Guitar pedal board shuts-off after a few seconds off bootup

Discussion in 'Technical Repair' started by Yami, Jun 10, 2019.

  1. Yami

    Thread Starter Member

    Jan 18, 2016
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    3
    Hi guys, I've got a guitar effects pedal board in for repair. Basically what happens is when the power switch is pressed, the boot-up sequence starts and then quickly turns off. The switch doesn't turn on the power supply but it turns on the processor(the switch doesn't latch). When the mains is connected to the device there always is standby power.
    Anyway first I thought it was something to do with the power supply - it checked out ok. No hiccups whats so ever, However I found out the following - there is a voltage regulator on the main board. I measured the output of the regulator and it measures 12V, but one the power on switch is pressed the voltage goes down until the device turns off and then the voltage at the regulator output climbs back to 12V. I figured there was a short somewhere on the main board, how I came to that conclusion - I removed all the wires going from the main board to other boards and still get the same behavior when the power switch is pressed.
    The model of the pedal board is very new so there are no service manual/schematic. So tracing where the problem could be is very hard. However I came across a potential breakthrough, I noticed some corrosion at the USB chassis pin. I measured resistance between pins and noticed that between D+ and Gnd pin I got a reading of 4.6 ohms. That seems a bit odd, could this be the issue?
    Any help on the issue will greatly appreciated.
    Thanks in advance.
    Headrush 2.jpg
    Headrush 1.jpg
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    When it's powered up what is the voltage from the regulator before and after reset, and the voltage at the usb?
     
  3. Yami

    Thread Starter Member

    Jan 18, 2016
    196
    3
    The output of the power supply is just above 12.8V unregulated , The regulator is on the main board its input is the 12.8V and at the output of the regulator its 11.98V. Cannot measure anything at the USB,the processor doesn't stay on enough time for me to meaure it. Only the front leds lights up and then the unit turns off.
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Does the regulator have a type marking?
     
  5. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    So does the regulator stay at 12V when booting up?

    Can you measure the USB 5V rail ,keep your dvm on the USB 5V then boot it up.
     
  6. Yami

    Thread Starter Member

    Jan 18, 2016
    196
    3
    @Alec_t the marking on the regulator is:
    AH02AD
    FDD
    4685
    That's the way its written on the package, I couldn't find any datasheet that indicates that its actually a voltage regulator. But from following the traces it looks and acts as a voltage regulator.

    @Dodgydave the regulator's output goes low as soon as the power button is pressed, it goes down to about 3V and comes back up when the device turns off by itself.
    I measured the USB 5V rail and there was no voltage there before or during boot up.
     
  7. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    So the 12V rail is going down to 3V, it's taking over 1.5Amp at reset due to some chips being fried..it could be any of the buck converter regulators for the 3 and 5V rails, or the processor.
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Microchip do a MIC4685 3A switch-mode buck regulator and ONSemi do a NCP4685 2.5V 150mA low dropout linear regulator. Does either seem a likely candidate?
     
  9. Yami

    Thread Starter Member

    Jan 18, 2016
    196
    3
    Just an update, I tried removing the USB connector but failed to do so - just too damn hard. I checked again with the power on to see if it had any effect (due to the heating of the board) now the voltage at the output of the regulator doesn't go down slowly but the moment the switch is pressed it goes down to 0V instantly and climbs back up when the device turns off on its own.

    @Dodgydave I would like to know how you calculated 1.5A? Thanks
     
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