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Thread: PROP_LC Hot Amp Chip

  1. #1
    Senior Member+ defragster's Avatar
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    PROP_LC Hot Amp Chip

    Connecting a new Prop_LC I discovered the AMP end chip would quickly heat to 'too hot' to touch a few seconds after powering up and pin 5 was set output HIGH.

    Using the Beta PROP shield I got - with the same wires to the same Teensy that device does not display this behavior.

    Note: Using TALKIE and the correct Teensy DAC I can hear the audio clear and true, but the AMP chip goes hot in ~10 seconds. Pulling the 5V line kills the sound and the heat. A quick check of a couple pins doesn't show any power/GND shorts. I saw this behavior the other week and set it aside. I just Brushed solder resin off with 90% isopropyl and it is much cleaner - and there may have been some spatter on the side of the '473' chip adjacent the AMP pin that is clear now. But after drying it repeats the hot behavior.

    End pins soldered underneath and pins clipped are GND/PGM/DAC and seem clear of anything that would affect this.

    Here is a pic of the topside where I put the side headers on - I don't see anything obvious:
    Click image for larger version. 

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    Anything else I can check or fix? Something obvious I did to affect it?

  2. #2
    Senior Member+ Frank B's Avatar
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    I don't know.
    But, on this picture, the 5V-pin is not connected - or is there a teensy under that board ?

  3. #3
    Senior Member+ defragster's Avatar
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    Correct - I pulled 5V after it heated and saw the heat stop - I switched off the Teensy - but it was just a red jumper to VIN from the USB.

    I laid a second new Prop_LC on that board unsoldered and pushed the same jumper wires through (including 5V) into breadboard and headphone connection and it worked with no heat - just like the working Beta Prop I have.

    The Prop_LC is bottom pinned to fit a top headered Teensy - when it was placed into those headers is when I noticed the heating. That was using an alternate Analog pin for the DAC signal and the sound was horrid - now found to be unrelated to the heating problem as on the right DAC pin for this Teensy it sounds great - but HOT.
    Last edited by defragster; 07-10-2016 at 10:07 AM.

  4. #4
    Senior Member+ defragster's Avatar
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    Paul: I ordered two of the Prop_LC's and the second is powered and working properly- like the Beta Prop_WIT does. Wondering if there was something I did to put the AMP into the high heat state or if it was somehow defective?

  5. #5
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by defragster View Post
    Wondering if there was something I did to put the AMP into the high heat state or if it was somehow defective?
    I'm wondering the same. From only the description here, I can't even know what's really happening.

    Did you try leaving the 5V power connected while it's doing this, but disconnect the speaker? It's really be helpful to know whether the chip is getting hot when no load is connected. Likewise if the problem continues when pin 5 puts it into low power mode.

  6. #6
    Senior Member+ defragster's Avatar
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    Happy to try whatever you suggest or even mail it back - I have it here and know less than you On K66 non-DAC pin Talkie sounded awful - felt the heat radiating to my hand as I was checking wires.

    Fully connected to Teensy, 'but disconnect the speaker' : Result :: HOT in under 10 secs. { same with speaker 0.5W@8ohm, or headphones }

    I think this should answer: "when pin 5 puts it into low power mode"
    Code:
      pinMode(5, OUTPUT);
      delay(20000);
      Serial.println("NOT HOT");
      digitalWrite(5, 1);
      delay( 10000 );
      Serial.println("quite HOT"); // I could barely keep my finger in place only knowing the pain would end
      digitalWrite(5, 0);
      delay( 5000 );
      Serial.println("NOT HOT");
    <edit>: This Prop_LC was unpacked and soldered for K66 first use - the only thing I might have done wrong was in pulling incoming DAC pin from p33/A14 (old T_3.1 number) - but there is nothing down there that looks dangerous as an input if I was off by one? The amp works normally as far as I can hear - just hot? Are both GND and 3.3V in the normal range of inputs to Prop_LC?
    Last edited by defragster; 07-12-2016 at 05:15 AM.

  7. #7
    Senior Member+ defragster's Avatar
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    Paul/?: With p#6 test notes - Any ideas for further tests ( to identify or correct the operation ) or how I might have caused this state of tremendous heat versus defect?

    Would better pics indicate if I've got a solder overrun on some vital component?

    I have no concept of the AMP operation - how it could arrive at this state or maintain function at a usable/normal level. It was the first use of this device - and if not for poor sound quality owing to the needed software adjustment to the (K66) DAC pin - I might not have had my hand near enough to it to feel the radiant heat as I checked connections to explain he poor sound quality from A14, that is the T_3.2 DAC pin.

  8. #8
    Senior Member+ defragster's Avatar
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    Wondering about current draw I breadboarded the PROP_LC.

    Brought over GND and 3.3V from a T_3.1. Routed 5V through DVM on 200mA reading:

    < 2 mA with no 3.3v on AMP pin
    190 mA and Gets hot** when 3.3V applied to AMP pin with GND or Nothing on DAC input.

    **Not as hot (touch safe) and Not as fast (over 20 secs versus 10) as direct wired - I assume the breadboard and meter/wires are limiting the current.

    Just repeated with the 2nd pinned PROP_LC:
    < 2 mA with no 3.3v on AMP pin
    < 3.10 mA when 3.3V applied to AMP pin with GND or Nothing on DAC input.

    - I didn't measure across 3.3V as I assume the heat is from 5V?

  9. #9
    Senior Member+ defragster's Avatar
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    bump . . . this isn't critical in any way given the goings on . . . but I do wonder about this and don't want to forget it.

  10. #10
    Senior Member+ defragster's Avatar
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    ping ... emailed Robin . . .

  11. #11
    Senior Member+ defragster's Avatar
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    Paul - any ideas on getting to the bottom of this PROP_LC with the dysfunctional AMP?

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