMI chip is shorted to ground. That's not the case on the original board (with the "well ventilated" MCU), so there's still hope for that one Either way, all the series ferrites for the 3.3v HDMI supplies are removed for the moment, to eliminate that variable.

    Then i decided to remove the MCU board altogether, and force the mains relay on (wire jumper between the MPOWERD pin and ground, on the wire harness coming from the stand-by PSU) - see pages 40-41 in the attached service manual. After a few seconds, a wisp of smoke came out from somewhere on the main board once again, but from a different place.

    I removed the "mid board" (containing the speaker relays & binding posts and some analog I/O), and forced the thing on again. No more smoke, but i noticed some 1/2w resistors getting quite hot. These turned out to be the dropper resistors upstream of a pair of 6.8v zeners, which form the local regulation for Q4001, the audio mux / volume control chip - a Renesas R2A15218FP. Measurements showed the outputs of those to be -3.3v / +2.5v respectively. Safe to assume that's toast as well

    Luckily those rails go through two wire jumpers on the top of the board (i haven't removed it yet again from the case). After snipping those two apart there's no more heat, and the 7912's LED doesn't wink out in-between the on/off cycles, with the MCU board reconnected.

    So, as it stands:
    - the 3.3v HDMI supplies are disconnected (pending chip swap)
    - the +/-6.8v supplies to the R2A15218FP chip are disconnected (chip's toasted)
    - the MCU "still" cycles the power on/off when connected
    - the thermal fuse inside the power transformer is perfectly fine (getting about 6 ohm across the primary both before and after the fuse)

    I don't suppose anyone has any idea of some "checklist" of causes which might make the MCU turn the main relay on & off, right?
    Attached Files
    Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans
  • Khron
    Badcaps Legend
    • Sep 2006
    • 1350
    • Finland

    #2
    Re: Onkyo TX-NR525 clicking on & off

    I took another closer look at the original MCU board from this thing, and i noticed that the edge of the die is visible through the "vent hole" in the MCU.

    Now, on a tqfp, it's quite logical that they laid out the chip so the bond wires could go our radially, in order, right? So i took a blind shot and assumed that the blown up area is pretty much "in line" with whatever pins / signals are at the edge of the chip.

    Lo and behold, a 4-resistor array no longer has any visible markings - R638, connected to pins 117-120 of the MCU. I'd call that bingo!

    That used to be a 4x 47ohm resistor pack, going out to signals MIC_MUT, /MICDET (/ = active low), LEDTHRU and LEDPURE. The first three are blown wide open; the last one isn't.

    Now, coupling this with that blown ground trace associated to the AUX_L input, i'm starting to get a hunch that something went very wrong in the I/O area on the front panel. Is it a coincidence that the "AUX" input as well as the mic input are in vicinity of each other? They're even next to each other on the schematic (page 39, right edge)...

    http://www.eu.onkyo.com/assets/2/4/8...9x9999.png.jpg

    On one hand, i'm growing increasingly tempted to transplant the MCU from the "new" board (the ebay one with the shorted HDMI chip) to the original board. The MCU's 3.3v rail isn't shorted to ground.

    On the other hand, i definitely gotta remove the pcb('s) from the front panel and see if there's any visible damage there...
    Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

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    • Khron
      Badcaps Legend
      • Sep 2006
      • 1350
      • Finland

      #3
      Re: Onkyo TX-NR525 clicking on & off

      Sure enough, there are some indications of "foul play" on the front panel as well, as i was starting to suspect.

      The sooty RCA is the AUX_L - photo 1.

      In photo 2, yet again the ground trace for the AUX input is blown to high hell.
      I see the MICMUT signal is preeeeetty near to said ground trace.

      What're the odds that, whatever scorched the RCA socket and blew that ground trace, just arc'ed over to the MICMUT line, which goes straight into the MCU?
      Last edited by Khron; 10-18-2016, 04:09 AM.
      Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

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      • Khron
        Badcaps Legend
        • Sep 2006
        • 1350
        • Finland

        #4
        Re: Onkyo TX-NR525 clicking on & off

        Oops, seems i forgot to actually ATTACH the photos

        Now that i took another look at the 2nd photo, i just noticed that the -15v pin's in that same area, and the 7915, as you can read in the first post, was also blown.

        Coincidence?..
        Attached Files
        Last edited by Khron; 10-18-2016, 09:26 AM.
        Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

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